A Learner's Guide To Programming Using The Python Language

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Advance Praise for Head First Programming

“Head First Programming does a great job teaching programming using an iterative process. Add a little,

explain a little, make the program a little better. This is how programming works in the real world

and Head First Programming makes use of that in a teaching forum. I recommend this book to anyone

who wants to start dabbling in programming but doesn’t know where to start. I’d also recommend this

book to anyone not necessarily new to programming, but curious about Python. It’s a great intro to

programming in general and programming Python specifically.”

— Jeremy Jones, Coauthor of Python for Unix and Linux System Administration

“David Griffiths and Paul Barry have crafted the latest gem in the Head First series. Do you use a

computer, but are tired of always using someone else’s software? Is there something you wish your

computer would do but wasn’t programmed for? In Head First Programming, you’ll learn how to write

code and make your computer do things your way.”

— Bill Mietelski, Software Engineer

“Head First Programming provides a unique approach to a complex subject. The early chapters make

excellent use of metaphors to introduce basic programming concepts used as a foundation for the rest

of the book. This book has everything, from web development to graphical user interfaces and game

programming.”

— Doug Hellmann, Senior Software Engineer, Racemi

“A good introduction to programming using one of the best languages around, Head First Programming

uses a unique combination of visuals, puzzles, and exercises to teach programming in a way that is

approachable and fun.”

— Ted Leung, Principal Software Engineer, Sun Microsystems

Praise for other Head First books

“Kathy and Bert’s Head First Java transforms the printed page into the closest thing to a GUI you’ve ever

seen. In a wry, hip manner, the authors make learning Java an engaging ‘what’re they gonna do next?’

experience.”

— Warren Keuffel, Software Development Magazine

“Beyond the engaging style that drags you forward from know-nothing into exalted Java warrior status, Head

First Java covers a huge amount of practical matters that other texts leave as the dreaded ‘exercise for the

reader....’ It’s clever, wry, hip and practical—there aren’t a lot of textbooks that can make that claim and live

up to it while also teaching you about object serialization and network launch protocols.”

— Dr. Dan Russell, Director of User Sciences and Experience Research

IBM Almaden Research Center (and teaches Artiﬁcial Intelligence at

Stanford University)

“It’s fast, irreverent, fun, and engaging. Be careful—you might actually learn something!”

— Ken Arnold, former Senior Engineer at Sun Microsystems

Coauthor (with James Gosling, creator of Java), The Java Programming

Language

“I feel like a thousand pounds of books have just been lifted off of my head.”

— Ward Cunningham, inventor of the Wiki and founder of the Hillside Group

“Just the right tone for the geeked-out, casual-cool guru coder in all of us. The right reference for practi-

cal development strategies—gets my brain going without having to slog through a bunch of tired, stale

professor -speak.”

— Travis Kalanick, Founder of Scour and Red Swoosh

Member of the MIT TR100

“There are books you buy, books you keep, books you keep on your desk, and thanks to O’Reilly and the

Head First crew, there is the penultimate category, Head First books. They’re the ones that are dog-eared,

mangled, and carried everywhere. Head First SQL is at the top of my stack. Heck, even the PDF I have

for review is tattered and torn.”

— Bill Sawyer, ATG Curriculum Manager, Oracle

“This book’s admirable clarity, humor and substantial doses of clever make it the sort of book that helps

even non-programmers think well about problem-solving.”

— Cory Doctorow, co-editor of Boing Boing

Author, Down and Out in the Magic Kingdom

and Someone Comes to Town, Someone Leaves Town

Praise for other Head First books

“I received the book yesterday and started to read it... and I couldn’t stop. This is definitely très ‘cool.’ It

is fun, but they cover a lot of ground and they are right to the point. I’m really impressed.”

— Erich Gamma, IBM Distinguished Engineer, and co-author of Design Patterns

“One of the funniest and smartest books on software design I’ve ever read.”

— Aaron LaBerge, VP Technology, ESPN.com

“What used to be a long trial and error learning process has now been reduced neatly into an engaging

paperback.”

— Mike Davidson, CEO, Newsvine, Inc.

“Elegant design is at the core of every chapter here, each concept conveyed with equal doses of

pragmatism and wit.”

— Ken Goldstein, Executive Vice President, Disney Online

“I ♥ Head First HTML with CSS & XHTML—it teaches you everything you need to learn in a ‘fun coated’

format.”

— Sally Applin, UI Designer and Artist

“Usually when reading through a book or article on design patterns, I’d have to occasionally stick myself

in the eye with something just to make sure I was paying attention. Not with this book. Odd as it may

sound, this book makes learning about design patterns fun.

“While other books on design patterns are saying ‘Buehler… Buehler… Buehler…’ this book is on the

float belting out ‘Shake it up, baby!’”

— Eric Wuehler

“I literally love this book. In fact, I kissed this book in front of my wife.”

— Satish Kumar

Other related books from O’Reilly

Learning Python

Programming Python

Python Cookbook

Other books in O’Reilly’s Head First series

Head First JavaTM

Head First Object-Oriented Analysis and Design (OOA&D)

Head First HTML with CSS and XHTML

Head First Design Patterns

Head First Data Analysis

Head First Servlets and JSP

Head First EJB

Head First PMP

Head First SQL

Head First Software Development

Head First JavaScript

Head First Ajax

Head First Physics

Head First Statistics

Head First Rails

Head First PHP & MySQL

Head First Algebra

Head First Web Design

Beijing • Cambridge • Farnham • Kln • Sebastopol • Taipei • Tokyo

Head First

Programming

Wouldn‛t it be dreamy if

there were an introductory

programming book that didn‛t

make you wish you were anywhere

other than stuck in front of your

computer writing code? I guess it‛s

just a fantasy...

Paul Barry

David Grifths

Head First Programming

by Paul Barry and David Griffiths

Printed in the United States of America.

Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472.

O’Reilly Media books may be purchased for educational, business, or sales promotional use. Online editions are

also available for most titles (safari.oreilly.com). For more information, contact our corporate/institutional sales

department: (800) 998-9938 or corporate@oreilly.com.

Series Creators: Kathy Sierra, Bert Bates

Series Editor: Brett D. McLaughlin

Editor: Brian Sawyer

Cover Designers: Steve Fehler

Production Editor: Scott DeLugan

Proofreader: Colleen Toporek

Indexer: Angela Howard

Page Viewers: David: Dawn; Paul: Deirdre, Joseph, Aaron, and Aideen

Printing History:

November 2009: First Edition.

The O’Reilly logo is a registered trademark of O’Reilly Media, Inc. The Head First series designations,

Head First Programming, and related trade dress are trademarks of O’Reilly Media, Inc.

Many of the designations used by manufacturers and sellers to distinguish their products are claimed as

trademarks. Where those designations appear in this book, and O’Reilly Media, Inc., was aware of a trademark

claim, the designations have been printed in caps or initial caps.

While every precaution has been taken in the preparation of this book, the publisher and the authors assume no

responsibility for errors or omissions, or for damages resulting from the use of the information contained herein.

No surfers or snakes were harmed in the making of this book.

ISBN: 978-0-596-80237-0

[C]

This book uses RepKover™

, a durable and ﬂexible lay-ﬂat binding.

Aideen (future

singer/songwriter)

Paul

Deirdre

(super mom)

Aaron and Joseph

(real-life surfer

dudes)

Dawn (without

whom the book

would never have

been finished)

We dedicate this book to the first person who looked at a computer

and then asked the question, “I wonder how I make it do this... ?”

And to those that made programming complex enough that people

need a book like ours to learn it.

David: To Dawn. The smartest person I know.

Paul: This one’s dedicated to my father, Jim Barry, who, 25 years

ago—when I needed a push—pushed me toward computing. That

was a good push.

viii

the authors

Authors of Head First Programming

Paul Barry recently worked out that he has

been programming for close to a quarter century, a

fact that came as a bit of a shock. In that time, Paul

has programmed in lots of different programming

languages, lived and worked in two countries on two

continents, got married, had three kids (well... his wife

Deirdre actually had them, but Paul did play his part),

completed a B.Sc. and M.Sc. in Computing, written

two other books, written a bunch of technical articles

for Linux Journal, and managed not to lose his hair... a

situation that, sadly, may in fact be changing.

When Paul first saw Head First HTML with CSS &

XHTML, he loved it so much he knew immediately that

the Head First approach would be a great way to teach

programming. He is only too delighted, together with

David, to create this book in an attempt to prove his

hunch correct.

Paul’s day job is working as a lecturer at The Institute

of Technology, Carlow in Ireland. As part of the

Department of Computing & Networking, Paul gets

to spend his day exploring, learning, and teaching

cool programming technologies, which is his idea of

fun (and further proof that Paul probably needs to get

out more). Paul hopes his students think the stuff he

teaches is fun, too.

Paul Barry

David Griffiths

David Griffiths began programming at age 12,

when he saw a documentary on the work of Seymour

Papert. At age 15, he wrote an implementation of

Papert’s computer language LOGO. After studying

Pure Mathematics at University, he began writing code

for computers and magazine articles for humans. He’s

worked as an agile coach, a developer, and a garage

attendant, but not in that order. He can write code in

over 10 languages and prose in just one, and when not

writing, coding, or coaching, he spends much of his

spare time travelling with his lovely wife—and fellow

Head First author—Dawn.

Before writing Head First Programming, he wrote another

book called Head First Rails, which is an excellent read

and would make a thoughtful gift for any close friend or

family member.

You can follow him on Twitter at:

http://twitter.com/dgriffiths

table of contents

Table of Contents (Summary)

Table of Contents (the real thing)

Your brain on Programming. Here you are trying to learn something,

while here your brain is doing you a favor by making sure the learning doesn’t stick.

Your brain’s thinking, “Better leave room for more important things, like which wild

animals to avoid and whether naked snowboarding is a bad idea.” So how do you

trick your brain into thinking that your life depends on knowing Programming?

Intro

Who is this book for? xxiv

We know what you’re thinking xxv

Metacognition xxvii

Bend your brain into submission xxix

Read me xxx

The technical review team xxxii

Acknowledgments xxxiii

Intro xxiii

1 Starting to Code: Finding Your Way 1

2 Textual Data: Every String Has Its Place 37

3 Functions: Let’s Get Organized 77

4 Data Files and Arrays: Sort It Out 113

5 Hashes and Databases: Putting Data in Its Place 145

6 Modular Programming: Keeping Things Straight 177

7 Building a Graphical User Interface: Going All Gooey 215

8 GUIs and Data: Data Entry Widgets 257

8½ Exceptions and Message Boxes: Get the Message? 293

9 Graphical Interface Elements: Selecting the Right Tool 313

10 Custom Widgets and Classes: With an Object in Mind 349

i Leftovers: The Top Ten Things (We Didn’t Cover) 385

table of contents

Programming lets you do more 2

So how do you run your code? 5

Create a new program file 6

Prepare and run your code 7

A program is more than a list of commands 12

Codeville: Your program is like a network of roads 13

Branches are code intersections 14

if/else branches 15

The Python code needs interconnecting paths 20

Python uses indents to connect paths 21

Loops let you run the same piece of code over and over again 28

Python’s while loop 29

Your Programming Toolbox 35

Finding your way

1starting to code

Writing programs gives you the power to control your PC.

Almost everyone knows how to use a computer, but few people take the next step and

learn how to control it. If you use other people’s software, you will always be limited by

what other people think you want to do. Write your own programs and the only limit will

be your own imagination. Programming will make you more creative, it will make you

think more precisely, and it will teach you to analyze and solve problems logically.

Do you want to be programmed or be the programmer?

guess <= 5?

table of contents

Every string has its place

Imagine trying to communicate without words.

All programs process data, and one of the most important types of data is text. In this

chapter, you’ll work through the basics of textual data. You’ll automatically search

text and get back exactly what you’re looking for. Along the way, you’ll pick up key

programming concepts such as methods and how you can use them to bend your

data to your will. And finally, you’ll instantly power up your programs with the help of

library code.

textual data

Your new gig at Starbuzz Coffee 38

Here’s the current Starbuzz code 39

The cost is embedded in the HTML 41

A string is a series of characters 41

Find characters inside the text 42

But how do you get at more than one character? 43

The String Exposed 48

Beans’R’Us is rewarding loyal customers 50

Searching is complex 52

Python data is smart 54

Strings and numbers are different 64

The program has overloaded the Beans’R’Us Server 67

Time... if only you had more of it 68

You’re already using library code 69

Order is restored 74

Your Programming Toolbox 75

table of contents

xii

Starbuzz is out of beans! 78

What does the new program need to do? 79

Don’t duplicate your code... 81

Reuse code with functions 82

Always get things in the right order 84

Return data with the return command 87

Use the Web, Luke 93

The function always sends the same message 94

Use parameters to avoid duplicating functions 96

Someone decided to mess with your code 102

The rest of the program can’t see the password variable 104

When you call a function, the computer creates a fresh list of variables 105

When you leave a function, its variables get thrown away 106

Starbuzz is fully stocked! 110

Your Programming Toolbox 111

Let’s get organized

3functions

As programs grow, the code often becomes more complex.

And complex code can be hard to read, and even harder to maintain. One way of

managing this complexity is to create functions. Functions are snippets of code

that you use as needed from within your program. They allow you to separate out

common actions, and this means that they make your code easier to read and

easier to maintain. In this chapter, you’ll discover how a little function knowledge

can make your coding life a whole lot easier.

table of contents

xiii

Sort it out

As your programs develop, so do your data handling needs.

And when you have lots of data to work with, using an individual variable for each piece

of data gets really old, really quickly. So programmers employ some rather awesome

containers (known as data structures) to help them work with lots of data. More times

than not, all that data comes from a file stored on a hard disk. So, how can you work

with data in your files? Turns out it’s a breeze.

data files and arrays

Hey, dude, it‛s

Chapter 4... time

for a break - let‛s

catch some waves.

Surf ’s up in Codeville 114

Find the highest score in the results file 115

Iterate through the file with the open, for, close pattern 116

The file contains more than numbers... 120

Split each line as you read it 121

The split() method cuts the string 122

But you need more than one top score 126

Keeping track of 3 scores makes the code more complex 127

An ordered list makes code much simpler 128

Sorting is easier in memory 129

You can’t use a separate variable for each line of data 130

An array lets you manage a whole train of data 131

Python gives you arrays with lists 132

Sort the array before displaying the results 136

Sort the scores from highest to lowest 139

And the winner is...? 142

You somehow forgot the surfer names 143

Your Programming Toolbox 144

table of contents

Who won the surfing contest? 146

Associate the name with the score 150

Associate a key with a value using a hash 153

Iterate hash data with for 154

The data isn’t sorted 158

When data gets complex 160

Return a data structure from a function 164

H ’s your new board! 168

while, down at the studio... 169

code remains the same; it’s the function that changes 170

’s data is on the money! 174

Programming Toolbox 175

Putting data in its place

5hashes and databases

Arrays aren’t the only show in town when it comes to data.

Programming languages come with other data-arranging goodies too, and our

chosen tool, Python, is no exception. In this chapter, you’ll associate values

with names using a data structure commonly called the hash (better known as

dictionary to Python-folk). And when it comes to working with stored data, you’ll

read data from an external database system as well as from regular text-based

files. All the world’s awash with data, so turn the page and start applying your ever-

expanding programming skills to some cool data-processing tasks.

table of contents

Keeping things straight

The code that you write will make its way into many programs.

And, although sharing is good, you need to be careful. One programmer might take

your code and use it in an unexpected way, while another might change it without even

letting you know. You might want to use one function in all your programs and, over

time, that function’s code might change to suit your needs. Smart programmers take

advantage of modular programming techniques to keep their workload manageable.

modular programming

Head First Health Club is upgrading some systems 178

The program needs to create a transaction file 179

Use strings to format strings 180

The Format String Exposed 186

A late night email ruins your day 187

$50,000... for a donut?! 188

Only the sales from your program were rejected 189

The new bank uses a new format 190

Your coffee bar program still uses the old format 191

Don’t just update your copy 192

So how do you create a module...? 193

The transaction file is working great, too 199

The health club has a new requirement 200

The Starbuzz code 205

The two discount functions have the same name 206

Fully Qualified Names (FQNs) prevent your programs from getting confused 207

The discounts get the customers flooding in 213

Your Programming Toolbox 214

table of contents

xvi

Head First TVN now produces game shows 216

pygame is cross platform 220

pygame Exposed 229

0... 2... 1... 9... blast off ! 230

tkinter gives you the event loop for free 234

tkinter is packed with options 235

The GUI works, but doesn’t do anything 238

Connect code to your button events 239

The GUI program’s now ready for a screentest 244

But TVN is still not happy 246

Label it 249

Your Programming Toolbox 255

Going all gooey

building a graphical user interface

Your coding skills are great and getting better all the time.

It’s just a shame your programs are not that nice to look at. Displaying prompts

and messages on a text-based console is all well and good, but it’s so 1970s,

isn’t it? Add some green text on a black background and your retro look will be

complete. There has to be a better way to communicate with your users than

the console, and there is: using a graphical user interface or GUI (pronounced

“gooey”). Sounds cool, but complex, and it can be. But, don’t fret; learning a trick or

two will have your code all graphical in no time. Let’s get all gooey (sorry, GUI) in

this chapter.

table of contents

xvii

8Data entry widgets

GUIs don’t just process events. They also handle data.

Almost all GUI applications need to read user data, and choosing the right widgets can

change your interface from data entry hell to user heaven. Widgets can accept plain

text, or just present a menu of options. There are lots of different widgets out there,

which means there are lots of choices, too. And, of course, making the right choice can

make all the difference. It’s time to take your GUI program to the next level.

guis and data

Head-Ex needs a new delivery system 258

They’ve already designed the interface 259

Read data from the GUI 260

The Entry and Text widgets let you enter text data into your GUI 261

Read and write data to text fields 262

Large Text fields are harder to handle 263

One of the Head-Ex deliveries went astray 270

Users can enter anything in the fields 271

Radio buttons force users to choose a valid depot 272

Creating radio buttons in tkinter 273

The radio buttons should work together 275

The radio buttons can share a model 276

The system tells the other widgets when the model changes 277

So how do you use models in tkinter? 278

Head-Ex’s business is expanding 282

There are too many depots on the GUI 283

An OptionMenu lets you have as many options as needed 284

The model stays the same 285

Things are going great at Head-Ex 291

Your Programming Toolbox 292

Look, I don‛t care

what you guys do, I‛m

gonna stay selected.

Yeah,

me too.

Huh, and me.

table of contents

xviii

What’s that smell? 294

Someone changed the file permissions 295

When it couldn’t write to the file, the program threw an exception 296

Catch the exception 297

Watch for exceptions with try/except 298

There’s an issue with the exception handler 302

A message box demands attention 303

Creating message boxes in Python 304

Your Programming Toolbox 311

Get the message?

exceptions and message boxes

Sometimes things just go wrong. You just need to handle it.

There will always be things beyond your control. Networks will fail. Files will

disappear. Smart coders learn how to deal with those kinds of errors and make

their programs recover gracefully. The best software keeps the user informed

about the bad things that happen and what should be done to recover. By learning

how to use exceptions and message boxes, you can take your software to the

next level of reliability and quality.

8 /

table of contents

xix

Selecting the right tool

It’s easy to make your programs more effective for your users.

And when it comes to GUI applications, there’s a world of difference between a working

interface and one that’s both useful and effective. Selecting the right tool for the

right job is a skill that comes with experience, and the best way to get that experience

is to use the tools available to you. In this chapter, you’ll continue to expand your

GUI application building skills. There’s a bunch of truly useful widgets waiting to be

experienced. So, turn the page and let’s get going.

graphical interface elements

Time to mix it up 314

The music just kept on playing... 318

Not all events are generated by button clicks 319

Capturing the protocol event isn’t enough 326

Two buttons, or not two buttons? That is the question... 328

The checkbox is an on/off, flip/flop toggle 331

Working with checkboxes in tkinter 332

Pump up the volume! 336

Model a slider on a scale 337

Use pygame to set the volume 339

Use tkinter for everything else 340

The DJ is over the moon! 347

Your Programming Toolbox 348

table of contents

The DJ wants to play more than one track 350

Create code for each track as a function 351

The new function contains other functions 356

Your new function needs to create widgets and event handlers 357

The DJ is confused 362

Group widgets together 363

A frame widget contains other widgets 364

A class is a machine for creating objects 366

A class has methods that define behavior 367

But how does an object call a method? 369

The SoundPanel class looks a lot like the create_gui() function 370

class = methods + data 372

The Class Exposed 373

The DJ has an entire directory of tracks 378

It’s party time! 382

Your Programming Toolbox 383

Leaving town... 384

It’s been great having you here in Codeville! 384

With an object in mind

10 custom widgets and classes

Requirements can be complex, but programs don’t have

to be.

By using object orientation, you can give your programs great power without

writing lots of extra code. Keep reading, and you’ll create custom widgets that do

exactly what you want and give you the power to take your programming skills

to the next level.

table of contents

xxi

The Top Ten Things (we didn’t cover)

You’ve come a long way.

But learning how to program is an activity that never stops. The more you code, the

more you’ll need to learn new ways to do certain things. You’ll need to master new

tools and new techniques, too. There’s just not enough room in this book to show you

everything you might possibly need to know. So, here’s our list of the top ten things we

didn’t cover that you might want to learn more about next.

leftovers

#1: Doing things “The Python Way” 386

#2: Using Python 2 387

#3: Other programming languages 388

#4: Automated testing techniques 389

#5: Debugging 390

#6: Command-line execution 391

#7: Ooops... we could’ve covered more OOP 392

#8: Algorithms 393

#9: Advanced programming topics 394

#10: Other IDEs, shells and text editors 395

xxiii

how to use this book

Intro

In this section we answer the burning question:

“So why DID they put that in a programming book?”

I can‛t believe

they put

that

in a

programming book.

xxiv intro

how to use this book

Who is this book for?

Who should probably back away from this book?

If you can answer “yes” to all of these:

If you can answer “yes” to any of these:

this book is for you.

this book is not for you.

[Note from marketing: this book

is for anyone with a credit card...

we’ll accept a check, too.]

Do you prefer actually doing things and applying the stuff

you learn over listening to someone in a lecture rattle on

for hours on end?

Do you want to learn how to program, so you can create

the next big thing in software, make a small fortune, and

retire to your own private island?

Are you looking for a quick introduction or reference book

to Python?

Do you wish you had the know-how to control your

computer and make it do new things?

Are you a seasoned programmer? Do you already know

how to program?

Would you rather have your toenails pulled out by 15

screaming monkeys than learn something new? Do you

believe a programming book should cover everything and

if it bores the reader to tears in the process then so much

the better?

OK, maybe that one’s a little

far-fetched. But, you gotta

start somewhere, right?

you are here 4 xxv

“How can this be a serious Programming book?”

“What’s with all the graphics?”

“Can I actually learn it this way?”

Your brain craves novelty. It’s always searching, scanning, waiting for something

unusual. It was built that way, and it helps you stay alive.

So what does your brain do with all the routine, ordinary, normal things

you encounter? Everything it can to stop them from interfering with the

brain’s real job—recording things that matter. It doesn’t bother saving the

boring things; they never make it past the “this is obviously not important”

filter.

How does your brain know what’s important? Suppose you’re out for a day

hike and a tiger jumps in front of you, what happens inside your head and

body?

Neurons fire. Emotions crank up. Chemicals surge.

And that’s how your brain knows...

This must be important! Don’t forget it!

But imagine you’re at home, or in a library. It’s a safe, warm, tiger-free zone.

You’re studying. Getting ready for an exam. Or trying to learn some tough

technical topic your boss thinks will take a week, ten days at the most.

Just one problem. Your brain’s trying to do you a big favor. It’s trying to

make sure that this obviously non-important content doesn’t clutter up scarce

resources. Resources that are better spent storing the really big things.

Like tigers. Like the danger of fire. Like how you should never have

posted those “party” photos on your Facebook page. And there’s no

simple way to tell your brain, “Hey brain, thank you very much, but

no matter how dull this book is, and how little I’m registering on the

emotional Richter scale right now, I really do want you to keep this

stuff around.”

We know what you’re thinking

We know what your

brain

is thinking

Your brain thinks

THIS is important.

Your brain thinks

THIS isn’t worth

saving.

Great. Only 464

more dull, dry,

boring pages.

xxvi intro

how to use this book

We think of a “Head First” reader as a learner.

So what does it take to

learn

something? First, you have to

get

it, then make sure

you don’t

forget

it. It’s not about pushing facts into your head. Based on the latest

research in cognitive science, neurobiology, and educational psychology,

learning

takes a lot more than text on a page. We know what turns your brain on.

Some of the Head First learning principles:

Make it visual. Images are far more memorable than words alone, and make learning much

more effective (up to 89% improvement in recall and transfer studies). It also makes things more

understandable. Put the words within or near the graphics they relate to, rather than on

the bottom or on another page, and learners will be up to twice as likely to solve problems related to the

content.

Use a conversational and personalized style. In recent studies, students performed up

to 40% better on post-learning tests if the content spoke directly to the reader, using a first-person,

conversational style rather than taking a formal tone. Tell stories instead of lecturing. Use casual language.

Don’t take yourself too seriously. Which would you pay more attention to: a stimulating dinner party

companion, or a lecture?

Get the learner to think more deeply. In other words, unless you actively flex your neurons,

nothing much happens in your head. A reader has to be motivated, engaged, curious, and inspired to

solve problems, draw conclusions, and generate new knowledge. And for that, you need challenges,

exercises, and thought-provoking questions, and activities that involve both sides of the brain and

multiple senses.

Get—and keep—the reader’s attention. We’ve all had the “I really want to learn this but I can’t

stay awake past page one” experience. Your brain pays attention to things that are out of the ordinary,

interesting, strange, eye-catching, unexpected. Learning a new, tough, technical topic doesn’t have to be

boring. Your brain will learn much more quickly if it’s not.

Touch their emotions. We now know that your ability to remember something is largely dependent

on its emotional content. You remember what you care about. You remember when you feel something.

No, we’re not talking heart-wrenching stories about a boy and his dog. We’re talking emotions like

surprise, curiosity, fun, “what the...?” , and the feeling of “I Rule!” that comes when you solve a puzzle, learn

something everybody else thinks is hard, or realize you know something that “I’m more technical than

thou” Bob from engineering doesn’t.

you are here 4 xxvii

Metacognition: thinking about thinking

I wonder how

I can trick my brain

into remembering

this stuff...

If you really want to learn, and you want to learn more quickly and more

deeply, pay attention to how you pay attention. Think about how you think.

Learn how you learn.

Most of us did not take courses on metacognition or learning theory when we

were growing up. We were expected to learn, but rarely taught to learn.

But we assume that if you’re holding this book, you really want to learn how

to program And you probably don’t want to spend a lot of time. If you want

to use what you read in this book, you need to remember what you read. And

for that, you’ve got to understand it. To get the most from this book, or any book

or learning experience, take responsibility for your brain. Your brain on this

content.

The trick is to get your brain to see the new material you’re learning as

Really Important. Crucial to your well-being. As important as a tiger.

Otherwise, you’re in for a constant battle, with your brain doing its best to

keep the new content from sticking.

So just how

you get your brain to treat

programming like it was a hungry tiger?

There’s the slow, tedious way, or the faster, more effective way. The

slow way is about sheer repetition. You obviously know that you are able to learn

and remember even the dullest of topics if you keep pounding the same thing into your

brain. With enough repetition, your brain says, “This doesn’t feel important to him, but he

keeps looking at the same thing over and over and over, so I suppose it must be.”

The faster way is to do anything that increases brain activity, especially different

types of brain activity. The things on the previous page are a big part of the solution,

and they’re all things that have been proven to help your brain work in your favor. For

example, studies show that putting words within the pictures they describe (as opposed to

somewhere else in the page, like a caption or in the body text) causes your brain to try to

makes sense of how the words and picture relate, and this causes more neurons to fire.

More neurons firing = more chances for your brain to get that this is something worth

paying attention to, and possibly recording.

A conversational style helps because people tend to pay more attention when they

perceive that they’re in a conversation, since they’re expected to follow along and hold up

their end. The amazing thing is, your brain doesn’t necessarily care that the “conversation”

is between you and a book! On the other hand, if the writing style is formal and dry, your

brain perceives it the same way you experience being lectured to while sitting in a roomful

of passive attendees. No need to stay awake.

But pictures and conversational style are just the beginning…

xxviii intro

how to use this book

Here’s what WE did:

We used pictures, because your brain is tuned for visuals, not text. As far as your brain’s

concerned, a picture really is worth a thousand words. And when text and pictures work

together, we embedded the text in the pictures because your brain works more effectively

when the text is within the thing the text refers to, as opposed to in a caption or buried in the

text somewhere.

We used redundancy, saying the same thing in different ways and with different media types,

and multiple senses, to increase the chance that the content gets coded into more than one area

of your brain.

We used concepts and pictures in unexpected ways because your brain is tuned for novelty,

and we used pictures and ideas with at least some emotional content, because your brain

is tuned to pay attention to the biochemistry of emotions. That which causes you to feel

something is more likely to be remembered, even if that feeling is nothing more than a little

humor, surprise, or interest.

We used a personalized, conversational style, because your brain is tuned to pay more

attention when it believes you’re in a conversation than if it thinks you’re passively listening

to a presentation. Your brain does this even when you’re reading.

We included more than 80 activities, because your brain is tuned to learn and remember

more when you do things than when you read about things. And we made the exercises

challenging-yet-do-able, because that’s what most people prefer.

We used multiple learning styles, because you might prefer step-by-step procedures, while

someone else wants to understand the big picture first, and someone else just wants to see

an example. But regardless of your own learning preference, everyone benefits from seeing the

same content represented in multiple ways.

We include content for both sides of your brain, because the more of your brain you

engage, the more likely you are to learn and remember, and the longer you can stay focused.

Since working one side of the brain often means giving the other side a chance to rest, you

can be more productive at learning for a longer period of time.

And we included stories and exercises that present more than one point of view,

because your brain is tuned to learn more deeply when it’s forced to make evaluations and

judgments.

We included challenges, with exercises, and by asking questions that don’t always have

a straight answer, because your brain is tuned to learn and remember when it has to work at

something. Think about it—you can’t get your body in shape just by watching people at the

gym. But we did our best to make sure that when you’re working hard, it’s on the right things.

That you’re not spending one extra dendrite processing a hard-to-understand example,

or parsing difficult, jargon-laden, or overly terse text.

We used people. In stories, examples, pictures, etc., because, well, because you’re a person.

And your brain pays more attention to people than it does to things.

you are here 4 xxix

So, we did our part. The rest is up to you. These tips are a

starting point; listen to your brain and figure out what works

for you and what doesn’t. Try new things.

6Drink water. Lots of it.

Your brain works best in a nice bath of fluid.

Dehydration (which can happen before you ever

feel thirsty) decreases cognitive function.

9Write a lot of code!

There’s only one way to learn to program: writing

a lot of code. And that’s what you’re going to

do throughout this book. Coding is a skill, and the

only way to get good at it is to practice. We’re going

to give you a lot of practice: every chapter has

exercises that pose a problem for you to solve. Don’t

just skip over them—a lot of the learning happens

when you solve the exercises. We included a solution

to each exercise—don’t be afraid to peek at the

solution if you get stuck! (It’s easy to get snagged

on something small.) But try to solve the problem

before you look at the solution. And definitely get it

working before you move on to the next part of the

book.

8Feel something.

Your brain needs to know that this matters. Get

involved with the stories. Make up your own

captions for the photos. Groaning over a bad joke

is still better than feeling nothing at all.

7Listen to your brain.

Pay attention to whether your brain is getting

overloaded. If you find yourself starting to skim

the surface or forget what you just read, it’s time

for a break. Once you go past a certain point, you

won’t learn faster by trying to shove more in, and

you might even hurt the process.

5Talk about it. Out loud.

Speaking activates a different part of the brain. If

you’re trying to understand something, or increase

your chance of remembering it later, say it out loud.

Better still, try to explain it out loud to someone else.

You’ll learn more quickly, and you might uncover

ideas you hadn’t known were there when you were

reading about it.

4Make this the last thing you read before bed.

Or at least the last challenging thing.

Part of the learning (especially the transfer to

long-term memory) happens

after

you put the book

down. Your brain needs time on its own, to do more

processing. If you put in something new during that

processing time, some of what you just learned will

be lost.

3Read the “There are No Dumb Questions”

That means all of them. They’re not optional

sidebars, they’re part of the core content!

Don’t skip them.

Cut this out and stick it

on your refrigerator.

Here’s what YOU can do to bend

your brain into submission

2Do the exercises. Write your own notes.

We put them in, but if we did them for you, that

would be like having someone else do your workouts

for you. And don’t just look at the exercises. Use a

pencil. There’s plenty of evidence that physical

activity

while

learning can increase the learning.

Don’t just read. Stop and think. When the book asks

you a question, don’t just skip to the answer. Imagine

that someone really is asking the question. The

more deeply you force your brain to think, the better

chance you have of learning and remembering.

Slow down. The more you understand, the

less you have to memorize.

xxx intro

how to use this book

Read Me

This is a learning experience, not a reference book. We deliberately stripped out everything

that might get in the way of learning whatever it is we’re working on at that point in the

book. And the first time through, you need to begin at the beginning, because the book

makes assumptions about what you’ve already seen and learned.

This is not Head First Python.

We use release 3 of the Python programming language throughout this book, but this

fact alone does not make this book Head First Python. We chose Python because it’s a great

programming language to start with and it’s also a great programming language to grow

with. In fact, Python might be the only programming language you’ll ever need to learn

and use (although your employer might think otherwise). Of course, you have to start with

something, and we can think of no better programming language to use than Python when

first learning how to program. That said, this book isn’t designed to teach you Python;

it’s designed to teach you programming, so most of the things we show you are designed to

hightlight the programming concept, not the Python feature.

You need to install Python 3 on your computer.

To run the programs in this book, you need to download and install Python 3 on your

computer. This isn’t as hard as it sounds. Pop on over to the Python download site and

select the option that fits best with the computer you are using. Just be sure to select release

3 of Python, not release 2: http://www.python.org/download.

We begin by teaching some basic programming concepts, then we

start putting programming to work for you right away.

We cover the fundamentals of programming in Chapter 1. That way, by the time you

make it all the way to Chapter 2, you are creating programs that actually do something

real, useful, and—gulp!—fun. We are guessing you’ll be amazed by how much you can do

with less than a dozen lines of code in Chapter 2. The rest of the book then builds on your

programming skills turning you from programming newbie to coding ninja master in no time.

you are here 4 xxxi

The activities are NOT optional.

The exercises and activities are not add-ons; they’re part of the core content of the book.

Some of them are to help with memory, some are for understanding, and some will help

you apply what you’ve learned. Don’t skip the exercises.

The redundancy is intentional and important.

One distinct difference in a Head First book is that we want you to really get it. And we

want you to finish the book remembering what you’ve learned. Most reference books

don’t have retention and recall as a goal, but this book is about learning, so you’ll see some

of the same concepts come up more than once.

The examples are as lean as possible.

Our readers tell us that it’s frustrating to wade through 200 lines of an example looking

for the two lines they need to understand. Most examples in this book are shown within

the smallest possible context, so that the part you’re trying to learn is clear and simple.

Don’t expect all of the examples to be robust, or even complete—they are written

specifically for learning, and aren’t always fully-functional.

We’ve placed a lot of the code examples on the Web so you can copy and paste them as

needed. You’ll find them at two locations:

http://www.headfirstlabs.com/books/hfprog/ and

http://programming.itcarlow.ie

The Brain Power exercises don’t have answers.

For some of them, there is no right answer, and for others, part of the learning

experience of the Brain Power activities is for you to decide if and when your answers

are right. In some of the Brain Power exercises, you will find hints to point you in the

right direction.

xxxii intro

the review teamthe review team

Doug Hellmann is a Senior Software Engineer

at Racemi and former Editor in Chief for Python

Magazine. He has been programming in Python since

version 1.4 and prior to Python worked mostly with C

on a variety of Unix and non-Unix platforms. He has

worked on projects ranging from mapping to medical

news publishing, with a little banking thrown in for

good measure. Doug spends his spare time working on

several open source projects; reading science fiction,

history, and biographies; and writing the Python Module

of the Week blog series.

Jeremy Jones is the coauthor of Python for Unix and

Linux System Administration and has been actively using

Python since 2001. He has been a developer, system

administrator, quality assurance engineer, and tech

support analyst. They all have their rewards and

challenges, but his most challenging and rewarding job

has been husband and father.

Ted Leung has been programming in Python since

2002 and is currently a Principal Software Engineer

at Sun Microsystems. He has a Bachelor of Science

degree in Mathematics from the Massachusetts Institute

of Technology and a Master’s of Science degree in

computer science from Brown University. Ted’s weblog

is available at http://www.sauria.com/blog

Bill Mietelski has been an Information Technology

geek for over 20 years. He’s currently a Software

Engineer at a leading national academic medical center

in the Chicagoland area, working on statistical research

studies. When he’s not at the office or tied to a computer,

you’ll find him at a golf course chasing a little white ball.

Technical Reviewers:

Jeremy Jones

Bill Mietelski

Ted Leung

The technical review team

Doug Hellman

you are here 4 xxxiii

Our editor:

Brian Sawyer was Head First Programming’s editor. When not editing

books, Brian likes to run marathons “for fun.” It transpires that this

was the perfect training for working on the marathon that producing

this book turned into. At times, Brian worked us very hard, and,

consequently, we have a much better book.

The O’Reilly team:

Acknowledgments

Brian Sawyer

Brett McLaughlin, the Series Editor, kept an eye on what we were up to and on more than once occasion came to

our rescue we when we found ourselves in trouble. Karen Shaner provided administrative support and very capably

coordinated our techical review process.

Friends and colleagues:

David and Paul are especially grateful to Lou Barr for first thinking that the two of them might like to work together

on this book, suggesting they do, and then working hard to get the idea approved at O’Reilly. Thanks, Lou!

David: My thanks to Kathy Sierra and Bert Bates for this extraordinary series of books. And to Andy Parker and

Joe Broughton and Carl Jacques and Simon Jones and the many other friends who have heard so little from me

whilst I was busy scribbling away.

Paul: My thanks to Nigel Whyte, Head of Department, Computing and Networking at The Institute of Technology,

Carlow for supporting my involvement in yet another writing project. Also at work, Dr. Christophe Meudec

reviewed the early chapters and offered some very welcome words of encouragement, together with suggestions as how

to make things just that little bit better. Joseph Kehoe also reviewed the early material and liked what he saw.

Family:

David: I owe a very particular thank you to my wife, the author of Head First Statistics, Dawn Griffiths. For her wit,

humor, patience, and ability to turn vague ideas into real chapters.

Paul: Thanks are due to my father, Jim Barry, who reviewed the early, draft material and (once again) pointed out

where my writing could be improved and clarified. Regretfully, my home life suffered as working on this book grew to

consume all of my free time. Deirdre, Joseph, Aaron, and Aideen had to bear the grunts and groans, huffs and

puffs, and more than a few roars as the pressure reached boiling point on more than one occasion. I sometimes wonder

how they put up with me, but somehow they do, and I’m very grateful for their ongoing love and support.

The without-whom list:

Our technical review team did an excellent job of keeping us straight and making sure what we covered was spot on.

Finally, we both owe a huge debt of gratitude to David’s wife, Dawn, who not only looks after David, but also got

involved in the production of this book at a time when things looked like we’d never finish. Without Dawn’s help, this

book would never have been done on time. Dawn is the Head First Programming guardian angel.

xxxiv intro

safari books online

Safari® Books Online

When you see a Safari® icon on the cover of your favorite

technology book that means the book is available online

through the O’Reilly Network Safari Bookshelf.

Safari offers a solution that’s better than e-books. It’s a virtual library that lets you

easily search thousands of top tech books, cut and paste code samples, download

chapters, and ﬁnd quick answers when you need the most accurate, current

information. Try it for free at http://safari.oreilly.com.

this is a new chapter 1

starting to code

I wish “sweet cheeks” here

would finish with the polishing.

I‛ve got to re-bore the cam

shaft and reprogram the EMS

for Saturday‛s drift race.

Finding your way

Writing programs gives you the power to control your PC.

Almost everyone knows how to use a computer, but few people take the next step and

learn how to control it. If you use other people’s software, you will always be limited by

what other people think you want to do. Write your own programs and the only limit will be

your own imagination. Programming will make you more creative, it will make you think

more precisely, and it will teach you to analyze and solve problems logically.

Do you want to be programmed or be the programmer?

2 Chapter 1

take control of your computer

Programming lets you do more

If only I could

program Twitter...

You want to do more with your computer. You want to take control.

Learning to program gives you the power to create and solve.

Learning to program puts you in charge.

But, how does programming work?

You’ve got problems to solve and work to do, but your existing software

doesn’t quite cut it. Even with all those programs on your computer,

you still need to do something different, something specific to you.

Let’s look at a simple game written in Python.

Whaddaya mean, that‛s

not the way the website

was programmed?!?

How do I get

my website to do

what I want?

Man, I‛m sooooo fed

up with the “same old,

same old” games.

you are here 4 3

starting to code

print("Welcome!")

g = input("Guess the number: ")

guess = int(g)

if guess == 5:

print("You win!")

else:

print("You lose!")

print("Game over!")

This code is a guessing-game program. Study it carefully, and

opposite each line of code in the program, write down what you

think the code does. If you’re not sure what a particular line of

code does, don’t worry, but try to guess anyway. One line has

already been entered to get you started:

Convert the input to a number.

This code is written in

release 3 of the Python

programming language, which

is used throughout this book.

4 Chapter 1

guessing games

This code is a guessing-game program. You were to write down

what you think the code does.

Don’t worry if your answers are

different from ours. If they are

similar, then everything is OK.

print("Welcome!")

g = input("Guess the number: ")

guess = int(g)

if guess == 5:

print("You win!")

else:

print("You lose!")

print("Game over!")

Convert the input to a number.

Display a welcome message.

Ask the user to input a guess.

Was the guessed number equal to 5?

Tell the user “You win!"

Otherwise...

...tell the user “You lose!"

Be careful with

= signs in code.

Programming

languages use =

signs for different

purposes. In most languages

(including Python), a double

equals (==) is a test for equality.

It means, “are these two things

equal?” In contrast, a single

equal (=) is an instruction

(known as assignment) that

means “set the value to.”

But what are g and guess?

You might be wondering what g and guess are in the code. They are

called variables and they’re used to keep track of data in the computer’s

memory.

A variable is really just a label for data. So if the user inputs “3” at the

keyboard, then guess will be set to the number 3, and whenever the

computer reads guess, it will read it as the value 3.

g = input("Guess the number: ")

guess = int(g)

A variable

The value entered will

be known as “g".

This creates a number-

version of the g-value and

calls it “guess".

End the program.

you are here 4 5

starting to code

So how do you run your code?

There are two things that you will need to run the guessing-game

program: an editor and an interpreter.

The editor saves the code you write into a file on your hard disk. The

code (sometimes called the source code) is just text, and it can be

written and read by humans.

But computers can’t process text meant for humans, at least not very well.

That’s why we need a tool to translate the human-friendly source code

into the binary 1s and 0s that computers do understand. That’s what an

interpreter does. In this book, an interpreter called Python is used.

So we need an editor and a Python interpreter. Fortunately, Python 3

comes with a built-in application called IDLE, which does both jobs and

more. IDLE enables you to write and edit Python code, it translates this

code into binary form, and finally, it runs the Python 3 program. Because

of this, IDLE is known as an Integrated Development Environment.

Let’s see these steps in action.

.py

Hmmm... looks

like a guessing

game...

The editor will save the

source code into a file.

The Python

interpreter

The interpreter translates

the text of the source

code into language that the

computer can understand.

The source code

6 Chapter 1

open the shell

Create a new program file

When you first start IDLE, it displays a window called the Python Shell.

Select the New Window option from the Python Shell File menu,

which creates a new edit window for you. Input your program code as text

into this edit window and you’ll be on your way.

Go ahead and open a new IDLE edit

window and type in the code from page 3.

This is IDLE's

Python Shell.

This is an IDLE

edit window.

The New Window menu option

creates an edit window.

The Python program is color-

coded within IDLE's edit

window. This color-coding

is referred to as “syntax

highlighting." Be sure to enter

the code in EXACTLY as it

appears here. Don't forget

those “:” characters.

This is how IDLE looks on our computer. Things

might look a little different on yours. Don't

worry: the IDLE menu system and IDLE's

behavior should be the same, regardless of which

operating system you're using.

Do this!

you are here 4 7

starting to code

Prepare and run your code

The next step is to prepare your program code for execution. To do this,

select File → Save from the menu to save your program code to a file.

Choose an appropriate name for your program.

Python programs are usually saved in files that end with .py, so let’s call

this program game.py.

It doesn’t really matter to IDLE which directory you save the file in. Some

coders like to create special directories for each new programming project.

But for now, just save the code in some directory that’s easy to remember.

Now, let’s see what happens when we run the program.

If you choose File → Save

from the menu, you can save

your code in a file.

Python program

filenames usually

end with “.py".

Click the Save button to

create and save the file.

In the IDLE edit window,

go to the “Run" menu and

choose “Run Module."

8 Chapter 1

test drive

To run the program, you need to make sure that the edit window for

the game.py program code is selected. Each time you run (or rerun)

the program, you need to click on the IDLE edit window and choose

the Run Module option from the Run menu. The word module is

a name that IDLE uses to refer to your program code.

Here’s what happens when you run the code:

A couple of incorrect guesses...

...and then the correct

answer.

When you run your code within IDLE,

any messages appear within the Python

Shell, not within the edit window.

IDLE automatically makes the shell the

currently selected window the instant

your program runs.

Congratulations! The program works.

Each time you run the code, it displays a “Welcome!” message,

accepts input from the keyboard, and then tells us whether or not

we guessed the right answer. That means the program is accepting

input, it’s processing the data, and then it generates output.

Test Drive

you are here 4 9

starting to code

Q: I’ve never heard of Python. Is it

popular?

A: Python is used in lots of cool places.

Google, Industrial Light & Magic, YouTube,

and NASA (to name a few) all use Python.

We think they know what they’re doing.

Q: So, when I’m done with this

book I’ll throw Python away and use

something else, like C# or Java?

A: Only if you want to. Python might be

the only programming language you’ll ever

need. But, yes, if you want to learn another

programming language, you can take

everything you learn about programming in

this book and apply it to any other language

with the minimum of effort.

Q: But a buddy of mine told me I

should learn Java or C#. Why are you

not using either of these programming

languages in this book?

A: Both Java and C# are great

programming technologies, but they can be

difficult to learn, especially when you are

just starting out. This is not the case with

Python. And, anyway, this is a book that’s

designed to teach you how to program, and

using Python as your first programming

language will help us to do just that.

Q: There seems to be many different

versions of Python. Which should I use?

A: There are two main releases of

Python: 2 and 3. This book is based on

release 3 of the language. Python 3 is the

future of the language; any new features

are guaranteed to be added to release 3

of the language, not release 2. Of course,

like all releases, Python 3 remains a free

download, which makes it a no-brainer

when decidiing if you can afford to use it.

Q: Will Python run on my phone, just

like Java?

A: That really depends on your phone.

Python is designed to run on lots of

different technologies and operating

systems. Running your own code on your

own phone is a very specific requirement,

and Java has that pretty well covered at

the moment. As a programming technology,

Java was initially designed to run on very

small devices, so it is no big surprise that

it is a strong and popular choice when it

comes to telephony.

Q: Why is the Python IDE called

IDLE?

A: It’s partly because it sounds like IDE,

but we suspect that it has more to do with

Eric Idle, one of the founding members of

the Monty Python’s Flying Circus comedy

group.

Q: Come again?!? Monty Python’s

Flying what?

A: Circus. Yes, we know: sounds silly,

doesn’t it? And, believe us, it is. It’s funny,

too. The creator of Python, Guido van

Rossum, is a big Monthy Python fan and

reportedly watched the show’s reruns

while he designed Python. You’ll find lots

of references to Monty Python folklore in

the Python community. Dead parrots are a

particular favorite.

Q: What does int(g) mean?

A: It tells Python to interpret the user’s

input as a number rather than a letter.

Within programming languages, the number

5 is different than the letter ‘5’.

Q: So what if we’d left it out?

A: The computer would have treated the

input entered by the user of the program as

a letter. If you ask the computer if a letter

is equal to a number, it gets confused and

tells you it isn’t.

Q: Why’s that?

A: Because if the computer thinks that

two pieces of information are of different

“types,” it assumes that there’s no way they

can be equal.

Q: So what if I had not typed a

number when I was asked for a guess?

What if I’d just entered my name or

something?

A: The code would have crashed with an

error. In fact, Python will complain that the

program crashed with a “ValueError” (more

on these error messages later in the book).

10 Chapter 1

give a hint

I don‛t get it. How am I

supposed to guess the winning

number? All the program tells

me is that my guess is right or

wrong. Come on, give me some

help here!

We need this program

to display messages that

are more informative.

One of your users

The program needs to do more.

At the moment, the guessing game tells the user whether his

guess is right or wrong, but nothing more than that. It might

be more helpful if the program displayed more informative

messages, such as whether the guess is higher or lower than

the correct answer. That would help the user to hone in on the

right answer the next time the program is run.

We can do this by changing the code. But in what way?

you are here 4 11

starting to code

Number entered Message to display

Program Modification

You need to decide what messages should be displayed to the

user. Below is a table showing some typical values the user might

enter. What do you think the message should say?

Think about the original code. You will need to use

more than just print() commands to provide

more informative feedback. What else will you

need?

12 Chapter 1

code is a highway

A program is more than a list of commands

Number entered Message to display

3 Too low

5 You win!

7 Too high

8 Too high

Program Modification

You could create a program that was simply a list of commands. But you

almost never will. This is because a simple list of commands can only be

run in one direction. It’s just like driving down a straight piece of road:

there’s really only one way of doing it.

But programs need to be much smarter than that.

("Howdy!")

("Come

again!")

print(), input(), and int()

are examples of commands

that you've already seen.

You needed to decide what messages should be displayed to the

user. Below is a table showing some typical values the user might

enter. What did you think the message should say?

you are here 4 13

starting to code

Codeville: Your program is like a network

of roads

Programs need to do different things under different circumstances. In

the game, the code displays “You win!” if the user guesses the number

correctly, and “You lose!” if not. This means that all programs, even really

simple programs, typically have multiple paths through them.

A path refers to the set of instructions that the computer will actually

follow (or execute). Your code is like a street network, with lots of sections

of code connected together just like the streets in a city. When you drive

through a city, you make decisions as to which streets you drive down by

turning left or right at different intersections. It’s the same for a program.

It also needs to make decisions from time to time as to which path to take,

but for your code, it is not like driving along a road, it’s executing a particular

path.

Let’s look in more detail at how a program decides

which path to take.

There are decision points in

every program that are like

road intersections.

There are many roads (or

paths) through the code.

The computer runs the

commands that it finds

on the “path" it takes

through the code.

14 Chapter 1

fork in the road

Driving down a street is easy. You need to make a decision only when

you get to an intersection. It’s the same for your program. When a

program has a list of commands, it can blindly execute them one after

another. But sometimes, your program needs to make a decision. Does

it run this piece of code or that piece of code?

These decision points are called branches, and they are the road

intersections in your code.

The branch

condition.

The true path..

The false path.

guess==5?

Your program makes a decision using a branch condition. A branch

condition has the value true or false. If the branch condition is true, it

runs the code on the true branch. And if the branch condition is false, it

runs the code on the false branch.

Branches are code intersections

The computer will take this path if

the branch condition is false−that

is, “guess" is something other than 5.

The computer will take this path if the

branch condition is true−that is, “guess"

is equal to 5.

Branches are like road

intersections.

you are here 4 15

starting to code

if/else branches

We’ve already seen a branch in the Python game program:

if guess == 5:

print("You win!")

else:

print("You lose!")

print("Game over!")

Python, like many languages, has if/else branches. In our example,

the branch condition is the if guess == 5 piece of code. This is a

test for equality and it will result in the value true or false.

The code on the true path is indented, and given after the if line. The

code on the false path is indented, and given after the else line:

You need to amend the game program to give more informative messages

to the user.

But what will the paths in the program look like?

guess==5?

("You win!")

("You lose!")

The true path

These commands run only

if they are on the path

the program takes.

The false path

("Game over!")

16 Chapter 1

start your engines

guess <= 5?

("Too high")

Race Track

Construction Kit

The countdown’s started on the Codeville Grand Prix. The

cars have arrived, they’re warming their tires on the grid,

and the race is about to start. Can you assemble the track

so that it displays the right feedback message? Note that

you might not need all the pieces of track.

Number entered Feedback message

3 Too low

5 You win!

7 Too high

8 Too high

The race

start line is

fixed here.

you are here 4 17

starting to code

("You win!")

guess==5?

guess > 5?

("Too low")

The race

finish line is

fixed here.

("Game over!")

18 Chapter 1

finish line

guess==5?

guess > 5?

("You win!")

Race Track

Construction Kit Solution

The countdown’s started on the Codeville Grand Prix. The

cars have arrived, they’re warming their tires on the grid,

and the race is about to start. Were you able to assemble

the track so that it displays the right feedback message?

Number entered Feedback message

3 Too low

5 You win!

7 Too high

8 Too high

you are here 4 19

starting to code

("Too high")

("Too low")

guess <= 5?

We didn't need this

section of track.

("Game over!")

20 Chapter 1

connect branches

The Python code needs

interconnecting paths

The solution’s mapped out, and now we know that the program code will

need to have paths that match this:

But isn’t there a problem here? In the design there are many interconnecting

paths, but so far, we have only written code that contains just one branch:

if guess == 5:

print("You win!")

else:

print("You lose!")

In the new code, we will need to connect two branches together. We need the

second branch to appear on the false path of the first.

guess 5?

guess > 5?

("You win!")

("Too high")

("Too low")

First branch

Second branch

("Game over!")

guess 5?

guess > 5?

We need to connect

these two paths

together

So how do you connect branches together in Python?

you are here 4 21

starting to code

Python uses indents to connect paths

The code inside the if and else statements is indented. This isn’t just

to make the code pretty. In Python, indents matter. Let’s consider a

different piece of example code: something that will decide if you can

drive downtown. Python uses indents to connect a sequence of commands

together to form paths.

if fuel > 3:

print("It's OK")

print("You can drive downtown.")

else:

if money > 10:

print("You should buy some gas.")

else:

print("You better stay at home.")

print("What's next?")

The first if branch

This second if branch

is connected to the

“false" path of the

first if branch.

if fuel > 3:

print("It's OK")

print("You can drive downtown.")

else:

print("Sorry")

print("You don't have enough fuel")

print("What's next?")

This is the

TRUE path.

This is the

FALSE path.

The INDENTs tell Python that

the commands are in the same path

This command is not on the

FALSE path because it is not

indented. So it will always run.

So how do you connect branches together? You simply indent the

second branch in by one more level.

Notice the extra

indentation.

You should now have enough information to go fix the code, but

before we do that, let’s take a look at how IDLE helps you indent

code.

Indents matter

in Python.

Be careful how

you indent code in

Python; if you don’t

indent your code correctly, your

code might do something wildly

different from what you expect.

22 Chapter 1

idle time

IDLE ...at a glance

When you type a “:" and

hit ENTER, Python will

automatically and correctly

indent the next line for you.

Before you type in “else:", make

sure you hit BACKSPACE to

move the indent back one level.

By default IDLE

idents using 4 SPACEs.

If you press TAB, IDLE

converts it into SPACE

characters.

You can change the way

IDLE uses colors and

indenting by clicking on the

Options menu (on the Mac

it's called Preferences).

The Help menu not only

gives you help about

IDLE, but it also lets you

read the built-in Python

documentation.

You’ll be using IDLE to enter all of the Python code in this book, so it’s

worth taking a little time to familiarize yourself with a few of its features.

Even though IDLE looks like a simple editor, it’s actually packed full

of smarts that will make Python programming much easier and faster

for you. It’s worth spending some time exploring IDLE’s menus and help

system, but for now here’s a few handy hints to help you feel at home.

you are here 4 23

starting to code

print("Welcome!")

g = input("Guess the number: ")

guess = int(g)

Pool Puzzle

Your task is to take the Python code

fragments from the pool and place

them into the blank lines in the

game. You may not use the same

code fragment more than once, and

you won’t need to use all the code

fragments. Your goal is to complete

the guessing game program.

Hint: Don’t forget to indent.

Note: each code

fragment from the

pool can be used only

once!

if guess == 5:

print("You win!")

else:

if guess > 5:

print("Too high")

else:

print("Too low")

if guess <= 5:

print("You lose!")

else:

print("Game over!")

24 Chapter 1

indent or else

print("Welcome!")

g = input("Guess the number: ")

guess = int(g)

Did you remember

to indent your

code deep enough?

All of this code is

indented under the

else part of the

original if part.

Pool Puzzle Solution

Your task was to take the Python code

fragments from the pool and place

them into the blank lines in the

game. You could not use the same

code fragment more than once,

and you didn’t need to use all

the code fragments. Your goal

was to complete the guessing game

program.

Hint: Don’t forget to indent.

if guess == 5:

print("You win!")

else:

if guess > 5:

print("Too high")

else:

print("Too low")

if guess <= 5:

print("You lose!")

else:

This piece of code

from the first version

of this program is no

longer needed.

print("Game over!")

you are here 4 25

starting to code

The program works! But, are the users any happier?

So, what happens if you run the new version of the program?

Let’s try a few tests. Remember, you will need to switch back to the

program window for each run and choose Run module from the menu.

Looks like the first guess was too

low, so we need to try again.

Now the guess is too high.

Let’s have another go, and...

...success! We’ve guessed the

correct answer.

Test Drive

26 Chapter 1

another guess

The users still don’t like it.

The program works, and now generates extra feedback, but

there’s a problem. If the users want to have another guess, they

have to run the program again. They really want the program

to keep asking them for another guess until they finally get the

correct answer.

Can you see what the problem is?

How do we get the computer to do something repeatedly? Should

we just make a copy of the code and paste it at the end of the file?

That would make sure the user is asked twice. But what if they

need to make 3 guesses? Or 4 guesses? Or 10,000 guesses?

What about the case where the guess is correct?

The guessing game program needs to be able to

run some code repeatedly.

Why do I have to keep

rerunning the program?

You mean I only get one

guess?????

you are here 4 27

starting to code

Wouldn‛t it be dreamy if

there were a way to get a piece

of code to run several times?

But I guess it‛s just a fantasy…

28 Chapter 1

loop around

FALSE

Loops let you run the same piece

of code over and over again

Programs often need to keep running the same piece of code

many times. In addition to branches, programming languages also

provide loops.

Loops are a little like branches. Just like branches, loops have a

condition (the loop condition) that is either true or false. Also,

like the if part of branches, if the loop condition is true, then

a loop will run a given piece of code. For a branch, this code is

called the body. For a loop, it’s called the loop body.

The big difference between a loop and a branch is how many

times it runs the code associated with it. A branch will run its code

only once. But a loop will run the loop body, then check the loop

condition again and, if it’s still true, it will run the loop body again.

And again. And again. In fact, it will keep running the loop body

until the loop condition becomes false.

When the program first reaches

the loop, it checks the value

of the loop condition before

deciding what to do next.

The “loop body”

If the loop condition is

true, the program runs

the code in the loop body.

At the end of the loop, the program

will go back to the start of the loop

and check the condition again...

answer="no"

answer =

input("Are

we there?")

("We’re

there!")

answer==“no”?

you are here 4 29

starting to code

Python’s while loop

Programming languages have lots of different ways of creating loops,

but one of the simplest ways in Python is to use a while loop. Here’s

an example:

answer = "no"

while answer == "no":

answer = input("Are we there? ")

print("We're there!")

This is what the loop looks like when you write it as a Python while loop.

The code keeps asking the question “Are we there?” until the user types

something other than no. This is what it looks like when it runs:

Did you notice that you had to set the value of the answer variable to

something sensible before you started the loop? This is important, because

if the answer variable doesn’t already have the value no, the loop

condition would have been false and the code in the loop body would

never have run at all.

Bear that in mind. It might be useful in this next exercise...

The loop condition

The loop body is the

indented code following

the “while” line.

We want to make

sure the loop runs

the first time.

The loop body is just one line of code

in this example, but the loop body can

be many lines of code. It might even

include branches and other loops.

As soon as we type something

other than “no", the loop

body ends.

30 Chapter 1

change the game

Write your

code here.

Now, it’s time to apply your programming mojo. Be warned: this exercise is kind of tricky.

You need to rewrite your game program so it keeps running until the user guesses the

correct answer. You will need to use all of the things you’ve learned in this chapter. You will

need to work out the conditions for each of the branches and loops that are required.

Remember: the program needs to keep asking the user for an answer while the current

guess is wrong.

Hint: If you need to test that two things have different values, use the != operator.

This is the “not

equal to" operator.

you are here 4 31

starting to code

Ready Bake

Code

If you add these two lines of code to the top of your program:

from random import randint

secret = randint(1, 10)

The secret variable will be set to a random number between 1 and 10. Modify your

program from the facing page so that instead of the answer always being 5, it will

instead use a random number from 1 to 10 as the answer.

Write the next version of your

program here. This version of your

program uses the value of the “secret"

variable as the correct answer.

32 Chapter 1

new game

print(“Welcome!")

guess = 0

while guess != 5:

g = input(“Guess the number: ")

guess = int(g)

if guess == 5:

print(“You win!")

else:

if guess > 5:

print(“Too high")

else:

print(“Too low")

print(“Game over!")

Did you remember to set the guess to

some sensible default value to make sure

the loop ran the first time?

All of this code is indented, which

means it's all inside the loop body.

Don't worry if your code doesn't look

exactly like this. The important thing is that

it works in the same way when you run it.

This part of the program is very

similar to what you had before.

We need to keep running

while the guess is wrong.

You needed to rewrite your game program so it keeps running until the user guesses the

correct answer. You needed to use all of the things you’ve learned in this chapter. You

needed to work out the conditions for each of the branches and loops that are required.

Hint: If you need to test that two things have different values, use the != operator.

you are here 4 33

starting to code

from random import randint

secret = randint(1, 10)

print(“Welcome!")

guess = 0

while guess != secret:

g = input(“Guess the number: ")

guess = int(g)

if guess == secret:

print(“You win!")

else:

if guess > secret:

print(“Too high")

else:

print(“Too low")

print(“Game over!")

Ready Bake

Code

If you add these two lines of code to the top of your program:

from random import randint

secret = randint(1, 10)

The secret variable will be set to a random number between 1 and 10. You were to

modify your program from the facing page so that instead of the answer always being

5, it will instead use a random number from 1 to 10 as the answer.

Here are the two

lines that create the

random number.

Now, instead of checking if the

answer's 5, we check it against

the random number, which is held

in the “secret" variable.

34 Chapter 1

test drive

So, what happens when you run the new version of your program?

Your users love the program.

And you created it all yourself. By carefully analyzing the problem,

deciding what the feedback needed to be, and working out the intricate

looping and branching logic, you’ve created something that really rocks.

Well done. You’re on your way to becoming a real code

jockey.

This game‛s pretty cool.

No matter how many

times I play it, I still have

to think to get the right

answer!

The program keeps asking for

another guess for as long as you

keep getting the answer wrong.

The program stops when

you get the answer right.

The correct answer is now a random

number and it should be different

each time you play the game.

Test Drive

you are here 4 35

starting to code

CHAPTER 1

Your Programming Toolbox

You’ve got Chapter 1 under your

belt. Let’s look back at what

you’ve learned so far.

Programming Tools

* Programs are created from code statements:

commands do things.

branches decide things.

loops repeat things.

* Conditionals help you decide if

something is True or False.

* Assignment sets a name to a value.

* A named value is stored in a “variable”.

Python Tools

* if/else branches

* while loops

* = assignment operator

* == equality operator

* != inequality operator

* > greater than operator

* print() displays a message on screen

* input() gets and returns user input

* int() converts characters to numbers

* randint() produces a random number

this is a new chapter 37

Every string

has its place

textual data

I keep shix - hic! - honest

working men. They taught me all I

knew - hic! Their names are What..

and Why and... Gus... and - hic! -

Jim, and Bob, and Lou.

Imagine trying to communicate without words.

All programs process data, and one of the most important types of data is text. In this

chapter, you’ll work through the basics of textual data. You’ll automatically search

text and get back exactly what you’re looking for. Along the way, you’ll pick up key

programming concepts such as methods and how you can use them to bend your data

to your will. And finally, you’ll instantly power up your programs with the help of library

code.

38 Chapter 2

bean counter

Your new gig at Starbuzz Coffee

Starbuzz Coffee has made a name for itself as the fastest growing

coffee shop around. If you’ve seen one on your local corner, look

across the street; you’ll see another one.

The Starbuzz CEO is always on the lookout for ways to boost profits,

and he’s come up with a great idea. He wants a program that will

show him the current price of coffee beans so that his buyers can

make informed decisions about when to buy.

I had a programmer do some

work for me, but they‛re not

answering their phone. They‛ve

disappeared! Think you can take

over? I ll let you have the code

they‛ve already come up with.

The Starbuzz CEO

you are here 4 39

textual data

Here’s the current Starbuzz code

The previous programmer has already made a head start on

the code, and we can use this as a basis. Here’s the existing

Python code, but what does it do?

import urllib.request

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

print(text)

Here’s the program code

in its current form.

Take a good look at the existing Starbuzz code. What do you think it

actually does?

40 Chapter 2

test drive

Type the code into IDLE, save the program, and run it.

Here’s the program

code as entered

into IDLE.

When executed,

the program

produces this.

This looks like a chunk of “raw"

HTML text, which is the format

used to create web pages.

Hey, that‛s not right! I only

need to see the current

price of coffee beans, not all

that other stuff. Think you

can give me just the cost?

Test Drive

The code you’ve been left goes to the prices page on the

Beans’R’Us website to get the current price of coffee

beans. But instead of giving just the cost, it gives you all

of the HTML text used to create the web page itself.

you are here 4 41

textual data

The cost is embedded in the HTML

Take a closer look at the results of the program. The current price

of beans is right in the middle of the output:

The Starbuzz CEO would find it a lot easier if you could extract

the price of beans and just display that, rather than have to look

for it in the HTML. But how do you do that?

A string is a series of characters

The output of the Starbuzz program is an example of a string.

In other words, it’s a series of characters like this:

Somewhere within the string is the price of coffee beans. To

retrieve just the price, all you need to do is go to the right bit

of the string, retrieve the characters that give the price, and

display just those characters. But how?

You only need these few characters.

This is the HTML code

from the price web page.

This is the price you need

to extract from the page.

42 Chapter 2

offset values

Find characters inside the text

The computer keeps track of individual characters by using two pieces

of information: the start of the string and the offset of an individual

character. The offset is how far the individual character is from the start of

the string.

The first character in a string has an offset of 0, because it is zero

characters from the start. The second character has an offset of 1, and

so on:

The offset value is always 1 less than the position. Python lets you read a single

character from a string by providing the offset value in square brackets

after the variable name. Because the offset value is used to find a character,

it is called the index of the character:

Remember: the first character

has index 0, so we are “off

by one" when referring to an

individual character.

As we start counting from 0, the

offset values are always one less

than their actual position.

Offset

values

We are at the start of the

string and, as we have moved

zero places, the offset is 0.

Offset of 1

Offset of 2

you are here 4 43

textual data

But how do you get at more than

one character?

For Starbuzz, you don’t just need a single character. You need to extract

the price from the string of HTML, and the price is made up of several

characters.

You need to extract a smaller substring from a bigger string. A substring

is a sequence of characters contained within another string. Specifying

substrings in Python is a little like reading single characters from a string,

except that you provide two index values within the square brackets:

s[14] s[138:147]

s[5:9]

s[10:12]

s[13:18]

In general, if you specify a substring using s[a:b], then:

This will read the smaller

substring from the entire

string contained within “s".

If you provide two index values, you extract a

group of characters from the first index up to

(but not including) the second index.

Let’s work out what each of the following substring specifications

mean. Imagine the variable s is set to the string below. Your job

is to determine what each of the substrings provide.

a is b is

If you provide a single index

after the variable name, you

get a single character.

Write down what you think “a"

and “b" represent here.

44 Chapter 2

here is waldo

“here"

“is"

“Waldo"

the index of the first character the index after the last character

Up to, but not including.

s[5:9]

s[10:12]

s[13:18]

a is b is

The second

index value is

after the last

character in

the substring

This is even though the

first index value is the start

character of the substring.

Imagine the variable s is set to the string below. Your job was to

determine what each of the substrings provide.

In general, if you specify a substring using s[a:b], then:

s[13:18]

Even though index 18

is mentioned in the

substring specification,

it’s not included in the

extracted substring.

9 minus 5 = 4 characters

12 minus 10 = 2 characters

18 minus 13 = 5 characters

you are here 4 45

textual data

Remember: the 235th

character in the string has

index value 234.

You need to update the program to extract the price starting at

the 235th character of the string. The price is four characters long.

Store the price substring in a variable called price. Write the

new version of the program here:

import urllib.request

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

print(text)

Here’s the current

version of the code.

Write the new

code here.

46 Chapter 2

extract the price

import urllib.request

page = urllib.request.urlopen(“http://www.beans-r-us.biz/prices.html")

text = page.read().decode(“utf8")

price = text[234:238]

print(price)

You store the substring in

the “price" variable.

Now, instead of printing

“text" (the entire web

page), you are just

printing “price" (the

extracted substring).

The substring includes characters at indexes 234, 235,

236, and 237. Note how you’ve specified the substring

with two index values, separated by the “:" character.

This is the first character

in the required substring,

at index 234.

238 is the second index value

used in the substring specification

because it is after the last

character to be extracted.

You needed to update the program to extract the price starting at

the 235th character of the string. The price is four characters long

and stored in a variable called price.

This is the HTML text

leading up to the price.

Here it is: the substring to

extract and the current

price of coffee.

4 47

textual data

That looks a lot cleaner. Now, instead of displaying the entire

HTML text of the web page, you’ve cut it down to just the

piece of the string (the substring) that you need.

And there it is: the current price of coffee

from the Beans’R’Us web page.

Type the code into IDLE, save the program (choose a name for

the program that is meaningful to you), and run it.

Test Drive

That‛s great. It‛s exactly

what I need! You have no

idea how much time and money

this is going to save me...

48 Chapter 2

string theory

Head First: String, it’s so good of you to find the

time to speak to us.

String: Please, the honor is mine. Sit. Sit. Make

yourself at home. Did you eat yet?

Head First: I’m fine, thank you. String, where

should I begin? You are known the world over for

your work. In your time you’ve carried the works of

Shakespeare, Geothe...

String: Dan Brown.

Head First: ...all the great works of literature. And

even mundane things like names and addresses. Tell

me, how did you become so popular?

String: It’s a question of character. Well, characters.

See, before I existed, computer systems used to

record text one character at a time.

Head First: That must have been rather

inconvenient.

String: Inconvenient? It was a royal pain in the

tuchis.

Head First: Quite.

String: Without me, handling text was like riding a

pedal cycle without a saddle.

Head First: In what way?

String: It was possible to get somewhere, but the

journey was kind of stressful.

Head First: You simplify things.

String: Certainly. I simplify. Instead of keeping

track of a hundred, or a thousand, or a million

letters, you just need keep an eye on one thing. Me!

Head First: That’s a good point.

String: I like to think of myself as a front. An agent,

you might say, for all the characters I work with.

Head First: People deal with you, so they don’t

have to deal with individual characters in memory.

String: Exactly. I’m an organizer. I keep an eye on

the day to day business of the letters. If I need to be

shorter or longer, I arrange for the characters to be

made available.

Head First: Tell me about your substrings.

String: Ah, my substrings. Like chips off the old

block. That a humble datatype should be so blessed!

Head First: A tissue?

String: Bless you. <blows nose>. Those boys are

so close to me. Here’s a photo. Can you see the

resemblance?

Head First: Why he looks just like...

String: Ah, you guessed! Yes, my character

sequence from 137 to 149. Exactly. Just like his old

man. But shorter. Little more hair.

Head First: Your substrings are strings as well.

String: Certainly. Strings just like me. And they, I

hope, should one day be able to produce their own

substrings as well.

Head First: Yet some people are confused by your

indexing.

String: What can I say? I started with nothing!

Head First: String, thank you.

String: A pleasure. Are you sure you ate?

The String Exposed

This week’s interview:

We ask the String what it’s like

being the world’s most eligible

datatype.

you are here 4 49

textual data

Q: So, I can put any web address into

this code and grab the associated web

page from the Internet?

A: Yes, feel free to try it out for yourself.

Q: Don’t I need a web browser to

view web pages?

A: Yes, to view a web page in all its

formatted glory−with embedded pictures,

music, videos and the like−a web browser

is a must-have. However, if all you want to

see is the “raw” HTML, a browser is overkill.

Q: What does the import line of code

do?

A: It gives the program the ability to talk

to the Internet. The

urllib.request code comes as

standard with Python 3.

Q: And I guess that call to urlopen()

goes and gets the web page?

A: That’s right! The provided web

address (or “URL” to use the proper web-

speak) is fetched from the Internet and

returned by the call to urlopen().

In this code, the fetched web page is

assigned to the page variable.

Q: And the urllib.request bit?

A: That just tells the program to use the

urlopen() function that comes as

standard with Python 3’s Internet page—

reading technology. We’ll have more to say

about urllib.request in a little bit.

For now, just think how lucky we all are not

to have to write code to fetch web pages

from the Internet.

Q: I get that the call to read() actually

reads the web page from the page

variable, but what’s that decode(“utf8”)

thing?

A: When the web page is fetched from

the Internet, it is in a “raw” textual format.

This format can be a little hard for humans

to read. The call to decode() converts

the raw web page into something that looks

a little easier on the eye.

To see what we mean, try removing the

call to decode() from the program and

running the code again. Looks a little weird,

doesn’t it? (Don’t forget to put the call to

decode() back in before continuing.)

You can download the HTML of a

web page as a textual string.

A string is a sequence of characters.

You can access individual characters

in a string using an offset.

The offset is known as the index

value of the character (or just index

for short).

Strings within strings are called

substrings.

Substrings are specified using

two index values−for example:

text[10:20].

The first index value is the location of

the first character of the substring.

The second index value is the

location after the last character of the

substring (up to, but not including).

Subtract the second index from

the first to work out how long the

substring should be.

50 Chapter 2

change of address

import urllib.request

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices-loyalty.html")

text = page.read().decode("utf8")

price = text[234:238]

print(price)

Beans'R'Us is rewarding loyal customers

The CEO just got great news from the beans supplier.

The supplier actually maintains two prices: one for regular customers

and one for loyalty program customers. The different prices are published on

different web pages:

http://www.beans-r-us.biz/prices.html

http://www.beans-r-us.biz/prices-loyalty.html

That means you need to change the web page address in the code:

Let’s run it to make sure everything works OK.

Regular customers get

their prices here.

Loyalty program

customers get

their prices here.

This is the new

address.

The supplier is so happy

about all the business we‛re giving

them that they are going to make

us members of their loyalty discount

program. They say it should be a

simple fix. Can you look into it?

you are here 4 51

textual data

What’s going on here?

This time when you run it, this happens...

The program is no longer displaying a price. So what happened?

The price moved

The web page for loyalty customers is much more dynamic that the old

web page. The page for regular customers always displays the price in a

substring beginning at index 234. That’s not true for the loyalty program

web page. The price on that page can be almost anywhere. All you know

for sure is that the price follows the substring >$:

You need to search for the price string.

Test Drive

All you know is that the price

follows these two characters.

The price could

appear ANYWHERE

in the string.

52 Chapter 2

searching for substrings

Searching is complex

You already know how to find a substring, so you could run through the

entire web page and check each two characters to see if they match >$,

like this:

You could do it this way... but should you?

There’s a lot to worry about. Which two characters are you currently

comparing? Where in the string are you right now? What if “>$” isn’t

found? Searching for substrings in strings is a little more complex than it

first appears...

But if you don’t want to write code to search the

string, what else could you do?

We’ve found “>$"!

So these next 4

characters must be

the price, right?

We could search through the

whole string for “>$", looking

at 2 characters at a time.

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Wouldn‛t it be dreamy if there were

a simple way to search a string for a

substring? But I suppose that‛s just a

fantasy...

54 Chapter 2

smart data

Python data is smart

The more code you write, the more you will find that you need to do the

same kind of things to the data in your variables all the time. To prevent

you from having to create the same code over and over, programming

languages provide built-in functionality to help you avoid writing

unnecessary code. Python data is smart: it can do things.

Let’s look at an example.

Imagine you have a piece of text in a variable that you want to display in

uppercase (all CAPITAL letters):

msg = "Monster truck rally. 4pm. Monday."

You could write code that read through each character in the string and

printed out the matching uppercase letter. But if you’re programming in a

language like Python, you can do this:

But what does msg.upper() mean?

Well, msg is the string containing our piece of text. The .upper() that

follows it is called a string method. A method is just an instruction for the

string. When you call msg.upper(), you are telling the string to give

you an UPPERCASE version of its data.

But is there a string method that can help you search

for a substring within a string object?

print(msg.upper())

MONSTER TRUCK RALLY. 4PM. MONDAY.

Here’s what gets

displayed, the value of

the “msg" variable in

UPPERCASE.

The dot means

“call the method on"

the specified variable.

“upper()" is a string method.

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textual data

Which of the above methods do you need to use to locate the

price substring within the Beans’R’Us web page?

text.endswith(".jpg")

Return the first index value when the given substring is

found.

text.upper():

Return a copy of the string converted to uppercase.

text.lower():

Return a copy of the string converted to lowercase.

text.replace("tomorrow", "Tuesday"):

Return a copy of the string with all occurrences of one

substring replaced by another.

text.strip():

Return a copy of the string with the leading and

trailing whitespace removed.

text.find("python"):

text.startswith("<HTML>")

Return the value True if the string has the given

substring at the beginning.

Return the value True if the string has the given

substring at the end.

What the method does

Method

These are some of the many built-in string methods that come with

Python. Match each method to what it does. We’ve done one for you

already.

56 Chapter 2

find, the right method

Which of the above methods do you need to use to locate the

price substring within the Beans’R’Us web page?

What the method does

Method

The “find()" method

text.endswith(".jpg")

Return the first index value when the given substring is

found.

text.upper():

Return a copy of the string converted to uppercase.

text.lower():

Return a copy of the string converted to lowercase.

text.replace("tomorrow", "Tuesday"):

Return a copy of the string with all occurrences of one

substring replaced by another.

text.strip():

Return a copy of the string with the leading and

trailing whitespace removed.

text.find("python"):

text.startswith("<HTML>")

Return the value True if the string has the given

substring at the beginning.

Return the value True if the string has the given

substring at the end.

These are some of the many built-in string methods that come with

Python. You were to match each method to what it does.

SOlUTion

you are here 4 57

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You need to update your price-grabbing program so that it extracts the four-character substring

that follows the occurence of the “>$” characters. Write the new version of your code in the

space provided.

Hints: Don’t forget that the find() method finds the starting position of a substring. Once

you’ve found “>$”, use Python’s addition operator to calculate where in the string you want to

extract the substring. The addition operator is the “+” symbol.

coffee beans = <strong>$5.49</

strong></p><p>Price valid for

Search for this 2-character

combination.

Here’s what

you’re really

looking for.

58 Chapter 2

finding the deal

You needed to update your price-grabbing program so that it extracts the four-character

substring that follows the occurence of the “>$” characters.

Hints: Don’t forget that the find() method finds the starting position of a substring. Once

you’ve found “>$”, use Python’s addition operator to calculate where in the string you want to

extract the substring. The addition operator is the “+” symbol.

import urllib.request

page = urllib.request.urlopen(“http://www.beans-r-us.biz/prices.html")

text = page.read().decode(“utf8")

where = text.find(‘>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

price = text[start_of_price:end_of_price]

print(price)

This code

hasn’t

changed.

Search for the index location

of the “>$" combination.

The start of the actual price

is another 2 index positions

along the string, while the end

of the price is another 4.

With the start and end index

locations known, it’s easy to

specify the substring required.

This is the addition operator.

Did you remember to

print out the price

once you’d found it?

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OK, so your program should now be able to find the price, no matter

where it appears in the page.

It works! By adding very little extra code, you have made the program

much smarter and more useful.

Version 3 of

your program

The price

extracted from

the larger string

of HTML

That was quick! We‛re

back to saving money

once more. Now,

there‛s just one thing...

Test Drive

60 Chapter 2

frugality feature

The new version of the program works, but

now there′s a design issue.

The Starbuzz CEO wants to know when the price of

the beans falls below $4.74. The program needs to keep

checking the Beans’R’Us website until that happens. It’s

time to restructure the program to add in this new feature.

Let’s add a loop to the program that stops

when the price of coffee is right.

I forgot to say that I

only need to know the

price when it‛s $4.74 or

lower. I don‛t want it to

bug me when it isn‛t.

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textual data

Code Magnets

The program code to add the feature is sitting on the fridge door.

Your job is to arrange the magnets so that the program loops until

the price falls to $4.74 or lower.

import urllib.request

while price > 4.74:

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find(‘>$’)

end_of_price = start_of_price + 4

start_of_price = where + 2

price = text[start_of_price:end_of_price]

print ("Buy!")

price = 99.99

62 Chapter 2

Code Magnets Solution

The program code to add the feature was sitting on the fridge door.

You were asked to arrange the magnets so that the program loops

until the price falls to $4.74 or lower.

import urllib.request

while price > 4.74:

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find(‘>$’)

end_of_price = start_of_price + 4

start_of_price = where + 2

price = text[start_of_price:end_of_price]

print ("Buy!")

price = 99.99

Did you

remember

to indent

these lines?

They are

inside the

loop.

This line shouldn’t

be indented, as it's

outside the loop.

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textual data

Enter the new version of the program code into an IDLE edit window and

run it.

It looks like something’s gone wrong with the program. What does

TypeError mean? What’s happened?

Here’s your

program code

typed into

IDLE.

But what's this?

Test Drive

Look at the error message in detail. Try to identify which line in the code

caused the crash and guess what a TypeError might be. Why do you

think the code crashed?

64 Chapter 2

type differences

“A”

Strings and numbers are different

The program crashed because it tried to compare a string with

a number, which is something that doesn’t make a lot of sense

to a computer program. When a piece of data is classified as a

string or a number, this refers to more than just the contents of the

variable. We are also referring to its datatype. If two pieces of

data are different types, we can’t compare them to each other.

Think back to the previous chapter. You’ve seen this problem before,

back when you were working on the guessing game program:

guess = int(g)

In the guessing-game program, you needed to convert the user’s guess

into an integer (a whole number) by using the int() function.

But coffee bean prices aren’t whole numbers, because they contain

numbers after a decimal point. They are floating point numbers

or floats, and to convert a string to a float, you need to use a

function other than int(). You need to use float():

float("4.99")

This variable will be

set to a number.

“g" is a string.

The int() function converts the “g"

string into an integer, which is then

assigned to “guess".

You‛re just

not my type.

Like int(), but works with numbers

that contain a decimal point.

you are here 4 65

textual data

This should be a pretty quick fix. If you use the float() function to

convert the price substring, you should then be able to compare the

price to 4.74:

That’s much better. Your program now waits patiently until the

price falls to the right level and only then does it tell the CEO that

the time is right to buy a fresh batch of coffee beans.

This is great! Now I can get on with

the rest of my day and I only hear

when the price of beans drops to the

right level. This‛ll save millions! I‛ll tell

every outlet to use it worldwide.

The updated code.

The program runs

with no problems

this time.

Test Drive

66 Chapter 2

traffic jam

From: The Department of Webland Security

Secret Service - Corporate Enforcement Unit

To Whom It May Concern:

A recent investigation into an apparent Distributed Denial

of Service (DDoS) attack on the www.beans-r-us.biz domain

showed that much of the traffic originated from machines

located in various Starbuzz outlets from around the world.

The number of web transactions (which reached a peak of

several hundred thousand requests worldwide) resulted in

a crash of the Beans'R'Us servers, resulting in a

significant loss of business.

In accordance with the powers invested in this office

by the United States Attorney General, we are alerting the

developer of the very dim view we take of this kind of

thing. In short:

We're watching you, Bud. Consider yourself on notice.

Yours faithfully,

Head of Internet Affairs

The Department of Webland Security

37 You don’t need

To know where we are (but we know

where you live)

Washington, D.C.

That sounds weird. What happened?

you are here 4 67

textual data

The program has overloaded

the Beans’R’Us Server

It looks like there’s a problem with the program. It’s sending so many

requests that it overwhelmed the Beans’R’Us website. So why did that

happen? Let’s look at the code and see:

If the value of price isn’t low enough (if

it’s more than 4.74), the program goes back

to the top of the loop immediately and sends

another request.

With the code written this way, the program

will generate thousands of requests per

hour. Multiply that by all the Starbuzz

outlets around the world, and you can start

to see the scale of the problem:

You need to delay the pricing

requests. But how?

import urllib.request

price = 99.99

while price > 4.74:

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find(‘>$’)

start_of_price = where + 2

end_of_price = start_of_price + 4

price = float(text[start_of_price:end_of_price])

print ("Buy!")

Here’s the code

as it currently

stands.

The Beans 'R' Us

server can't cope

with all the requests.

68 Chapter 2

Time... if only you had more of it

Just when you’re feeling completely lost, you get a phone call

from the Starbuzz coder who wrote the original version of

the program:

It seems that she can’t get back because of a storm in the

mountains. But she does make a suggestion. You need to regulate

how often you make a request of the Beans’R’Us web server. One

way to do this is to use the time library. This will apparently

make it possible to send requests every 15 minutes or so, which

should help to lighten the load.

There’s just one thing: what’s a library?

Zzzkzzkkvkk...

Sorry, dude... vvzzz...

Heavy snow... ffzzkk... Phone

connection... pzzzkkkvkk.... I think

you need the.... vzzzkkkk.... time

library!

you are here 4 69

textual data

You’re already using library code

Look at the first line of the original code:

import urllib.request

page = urllib.request.urlopen("http://...")

The import line tells Python that you intend to use some library code

called urllib.request. A library is a bunch of prewritten code that

you can use in your own programs. In this case, the urllib.request

code accesses data on the Web. This is a library that comes as standard with

Python.

To see how the code is used, look at this line:

The code that follows the = sign is calling a function in urllib.

request called urlopen(). Notice how we say we want the code:

urllib.request, followed by a “.”, then the name of the function.

urlcleanup()

urlopen()

urlretrieve()

The urllib.request code

<library-name>

<function-name>

But how will the time library help us? Let’s see...

This says that we are

going to use code stored in

the “urllib.request” library.

Every library contains

functions that you can

use in your own program.

Library name.

70 Chapter 2

find the time

Python Library Documentation: time

time.clock()

The current time in seconds, given as a floating

point number.

time.daylight()

This returns 0 if you are not currently in

Daylight Savings Time.

time.gmtime()

Tells you current UTC date and time (not affected

by the timezone).

time.localtime()

Tells you the current local time (is affected by

your timezone).

time.sleep(secs)

Don’t do anything for the specified number of

seconds.

time.time()

Tells you the number of seconds since January 1st,

1970.

time.timezone()

Tells you the number of hours difference between

your timezone and the UTC timezone (London).

You need to use one of these functions to help you fix your code.

But which one? Draw a circle around the function you think you might need.

These are some of the functions provided by Python’s built-in time library:

you are here 4 71

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With the appropriate function identified, amend the code to control how often the request for

the web page is sent to the server. The Beans’R’Us webmaster has been in touch to say that

their web-based pricing information is updated every 15 minutes. Fill in the blanks in the code

as indicated by the dashed lines.

Hints: 15 minutes equates to 15 multiplied by 60 seconds, which is 900 seconds. Also: to use

the functionality provided by a library, remember to import it first.

import urllib.request

price = 99.99

while price > 4.74:

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

price = float(text[start_of_price:end_of_price])

print ("Buy!")

72 Chapter 2

time is on our side

Python Library Documentation: time

time.clock()

The current time in seconds, given as a floating

point number.

time.daylight()

This returns 0 if you are not currently in

Daylight Savings Time.

time.gmtime()

Tells you current UTC date and time (not affected

by the timezone).

time.localtime()

Tells you the current local time (is affected by

your timezone).

time.sleep(secs)

Don’t do anything for the specified number of

seconds.

time.time()

Tells you the number of seconds since January 1st,

1970.

time.timezone()

Tells you the number of hours difference between

your timezone and the UTC timezone (London).

You need to use one of these functions to help you fix your code.

But which one? You were to draw a circle around the function you thought you

might need.

These are some of the functions provided by Python’s built-in time library:

This looks

like the best

function to use.

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With the appropriate function identified, you were to amend the code to control how often the

request for the web page is sent to the server. The Beans’R’Us webmaster has been in touch

to say that their web-based pricing information is updated every 15 minutes.You were to fill in

the blanks in the code as indicated by the dashed lines.

Hints: 15 minutes equates to 15 multiplied by 60 seconds, which is 900 seconds. Also: to use

the functionality provided by a library, remember to import it first.

import time

time.sleep(900)

Import the library at the top

of the program. This gives the

program access to all the built-in

functionality that the library

provides.

Use the facilities of the time

library to pause the program for

15 minutes between requests.

import urllib.request

price = 99.99

while price > 4.74:

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

price = float(text[start_of_price:end_of_price])

print ("Buy!")

74 Chapter 2

coffee all around

Order is restored

Starbuzz Coffee is off the blacklist, because their price-checking

programs no longer kill the Beans’R’Us web server. The nice

people at Webland Security have, rather quietly, gone away.

Coffee beans get ordered when the price is right!

I love the taste of

this coffee, and I just

love the cost of those

beans!

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CHAPTER 2

Your Programming Toolbox

You’ve got Chapter 2 under your

belt. Let’s look back at what you’ve

learned in this chapter:

Programming Tools

* Strings are sequences of individual characters.

* Individual string characters are referenced by index.

* Index values are offsets that start from zero.

* Methods provide variables with built-in functionality.

* Programming libraries provide a collection of related

pre-built code and functions.

* As well as having a value, data in variables also have a

“data type."

* Number is a data type.

* String is a data type.

Python Tools

* s[4] - access the 5th character of the variable “s",

which is a string

* s[6:12] - access a sub-string within the string “s" (up

to, but not including)

*s.find() method for searching strings

* s.upper() method for converting strings to

UPPERCASE

* float() converts strings to decimal point numbers

known as “floats"

* + addition operator

* > greater than operator

* urllib.request library for talking to the Web

* time library for working with dates/time

functions

Let’s get organized

As programs grow, the code often becomes more complex.

And complex code can be hard to read, and even harder to maintain. One way of

managing this complexity is to create functions. Functions are snippets of code that

you use as needed from within your program. They allow you to separate out common

actions, and this means that they make your code easier to read and easier to maintain.

In this chapter, you’ll discover how a little function knowledge can make your coding life

a whole lot easier.

@starbuzzceo Waiting

patiently for milk...

78 Chapter 3

emergency option

Starbuzz is out of beans!

When the coffee beans start to run low in an outlet,

the Starbuzz baristas need to be able to send an

emergency order to the CEO. The outlets need

some way of immediately requesting the purchase of

coffee beans at the current price, regardless of what

that price is. They also need the option of waiting for

the best price, too, just like in the current program.

The program needs an extra option.

We have a worldwide crisis! We‛ve

run out of coffee beans in some of our

stores, and we‛ve lost some customers, too.

My buyers are only buying coffee when

the cost is low, but if we run short on

coffee supplies, I ll pay any price.

The Starbuzz buyers love the program you created in the

last chapter. Thanks to your efforts, the Starbuzz CEO

is only buying coffee beans when the price drops below

$4.74, and his organization is saving money as a result.

But, now there’s a problem: some Starbuzz outlets have

run out of beans.

you are here 4 79

functions

import urllib.request

import time

price = 99.99

while price > 4.74:

time.sleep(900)

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

price = float(text[start_of_price:end_of_price])

print ("Buy!")

What does the new program need to do?

The new program for Starbuzz needs to give the user two options.

The first option is to watch and wait for the price of coffee beans to drop.

If the user chooses this option, the program should run exactly as it did

before.

The second option is for the user to place an emergency order. If the user

chooses this option, the program should immediately display the current

price from the supplier’s website.

Here‛s the existing code for Starbuzz. You need to modify the program

to add an emergency report feature that will immediately report the

current price. Which parts of the code can you reuse to generate the

emergency report? Grab your pencil and circle the part of the code you

think you might reuse. Why do you think you‛ll need to resuse this code?

80 Chapter 3

reuse code

If you just copy and paste the same code,

it could make your program very long. And

hard to maintain. That’s why you don’t want

to duplicate the code.

import urllib.request

import time

price = 99.99

while price > 4.74:

time.sleep(900)

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

price = float(text[start_of_price:end_of_price])

print ("Buy!")

Here‛s the existing code for Starbuzz. You needed to modify the program

to add an emergency report feature which will immediately report the

current price. Which parts of the code can you reuse to generate the

emergency report? You were to circle the part of the code you think you

might reuse as well as state why you might need to reuse it:

Whether you wait for the

right price or request an

emergency order, you'll need

this code in each case.

Does that mean we have to

duplicate the code for each

option? Is this a good idea?

Imagine if you

had to maintain a

program this length.

Here's the code you

can reuse.

you are here 4 81

functions

When you need to add a new feature to a program that’s similar to some

other code in the program, you might be tempted to just copy and paste

the code.

In practice, that’s actually a pretty bad idea, because it can lead to code

bloat. Code bloat means that you have more code in your program than

you actually need. Your programs will get longer, and they’ll get a lot

harder to maintain.

Programming languages all come with features that allow you to reuse

code. So what’s the difference between copying and reusing code?

If you copy code, you simply duplicate it. But when you reuse code, you

have a single copy of the code that you can call in all the places that

you need it. Not only will your programs be shorter, but it also means that

when you amend code, you will need to change it only once and in one

place only.

So code reuse is a good thing. But how do you do it?

Don't duplicate your code...

...Reuse your code instead

Here‛s the printout of the tic-tac-toe

game. If you really want me to change the

colors of the crosses, I‛ll just need to replace

the code that displays each of the 9 squares. That‛s

the same code in 9 different places. Oh, and if you

want me to change the 0s...

So you want a new gravity-bomb

launcher added to each of the

star-fighters? No problem. I‛ll change

a few lines of code and every craft in

the fleet will be updated.

You'll find that code bloat is

common in lots of programs,

even code written by

professional programmers.

82 Chapter 3

define your function

Reuse code with functions

Most programming languages let you create reusable, shareable code with

functions. A function is a chunk of code that you separate out from the

rest of your program, give a name, and then call from your code.

Different languages have different ways of creating functions. In Python,

use the def keyword to define a new function. Here’s some Python code

that defines a make_smoothie() function:

def make_smoothie():

juice = input("What juice would you like? ")

fruit = input("OK - and how about the fruit? ")

print("Thanks. Let's go!")

print("Crushing the ice...")

print("Blending the " + fruit)

print("Now adding in the " + juice + " juice")

print("Finished! There's your " + fruit + " and " + juice + " smoothie!")

In Python, it’s important that you define the function before you use it, so

make sure the code that calls (or uses) the function comes after the definition

of the function:

print("Welcome to smoothie-matic 2.0")

another = "Y"

while another == "Y":

make_smoothie()

another = input("How about another(Y/N)? ")

Every time that Python sees make_smoothie() in the code, it jumps

to the code in the make_smoothie() function. It runs the code in the

function until it gets to the end, and then returns to the next line in the

code that called it.

Let’s use functions to share code within your program.

When the computer first

encounters a call to the function,

it jumps to the start of the

function, runs the code it

finds there... then returns to

the calling piece of code. The

function “answers the call" to

run its code.

A function is a

boxed-up piece of

reusable code.

The parentheses are important,

so be sure to include them.

The code

you share is

indented.

Call the

function.

Note the

use of

parens.

Give the function a name.

make_

smoothie()

you are here 4 83

functions

Code Magnets

Before you amend the existing coffee bean program code, let’s

see if you can create a function to display the current bean price.

Rearrange the magnets in the correct order to create the function:

get_price()

import urllib.request

def

text[start_of_price:end_of_price]

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

get_price()

print(

)

We’ve given you a head start

by adding the first magnet.

84 Chapter 3

order matters

Always get things in the right order

When it comes to functions, the order in which you do things

really matters. The get_price() function needs to be

defined before you call it. And because the function relies upon

some code in the urllib.request library, you need to make

sure that the import line appears before the function, too.

Let’s see if your new code works.

You need a colon after

the function name.

The body of the function

needs to be indented.

The function needs

to be declared

before it's called.

This line isn't indented,

because it is part of

the main program.

You still need to import libraries

before using them in a function.

The function

definition

starts here.

The order in which

you do things is

really important.

Code Magnets Solution

Before you amend the existing coffee bean program code, let’s see

if you can create a function to display the current bean price. You

were to rearrange the magnets in the correct order to create the

function:

get_price()

import urllib.request

def

text[start_of_price:end_of_price]

get_price()

The function

definition ends here.

print( )

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

you are here 4 85

functions

Type the code on the opposite page into IDLE, and run it to see what

happens:

The price appears immediately. You now have a function that reads the

contents of the page from the supplier’s website and prints out the price

information.

You can reuse the function in lots of places in your program simply by

calling the get_price() function. Now all you have to do is modify

your existing program to use the new function.

Test Drive

When this code runs,

the price is displayed

right away. (Note: you

may see a different

price when you run your

code.)

Look back at the original program at the start of the chapter. You know that you

can use this function to produce emergency reports. But it will also need to

replace the existing price-watch code. Is there a problem? Why? Why not?

86 Chapter 3

limits of print

Your code prints out the price. Big

deal. It‛s just a pity you actually need

to GET the price and USE it. Your code

isn‛t much use, just printing out the

price like that, is it?

get_price()

Functions are great for reusing code, but they

really come into their own when they perform

an action for you, then give you back some

data to use in whichever way you want.

The current version of the get_price() function prints out

the price of coffee beans every time it is used, or called. This

is OK if that’s what you really want it to do. The trouble is, you

need the function to give you the price so that you can then

decide what you want to do with it.

...

discount = 0.9

print("The discounted price is: ")

price = get_price()

actual_price = price * discount

print(str(actual_price))

...

The “get_price()"

function. Even the

name tells you a little

about what it does.

What's the

current price?

5.51

you are here 4 87

functions

Return data with the return command

If you use the return() command within a function, you can send a

data value back to the calling code.

import urllib.request

def get_price():

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

print(text[start_of_price:end_of_price])

return(text[start_of_price:end_of_price])

Remove the call to

“print()"...

... and replace it with a

call to “return()" instead.

get_price()

...

price = get_price()

...

Call the “get_price()"

function.

5.51

Run the code

to determine

the current

price.

Return the price to

the caller.

The value assigned to “price"

is 5.51. The assignment

happens after the code in

the function executes.

88 Chapter 3

don't repeat yourself

Q: The return() command is just like print(), except nothing

appears on screen, right?

A: Well... sort of. The print() command is designed

to display (or output) a message, typically on screen. The

return() command is designed to allow you to arrange for a

function you write to provide a value to your program. Recall the

use of randint() in Chapter 1: a random number between

two values was returned to your code. So, obviously, when

providing your code with a random number, the randint()

function uses return() and not print(). In fact, if

randint() used print() instead of return(), it

would be pretty useless as a reusable function.

Q: So, it’s a case of return() letting a function give you

something back?

A: Yes, that’s it, exactly.

Q: I’m not sure I’m convinced about using functions. Isn’t

using copy’n’paste quick and easy?

A: No, using copy’n’paste is quick and dirty, with the emphasis

on the “dirty.” When you need to repeatedly use some code, it’s

always better to create a function to contain and name that code.

You then call (or invoke) the function as needed. If you later decide

to change how the repeated code works, it’s a no-brainer to change

the code in the function once. If, instead, you “quickly” performed

copy’n’paste five times, that’s five changes you now have to make,

and the chance of you missing one change or making a mistake are

actually pretty high. So, don’t copy’n’paste!

Q: So, using a function lets you share the repeated code in

a controlled way?

A: Yes, it does. There’s also a guiding principle among

prgrammers known as DRY: Don’t Repeat Yourself. Using functions

lets you keep your code DRY.

Q: What happens if the function omits the return()

command? Does each function have to have one?

A: No, the use of return() is not required. In fact, the

current version of your get_price() function doesn’t use

return() at all. But, your function feels like it gives you

something because it prints the current price on screen. When the

return() command is omitted, a function returns a special no

value. In Python, this value is called None.

Q: So, just to be clear, using return() is optional?

A: Yes, it is.

Q: Does return() always come at the end of the function?

A: Usually, but this is not a requirement, either. The

return() can appear anywhere within a function and, when it

is executed, control returns to the calling code from that point in the

function. It is perfectly reasonable, for instance, to have multiple

uses of return() within a function, perhaps embedded

with if statements which then provide a way to control which

return() is invoked when.

Q: Can return() send more than one result back to the

caller?

A: Yes, it can. return() can provide a list of results to the

calling code. But, let’s not get ahead of ourselves, because lists are

not covered until the next chapter. And there’s a little bit more to

learn about using return() first, so let’s read on and get back

to work.

you are here 4 89

functions

Using the new get_price() function, write a new version of the

price-checking program that does this:

1. Ask the user to indicate if the price is required immediately (Y/N).

2. If the user chooses “Y” for “yes,” find the current price and display it

on the screen.

3. Otherwise, check the price every 15 minutes until it falls below

$4.74, then (and only then), display the price on screen.

90 Chapter 3

buy it now

Your code may look a little

different from this, but that's

OK. As long as it does the same

thing, you're doing fine.

You need to ask the user

if the price is required

immediately.

If the user chooses “Y", display

the value that the get_price()

function gives you.

If the user decides to wait for

the price to drop, get the price

using the get_price() function,

then use the given value to

decide whether or not it's time

to buy coffee.

Using the new get_price() function, you were asked to write a

new version of the price-checking program that does this:

1. Ask the user to indicate if the price is required immediately (Y/N).

2. If the user chooses “Y” for “yes,” find the current price and display it

on the screen.

3. Otherwise, check the price every 15 minutes until it falls below

$4.74, then (and only then), display the price on screen.

import urllib.request

import time

def get_price():

page = urllib.request.urlopen(“http://www.beans-r-us.biz/prices.html")

text = page.read().decode(“utf8")

where = text.find(‘>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

return float(text[start_of_price:end_of_price])

price_now = input(“Do you want to see the price now (Y/N)? ")

if price_now == “Y":

print(get_price())

else:

price = 99.99

while price > 4.74:

time.sleep(900)

price = get_price()

print(“Buy!")

you are here 4 91

functions

See what happens when you run the new program. Make the required

changes in IDLE and take your new program for a spin:

Test Drive

The code has been

amended to include

the “get_price()"

function.

If you need the price

right now, this program

wastes no time in giving

it to you. Note: that's

an UPPERCASE “y”.

If you’re happy to wait, the program will

(eventually) tell you when the price is right.

Design principle: reuse code with functions.

92 Chapter 3

going mobile

The CEO wants the price sent to his

cell phone.

Rather than have the emergency report displayed

on a PC, the Starbuzz CEO would prefer to get

something more immediate while he’s on the

road. He needs messages to be sent directly to his

Twitter account.

This is great; it does just what

I want! In fact, it‛s so good it‛s given

me an idea. Since I‛m on the road a

lot, I‛d like the price sent to my cell.

Can your program tweet me?

Sending a message to a Twitter account feels like a tall order.

Where do you think you’d start looking for helpful suggestions and,

quite possibly, a solution to this new problem?

you are here 4 93

functions

def send_to_twitter():

msg = "I am a message that will be sent to Twitter"

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "...", "...")

http_handler = urllib.request.HTTPBasicAuthHandler(password_manager)

page_opener = urllib.request.build_opener(http_handler)

urllib.request.install_opener(page_opener)

params = urllib.parse.urlencode( {'status': msg} )

resp = urllib.request.urlopen("http://twitter.com/statuses/update.json", params)

resp.read()

To use the code you will first need

to sign up for a free Twitter account.

To register, go to:

https://twitter.com/signup

Use the Force, Luke

It’s pretty complicated to write a program that sends messages to a service

like Twitter. Fortunately, other people have already tackled problems like this

and posted their code on the Web. Here’s a Python function (found on the

Web) that is very similar to what you need:

Put your Twitter

username here.

This is the text of the

message that will be sent.

Do this!

Put your

Twitter

password

here.

This code looks like it could be useful. But is

there a problem?

Why can’t you just replace the print()

calls in our existing program with calls to this

function?

Web

This code looks complex but, for now, all you need to know is that it sends

a message to the Twitter service. An advantage of using functions (which is

illustrated here) is that they allow you to understand a program at a high level

without having to initially understand all the details. This is known as working

at a higher level of abstraction.

94 Chapter 3

the message stays the same

price_now = input("Do you want to see the price now (Y/N)? ")

if price_now == "Y":

print(get_price()) send_to_twitter()

else:

price = 99.99

while price > 4.74:

time.sleep(900)

price = get_price()

print("Buy!") send_to_twitter()

Replace the

calls to

“print()".

The function always sends the

same message

Call the “send_to_twitter()"

function instead.

The Starbuzz

CEO's homepage

on Twitter.

No matter which option is chosen by the user, the

“send_to_twitter()" function sends the same tweet to

Twitter.

you are here 4 95

functions

def send_to_twitter_price_low():

# original code here...

# ...but change message to

# buy at low price.

def send_to_twitter():

# original code here...

Well, duh, it‛s pretty obvious really...

you need a version of the function

that tweets the low price and

another that tweets an emergency

order. How hard is that?

Take the original

Twitter code and

create two new

functions.

Something doesn’t feel quite right about this solution.

Can you see the problem that this creates? Can you

think of a solution that fixes the problem?

def send_to_twitter_emergency():

# original code here...

# ... but change message to

# place an emergency order.

Lines that start with # are known

comments

in Python. Comments

are code annotations put there by a

coder and are meant to be read by

other coders working with the code.

Python ignores all comments, because

they aren't executable code.

96 Chapter 3

set parameters

Use parameters to avoid duplicating

functions

Just like it’s a bad idea to use copy’n’paste for repeated usages of code, it’s

also a bad idea to create multiple copies of a function with only minor

differences between them. Look again at the proposed send_to_

twitter_price_low() and send_to_twitter_emergency()

functions on the previous page; the only difference between them is the

message they send.

A parameter is a value that you send into your function. Think of it as

the opposite of what you get when you return a value from a function:

send_to_

twitter()

...

send_to_twitter('5.51')

...

Connect to Twitter and

send the message with the

price to the CEO.

Call the function

and send 5.51 as a

parameter.

Return control to the

calling code.

The parameter’s value works just like a variable within the function, except

for the fact that its initial value is set outside the function code:

...

send_to_twitter('5.51')

...

def send_to_twitter(msg):

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/st

http_handler = urllib.request.HTTPBasicAuthHandler(pas

page_opener = urllib.request.build_opener(http_handle

urllib.request.install_opener(page_opener)

params = urllib.parse.urlencode( {'status': msg} )

resp = urllib.request.urlopen("http://twitter.com/sta

In this version of the function,

the “msg" variable is set to

the value passed in by the call

to “send_to_twitter(‘5.51’)".

you are here 4 97

functions

Parameters Up Close

To use a parameter in Python, simply put a variable name between the

parentheses that come after the definition of the function name and before the colon.

Then within the function itself, simply use the variable like you would any other:

def shout_out(the_name):

return("Congratulations " + the_name + "!")

Later, invoke the function from your code with a different parameter value each

time you use the function:

print(shout_out('Wanda'))

msg = shout_out('Graham, John, Michael, Eric, and Terry by 2')

print(shout_out('Monty'))

The parameter name goes here.

Use the parameter's

value in your

function's code

just like any other

variable.

Grab your pencil and update your program. Do the following:

1. Modify the send_to_twitter() function so that the message text is

passed into the function as a parameter.

2. Update your code to make the approriate parameterized calls to

sent_to_twitter().

98 Chapter 3

ready to tweet

def get_price():

page = urllib.request.urlopen("http://www.beans-r-us.biz/prices.html")

text = page.read().decode("utf8")

where = text.find('>$')

start_of_price = where + 2

end_of_price = start_of_price + 4

return float(text[start_of_price:end_of_price])

price_now = input("Do you want to see the price now (Y/N)? ")

if price_now == "Y":

print(get_price())

else:

price = 99.99

while price > 4.74:

time.sleep(900)

price = get_price()

print("Buy!")

You were to grab your pencil and update your code to

incorporate a version of send_to_twitter() that supports

parameters:

def send_to_twitter():

msg = "I am a message that will be sent to Twitter"

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "...", "...")

http_handler = urllib.request.HTTPBasicAuthHandler(password_manager)

page_opener = urllib.request.build_opener(http_handler)

urllib.request.install_opener(page_opener)

params = urllib.parse.urlencode( {'status': msg} )

resp = urllib.request.urlopen("http://twitter.com/statuses/update.json", params)

resp.read()

import urllib.request

import time

def send_to_twitter(msg):

send_to_twitter(“Buy!")

send_to_twitter(get_price())

You just need to replace the print()

calls with send_to_twitter() calls.

The msg variable in the

code needs to become a

parameter of the function.

you are here 4 99

functions

Now you’ve amended the program code, it’s time to see if it works. Make

sure the amended code is in IDLE, and then press F5 to run your program.

To begin, let’s send an emergency message:

That worked. But what about the price-watch option...?

Test Drive

You asked for an

emergency price

and here it is.

100 Chapter 3

message received

Beep, beep... was that

someone’s phone?

Great, an emergency order!

I‛ll quickly place a call

before we run out of beans

again...

Test Drive

continued...

That works as well. You’re ready to go live!

You decide to wait and

(eventually) the tweet

with a message to buy

comes thru (when the

price is right).

No matter where the

Starbuzz CEO is, if he

has his cell nearby, he

gets the message.

you are here 4 101

functions

Q: Can I still call the Twitter function like this: send_to_

twitter()? Or do I always have to provide a value for the msg

parameter?

A: As it’s written, the parameter is required by the function. If

you leave it out, Python will complain and refuse to run your code

further.

Q: Can parameters to functions be optional?

A: Yes. In most programming languages (including Python), you

can provide a default value for a parameter, which is then used if

the calling code doesn’t provide any value. This has the effect of

making the parameter optional, in that it either takes its value from

the one provided by the caller, or uses the default value if the caller

does not provide anything.

Q: Can there be more than one parameter?

A: Yes, you can have as many as you like. Just bear in mind that

a function with a gazillion parameters can be hard to understand,

let alone use.

Q: Can all the parameters be optional?

A: Yes. As an example, Python’s built-in print() function

can have up to three optional parameters, in addition to the stuff to

print (which is also optional). To learn more, open up a Python Shell

prompt and type help(print) at the >>> prompt.

Q: Doesn’t all that optional stuff get kinda confusing?

A: Sometimes. As you create and use functions, you’ll get a

feel for when to make parameters mandatory and when to make

them optional. If you look at the description of print() again ,

you’ll see that in most usage scenarios print() takes a single

parameter: the thing to display. It is only when extra, less common,

functionality is required that the other parameters are needed.

Q: The description of print() mentions “keyword

arguments.” What are they?

A: The word “argument” is another name for “parameter,” and

it means the same thing. In Python, an argument can have

an optional “keyword” associated with it. This means that the

parameter has been given a name that the calling code can use to

identify which value in its code is associated with which parameter

in the function.

Continuing to use print() as an example, the sep, end,

and file parameters (a.k.a. keyword arguments) each have

a default value, so they are all optional. However, if you need to

use only one of them in the calling code, you need some way to

identify which one you are using, and that’s where the keyword

arguments come in. There are examples of these optional features

of print() and other such functions later in the book. Don’t

sweat the details right now, though.

102 Chapter 3

password changes

Someone decided to mess with your code

One of the Starbuzz coders decided that the password

should be set at the start of the program, where it can be

easily amended in the future. This is what she added:

import urllib.request

import time

def set_password():

password="C8H10N4O2"

set_password()

def send_to_twitter(msg):

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "starbuzzceo", password)

About time I changed the Twitter

password. Hmmm... Interesting code,

but it would be great if it printed a

message every time it sent a tweet. I

think I ll just improve it a little...

So, later in the program, the code uses the password

variable. That means that next time the password needs

to be changed, it will be easier to ﬁnd it in the code

because it is set right near the top of the ﬁle.

This is the new password.

The coder wants to set the

password at the top of the

file where it’s easy to find.

Use the value of

“password” here.

you are here 4 103

functions

Add the new password code to the top of the program and then

run it through IDLE:

The program has crashed and it can no longer send out

messages to the CEO. Stores across the globe are running

short on beans and it’s up to you to fix the code.

Test Drive

Look at the error message that was generated when the program crashed.

What do you think happened?

Oh no! It crashes! What happened?!? Our order

system has stopped working worldwide! If we don‛t

get information on where we need coffee orders soon,

this could be the end of Starbuzz... Help!

Yikes!

104 Chapter 3

add it to the stack

The rest of the program can’t see

the password variable

def set_password():

password="C8H10N4O2"

set_password()

def send_to_twitter(msg):

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "starbuzzceo", password)

The program crashed because, for some reason, the program

couldn’t find a variable called password. But that’s a little odd,

because you define it in the set_password() function:

So what happened? Why can’t the send_to_twitter()

function see the password variable that was created in the

set_password() function?

Programming languages record variables using a section of

memory called the stack. It works like a notepad. For example,

when the user is asked if she wants to send a price

immediately, her answer is recorded against the

price_now variable:

This code calls for the password to be set.

This code sets the password.

This code uses the

password... but for

some reason, it can’t

see it.

you are here 4 105

functions

When you call a function, the computer

creates a fresh list of variables

But when you call a function, Python starts to record any

new variables created in the function’s code on a new sheet

of paper on the stack:

def set_password():

password="C8H10N4O2"

This new sheet of paper on the stack is

called a new stack frame. Stack frames

record all of the new variables that are

created within a function. These are known

as local variables.

The variables that were created before the

function was called are still there if the function

needs them; they are on the previous stack frame.

But why does the computer record variables like this?

Your program creates a new stack frame each time it calls a function,

allowing the function to have its own separate set of variables. If

the function creates a new variable for some internal calculation, it

does so on its own stack frame without affecting the already existing

variables in the rest of the program.

This mechanism helps keep things organized, but

it has a side-effect that is causing problems...

New stack frame

LOCAL variables used

by the function

The calling code's

variables are still

here.

When a variable's

value can be seen by

some code, it is said

to be “in scope.”

106 Chapter 3

local garbage removal

When you leave a function, its

variables get thrown away

Each time you call a function, Python creates a new stack frame to

record new variables. But what happens when the function ends?

The computer throws away the

function’s stack frame!

Remember: the stack frame is there to

record local variables that belong to the

function. They are not designed to be used elsewhere

in the program, because they are local to the function. The whole reason

for using a stack of variables is to allow a function to create local

variables that are invisible to the rest of the program.

And that’s what’s happened with the password variable. The first

time Python saw it was when it was created in the set_password()

function. That meant the password variable was created on the

set_password() function’s stack frame. When the function ended,

the stack frame was thrown away and Python completely forgot about

the password variable. When your code then tries later to use the

password variable to access Twitter, you’re outta luck, because it can’t

be found anymore...

File this under G, for “garbage."

When a variable's

value CANNOT be

seen by some code,

it is said to be “out

of scope.”

you are here 4 107

functions

This is the start of the program. Write a modified version of this

code that will allow the send_to_twitter() function to see

the password variable.

Hint: you might not need to use a function.

import urllib.request

import time

def set_password():

password="C8H10N4O2"

set_password()

def send_to_twitter(msg):

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "starbuzzceo", password)

http_handler = urllib.request.HTTPBasicAuthHandler(password_manager)

page_opener = urllib.request.build_opener(http_handler)

urllib.request.install_opener(page_opener)

params = urllib.parse.urlencode( {'status': msg} )

resp = urllib.request.urlopen("http://twitter.com/statuses/update.json", params)

resp.read()

You need to rewrite this section.

Write your new

version here.

108 Chapter 3

create the password variable

This is the start of the program. You were to write a modified

version of this code that will allow the send_to_twitter()

function to see the password variable.

Hint: you might not need to use a function.

import urllib.request

import time

def set_password():

password="C8H10N4O2"

set_password()

def send_to_twitter(msg):

password_manager = urllib.request.HTTPPasswordMgr()

password_manager.add_password("Twitter API",

"http://twitter.com/statuses", "starbuzzceo", password)

http_handler = urllib.request.HTTPBasicAuthHandler(password_manager)

page_opener = urllib.request.build_opener(http_handler)

urllib.request.install_opener(page_opener)

params = urllib.parse.urlencode( {'status': msg} )

resp = urllib.request.urlopen("http://twitter.com/statuses/update.json", params)

resp.read()

You needed to rewrite this section.

This is all you need to do: just

create the variable.

password=“C8H10N4O2"

Because the “password" variable is created

outside a function, it is available anywhere

in the program. The “send_to_twitter()”

function should now be able to see it.

you are here 4 109

functions

Test Drive

The fixed version of the code has been loaded onto machines

in every Starbuzz store worldwide. It’s time to try out the code

and see if you can get the ordering system working again:

It works! Because you are creating the password outside

of a function, it is available globally throughout the Python

program file. The password variable will be recorded against

the initial stack frame, so the send_to_twitter() function

will now be able to see it.

Let’s see how the updated code is affecting

the rest of Starbuzz.

Sure enough, the emergency

order price gets through.

110 Chapter 3

full of beans

Starbuzz is fully stocked!

Phew! You really saved

the day! And the company!

For a while there, I thought

we were sunk... but you got

us back up and running.

From Cambridge to Cambodia, from Seattle to Sierra

Leone, the orders are being placed and the beans are

being delivered.

You did a great job. Your system tracks live prices from

the Web and automatically sends messages to the CEO

wherever he is on Earth. You are really using the power

of functions to keep your code clean, concise, and

clear. By correctly using variable scope, you even made

it easy to keep the password up-to-date.

Well done!

Show of hands who wants

a skinny latte?

Coffee

With the coffee beans fully

stocked, there's plenty of time for

the more important things in life ...

... and there are a lot

of happy Starbuzz

customers, too.

you are here 4 111

functions

CHAPTER 3

Your Programming Toolbox

You’ve got Chapter 3 under your

belt. Let’s look back at what

you’ve learned in this chapter:

Programming Tools

* Avoid code duplication with functions.

* Parameters are variables that you can pass to

functions.

* Functions can return values.

* Computers use stack frames to record and

track variables.

* When you call a function, a new stack frame is

created for the function to use.

* Stack frames (and local variables) are thrown

away when you exit a function.

* A variable is said to be “in scope” whenever it's

value can be seen by some code.

Python Tools

* Use “def" to create functions.

* Use return() to send a value

back to the code that called the

function.

* Pass parameters to functions by

placing them between parentheses.

this is a new chapter 113

data in files and arrays

Sort it out

As your programs develop, so do your data handling needs.

And when you have lots of data to work with, using an individual variable for each piece

of data gets really old, really quickly. So programmers employ some rather awesome

containers (known as data structures) to help them work with lots of data. More times

than not, all that data comes from a file stored on a hard disk. So, how can you work with

data in your files? Turns out it’s a breeze. Flip the page and let’s learn how!

Wow! Look, Mom! I didn‛t

know Daddy could surf the

green room... Ummmm... I just

hope he knows what

he‛s doing...

waiting for the waves

Surf’s up in Codeville

The annual Codeville Surf-A-Thon is more popular than ever

this year.

Because there are so many contestants, the organizers asked

you to write a Python program to process the scores. Eager to

please, you agreed.

The trouble is, even though the contest is over and the beach

is now clear, you can’t hit the waves until the program is

written. Your program has to work out the highest surfing

scores. Despite your urge to surf, a promise is a promise, so

writing the program has to come first.

114 Chapter 4

Surf-A-Thon

The scoreboard is

currently empty. Wonder

who won today's contest?

data in files and arrays

Find the highest score in the results file

After the judges rate the competitors, the scores are stored in a file called

results.txt. There is one line in the file for each competitor’s score. You

need to write a program that reads through each of these lines, picks out the

score, and then works out the highest score in the Surf-A-Thon.

you are here 4 115

It sounds simple enough, except for one small detail. You’ve written programs

to read data from the Web, and read data that’s been typed in at the keyboard,

but you haven’t yet written any code that reads data stored in a file.

Results are stored in

the results.txt file.

Score

8.65

9.12

8.45

7.81

8.05

7.21

8.31

Official Judging Dude

Brad

116 Chapter 4

shredding with the for loop

Iterate through the file with the open, for,

close pattern

If you need to read from a file using Python, one way is to use the built-in

open() command. Open a file called results.txt like this:

result_f = open("results.txt")

The call to open() creates a file handle, which is a shorthand that you’ll

use to refer to the file you are working with within your code.

Because you’ll need to read the file one line at a time, Python gives you the for

loop for just this purpose. Like while loops, the for loop runs repeatedly,

running the loop code once for each of the items in something. Think of a

for loop as your very own custom-made data shredder:

Each time the body of the for loop runs, a variable is set to a string

containing the current line of text in the file. This is referred to as iterating

through the data in the file:

Put the actual name of

the file to open here.

The opened file is

assigned to a file handle,

called “result_f" here.

result_f = open("results.txt")

for each_line in result_f:

print(each_line)

result_f.close()

Open the file and give

it a file handle.

The “each_line" variable is

set to the next line from

the file on each iteration.

The for loop stops when

you run out of lines to

read.

Close the file (through the file handle)

when you're done with it.

Do something with the thing you've just read from

the file. In this case, you print out the line. Notice

that the for loop's code is indented.

The entire file is

fed into the for loop

shredder...

The for loop shredderTM

...which breaks it up into one-

line-at-a-time chunks (which are

themselves strings).

Note: unlike a real

shredder, the for loop

shredderTM doesn't

destroy your data—it

just chops it into lines.

you are here 4 117

data in files and arrays

Code Magnets

You need to complete the code to find the highest score in the

results.txt file. Remember: the for loop creates a string from

each line in the file.

Hint: For the program to work, you will need to convert the string

into a number.

highest_score

float

line line

highest_score

highest_score = 0

result_f = open("results.txt")

for line in result_f:

if ( ) :

= ( )

result_f.close()

print("The highest score was:")

print(highest_score)

118 Chapter 4

getting the highest score

Code Magnets Solution

You needed to complete the code to find the highest score in the

results.txt file. Remember: the for loop creates a string from

each line in the file.

Hint: For the program to work, you needed to convert the string into

a number.

highest_score = 0

result_f = open("results.txt")

for line in result_f:

if ( ) :

= ( )

result_f.close()

print("The highest score was:")

print(highest_score)

highest_score

float

line

highest_score

Remember to

indent the code

inside the loop AND

again inside the

“if"statement.

Remember to convert the string to

a number with float(). Even though

the line is a number, it comes into

the program as a string.

The “highest_score" variable gets

updated every time you find a line that

contains a higher score.

After the loop runs, the “highest_score"

variable should have the best score from the

data file, so you can then go ahead and display

it on screen.

To successfully run this program, you need to grab a copy of the results.txt

data file from the Head First Programming website. Be sure to put the data file in

the same directory (or folder) that contains your code.

Load this!

you are here 4 119

data in files and arrays

Test Drive

It’s time to see if the program works. Use IDLE to create a new file using the code

from the previous page, save your program as high_score.py, and then run it by

pressing the F5 key:

Oh dear. It looks like something’s gone wrong! The program has

crashed with a ValueError, whatever that is.

Study the error message produced by Python. Is there something wrong with

the Python code? Is there a problem with the file? Is there something wrong

with the data? What do you think happened?

Here's the program

typed into IDLE.

Oh, no, something's

gone south here...

Look's like you are trying

to convert something that

didn't look like a number.

120 Chapter 4

names and numbers

The file contains more than numbers...

To see what happened, let’s take another look at the judge’s score sheet

to see if you missed anything:

The judges also recorded the name of each surf contestant next to his or

her score. This is a problem for the program only if the name was added

to the results.txt file. Let’s take a look:

Sure enough, the results.txt file also contains the contestant names.

And that’s a problem for our code because, as it iterates through the file,

the string you read is no longer just a number.

The results

file.

Johnny 8.65

Juan 9.12

Joseph 8.45

Stacey 7.81

Aideen 8.05

Zack 7.21

Aaron 8.31

The results in the file

look like this.

Name Score

Johnny 8.65

Juan 9.12

Joseph 8.45

Stacey 7.81

Aideen 8.05

Zack 7.21

Aaron 8.31

There are two pieces

of information on

each line: a name and

a number (the surfer's

score).

The judge's official ID

badge was covering up the

names.

Official Judging Dude

Brad

you are here 4 121

data in files and arrays

Split each line as you read it

Each line in the for loop represents a single string containing two

pieces of information:

You need to somehow extract the score from the string. In each

line, there is a name, followed by a space, followed by the score. You

already know how to extract one string from another; you did it for

Starbuzz back in Chapter 2. And you could do something similar

here using the find() method and index manipulation, searching

for the position of a space (' ') character in each line and then

extracting the substring that follows it.

Programmers often have to deal with data in strings that contain

several pieces of data separated by spaces. It’s so common, in fact,

that Python provides a special string method to perform the cutting

you need: split().

Python strings have a built-in split() method.

To isolate the score, you need to

cut each line in two.

Brad 4.03

Jim 7.91

Janet

7.49

Johnny 8.65

Juan 9.12

Joseph 8.45

Stacey 7.81

Aideen 8.05

Zack 7.21

Aaron 8.31

Each line contains a name and a number,

as a string.

The for loop shredderTM

And you'll find that other

programming languages have very

similar mechanisms for breaking up

strings.

122 Chapter 4

split the string

“Brian"

bass

The split() method cuts the string

Imagine you have a string containing several words assigned to a variable. Think

of a variable as if it’s a labeled jar:

rock_band = "Al Carl Mike Brian"

The rock_band string, like all Python strings, has a split() method that

returns a collection of substrings: one for each word in the original string.

Using a programming feature called multiple assignment, you can take

the result from the cut performed by split() and assign it to a collection of

variables:

Each of the return values from the split() on rock_band is assigned to its

own separately named variable, which allows you then to work with each word in

whatever way you want. Note that the rock_band variable still exists and that it

still contains the original string of four names.

Looks like you can use multiple assignment and split() to

extract the scores from the results.txt file.

(rhythm, lead, vocals, bass) = rock_band.split()

Al Carl Mike Brian

“Al"

rhythm

“Carl"

lead vocals

“Mike"

The right side of the

assignment operator

contains the call to

the split() method.

The left side of the

assignment operator lists

the variables to assign

values to.

A single variable is assigned...

...a single string, which

contains four words.

rock_band

“Al Carl

Mike Brian"

A variable, a

labeled jar.

A string,

contained

in a variable

(jar).

Multiple variables...

...each with its own

stringed value.

you are here 4 123

data in files and arrays

Here is the current version of the program:

highest_score = 0

result_f = open("results.txt")

for line in result_f:

if float(line) > highest_score:

highest_score = float(line)

result_f.close()

print("The highest score was:")

print(highest_score)

Write the extra code required to take advantage of the split() method and multiple

assignment in order to create variables called name and score. Then use them to complete

the program to find the highest score.

124 Chapter 4

find the top score

for line in result_f:

(name, score) = line.split()

if float(score) > highest_score:

highest_score = float(score)

Here is the current version of the program:

highest_score = 0

result_f = open("results.txt")

for line in result_f:

if float(line) > highest_score:

highest_score = float(line)

result_f.close()

print("The highest score was:")

print(highest_score)

You were to write the extra code required to take advantage of the split() method and

multiple assignment in order to create variables called name and score, then use them to

complete the program to find the highest score.

The only code changes

required are within the

for loop. The rest of

the program remains

unchanged.

Add in the call to the split() method

to cut the line in two, creating the

“name" and “score" variables.

You are no longer

comparing the line to

the highest score, so

be sure to compare the

“score" variable instead.

you are here 4 125

data in files and arrays

Test Drive

So what happens when you run this version of the code within IDLE ? Let’s

amend the code and see:

It works! The program reads each line from the file as a string, extracts the

score using the split() method, and then uses it to find the highest score

in the file. The organizers are so excited to hear that the program is finished

that they immediately display the result on the large scoreboard at the back

of the beach.

Surf-A-Thon

1. 9.12

2. ? ?

3. ? ?

The amended code, which

takes advantage of the

split() method.

This time, the program

produces a result you

can actually use.

The results are in!

Or are they... ?

Ummm ... those are

the top scores?!?!

126 Chapter 4

more top scores

But you need more than one top score

As soon as the top score appears, people start to wonder what the

second and third highest scores are:

It seems that the organizers didn’t tell you everything you

needed to know. The contest doesn’t just award a prize for the

winner, but also honors those surfers in second and third place.

Our program currently iterates through each of the lines in the

results.txt file and works out the highest score. But what it

actually needs to do is keep track of the top three scores, perhaps

in three separate variables:

Surf-A-Thon

1. 9.12

2. ???

3. ???

highest_

score

second_

highest

third_

highest

You are missing some

scores here... the 2nd

and 3rd scores are a

mystery.

Looks like you need extra

variables for the second and

third highest scores.

What about second

and third place?

“9.12"

you are here 4 127

data in files and arrays

Keeping track of 3 scores makes the code

more complex

So how will you keep track of the extra scores? You could do something

like this:

set the highest_score to 0

set the second_highest to 0

set the third_highest to 0

iterate through each of the scores:

if the score > highest_score:

set the third_highest to second_highest

set the second_highest to highest_score

set the highest_score to score

otherwise if the score > second_highest:

set the third_highest to second_highest

set the second_highest to score

otherwise if the score > third_highest:

set the third_highest to score

You can see that there’s a lot more logic here, because the program needs

to “think” a bit more. Unfortunately, turning this logic into code will

make the program longer and harder to change in the future. And, let’s be

honest, it’s somewhat more difficult to understand what’s actually going

on with the logic as shown here.

How could you make this simpler?

Think about what would make the program easier to write. Check

the box that you think would have the greatest impact:

If there were no names in the file, only numbers

If the scores came before the names in the file

If the data were ordered highest to lowest

If you knew exactly how many lines are in the file

This is NOT real

Python code. It's what

programmers call “pseudo-

code." They use it when

they are sketching out

ideas and working out a

program's logic.

128 Chapter 4

sort the data

An ordered list makes code much simpler

If you had some way of reading the data from the file and then producing an ordered copy

of the data, the program would be a lot simpler to write. Ordering data within a program

is known as “sorting:”

But how do you order, or sort, your data? What happens to the original data

in the file? Does it remain unsorted or is it sorted, too? Can the data even be

sorted on disk and, if so, does this make things easier, faster, or slower?

Sorting sounds tricky... is there a “best” way?

8.65

9.12

8.45

7.81

8.05

7.21

8.31

9.12

8.65

8.45

8.31

8.05

7.81

7.21

highest

_score

9.12

second_

highest

8.65

third_

highest

8.45

The unsorted data

An ordererd (or

sorted) copy of

the same data

The top three scores are just

the first three values in the

sorted data, extracted to

variables. Easy!

You were think about what would make the program easier to

write and check the box that you think would have the greatest

impact:

If there were no names in the file, only numbers

If the scores came before the names in the file

If the data were ordered highest to lowest

If you knew exactly how many lines are in the file

you are here 4 129

data in files and arrays

Sorting is easier in memory

If you are writing a program that is going to deal with a lot of data, you

need to decide where you need to keep that data while the program works

with it. Most of the time, you will have two choices:

Keep the data in memory

If you want to sort a lot of data, you will need to shuffle data around

quite a lot. This is much faster in memory than on disk.

Of course, before you sort the data, you need to read it into memory,

perhaps into a large number of individual variables:

Keep the data in files on the disk.

If you have a very large amount of data, the obvious place to put it is on

disk. Computers can store far more data on disk than they can in memory.

Disk storage is persistent: if you yank the power cord, the computer

doesn’t forget the information written on the disk. But there is one real

problem with manipulating data on disk: it can be very slow.

Keep the data in memory.

Data is much quicker to access and change if it’s stored in the computer’s

memory. But, it’s not persistent: data in memory disappears when your

program exits, or when the computer is switched off (unless you remember

to save it to a file, in which case it becomes persistent).

You are going to have a problem if you attempt to move all those

lines of data into the computer‛s memory. What type of problem do

you think you‛ll have?

You need to move

the lines of data

in the results.txt

file into memory

before trying to

sort it.

You have lots of lines of

data, so you'll need lots of

variables... right?!?

Do you have a big

cupboard?!? Because that's

a lot of jars...

130 Chapter 4

too many variables

But what if there were four scores in the file? Or five? Even worse, what

if there were 10,000 scores? You’d soon run out of variable names and

(possibly) memory in your computer, not to mention the wear and tear

on your fingers.

Sometimes, you need to deal with a whole bundle

of data, all at once. To do that, most languages give

you the array.

8.65

9.12

8.45

first_

score

8.65 second_

score

9.12

You can’t use a separate variable

for each line of data

Programming languages use variables to give you access to data in

memory. So if you are going to store the data from the results.txt

file in memory, it makes sense that you’ll need to use lots of variables to

access all the data, right?

But how many variables do you need?

Imagine the file just had three scores in it. You could write a program

that read each of the lines from the file and stored them in variables

called first_score, second_score, and third_score:

third_

score

8.45

Well, the As are done

and that‛s the first volume

of all the entries from the

Encyclopedia Galactica stored

in memory. Time to tackle the

Bs, then the Cs, Ds, Es....

Each line in the file gets a

variable of its own.

The data in

the file

you are here 4 131

data in files and arrays

An array lets you manage a whole train of data

So far, you’ve used variables to store only a single piece of data. But

sometimes, you want to refer to a whole bunch of data all at once. For

that, you need a new type of variable: the array.

An array is a “collection variable” or data structure. It’s designed to

group a whole bunch of data items together in one place and give them

a name.

Think of an array as a data train. Each car in the train is called an

array element and can store a single piece of data. If you want

to store a number in one element and a string in another, you

can.

Even though an array contains a whole bunch of data items, the array

itself is a single variable, which just so happens to contain a collection of data.

Once your data is in an array, you can treat the array just like any other

variable.

So how do you use arrays?

You might think that as you are storing all of that data in an array,

you still might need variables for each of the items it stores. But this is

not the case. An array is itself just another variable, and you can give it

its own variable name:

"24"

"Fish"

"hat stand"

my_

array

All aboard

the my_array

express!

my_array = [7, "24", "Fish", "hat stand"]

Here comes the

data train.

Each car holds

a single piece of

data.

132 Chapter 4

create and extend an array

my_words

"Dudes"

"and"

Python gives you arrays with lists

Sometimes, different programming languages have different

names for roughly the same thing. For example, in Python most

programmers think array when they are actually using a Python

list. For our purposes, think of Python lists and arrays as the

essentially same thing.

You create an array in Python like this:

Arrays can be extended

But what if you need to add some extra information to an array?

Like strings, arrays come with a bunch of built-in methods. Use the

append() method to add an extra element onto the end of the array:

This creates a two-item array and gives it the name my_words.

To look at individual items in the array, index the element

required:

>>> print(my_words[0])

Dudes

>>> print(my_words[1])

and

You can read individual pieces of data from inside the array using an

index, just like you read individual characters from inside a string.

As with strings, the index for the first piece of data is 0. The second

piece has index 1, and so on.

>>> my_words.append("Bettys")

>>> print(my_words[2])

Bettys

Python coders typically use the word “array" to

more correctly refer to a list that contains only

data of one type, like a bunch of strings or a

bunch of numbers. And Python comes with a built-

in technology called “array” for just that purpose.

However, as lists are very similar and much more

flexible, we prefer to use them, so you don't need

to worry about this distinction for now.

my_words = ["Dudes", "and"]

"Dudes"

"and"

"Bettys"

The array has grown

by one data element.

Give the

array a name.

Assign a list

of values to it.

you are here 4 133

data in files and arrays

Rewrite your program so that each time you read a new score you

append it to the end of an array called scores.

Then amend the code to print out the first 3 scores in the list.

Hint: To create an empty array of scores use scores = [].

highest_score = 0

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

if float(score) > highest_score:

highest_score = float(score)

result_f.close()

print("The highest score was:")

print(highest_score)

Here's the code as

it currently stands.

134 Chapter 4

high scores in the array

You were to rewrite the program so that each time you read a

new score you append it to the end of an array called scores.

You were then to amend the code to print out the first 3 scores in

the list.

highest_score = 0

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

if float(score) > highest_score:

highest_score = float(score)

result_f.close()

print("The highest score was:")

print(highest_score)

scores = []

result_f = open(“results.txt")

for line in result_f:

(name, score) = line.split()

scores.append(float(score))

result_f.close()

print(“The top scores were:")

print(scores[0])

print(scores[1])

print(scores[2])

Here's the code as

it currently stands.

Start with an

empty array.

Process the data in

the file as before...

...but, this time, append the

scores to an array.

With the data safely

stored in the array,

print out the first 3

array elements.

data in files and arrays

Test Drive

The program should now store all of the information from the file into

the scores array before displaying just the top three scores. Let’s run

the program within IDLE and see what happens:

Crap. The program is printing out the 3 scores all right, but it’s not

printing out the top 3 scores in order, and there’s confusion about who

won.

Which important step did the program forget to perform?

The code produces these

results, which flash up on

the scoreboard.

Surf-A-Thon

1. 8.65

2. 9.12

3. 8.45 Cool, dude!

That‛s my score...

I can‛t believe I

won.

136 Chapter 4

sort the array

boxes

01 2 3

box

Sort the array before

displaying the results

The array is storing the scores in the order they were read from the file.

However, you still need to sort them so that the highest scores appear first.

You could sort the array by comparing each of the elements with each of

the other elements, and then swap any that are in the wrong order.

That looks complicated. Aren‛t

we doing all this to make the

code simpler and easier to maintain?

I wonder if the array has any built—

in methods that might help...?

Arrays in Python have a whole host of methods that make

many tasks easier.

Let’s see which ones might help.

01 2 3

The boxes in the array are

stored in a random order.

By closely examining the array

of boxes, you can sort them by

arranging the boxes in biggest-

to-smallest order.

you are here 4 137

data in files and arrays

What the method does

Method

These are some of the methods that come built into every array. See

if you can match up the descriptions with the method names. Draw in

the missing connecting lines. We’ve done one for you to get you started:

count() Sorts the array into a specified order (low to high)

extend() Removes and returns the last array item

index() Adds an item at any index location

insert() Looks for an item and returns its index value

pop() Reverses the order of the array

remove() Tells you how many times a value is in the array

reverse() Adds a list of items to an array

sort() Removes and returns the first array item

Can you work out which two methods you need to employ to allow

you to sort the data in the order that you need?

138 Chapter 4

sort then reverse

What the method does

Method

These are some of the methods that come built into every array. You

were to match up the descriptions with the method names:

SOlUTion

count() Sorts the array into a specified order (low to high)

extend() Removes and returns the last array item

index() Adds an item at any index location

insert() Looks for an item and returns its index value

pop() Reverses the order of the array

remove() Tells you how many times a value is in the array

reverse() Adds a list of items to an array

sort() Removes and returns the first array item

You were to work out which two methods you needed to employ to

allow you to sort the data in the order that you needed.

The sort() and reverse() methods look the most useful. You

need to use reverse() after you sort() the data, because the

default ordering used by sort() is lowest-to-highest, the opposite

of what you need.

SOlUTion

you are here 4 139

data in files and arrays

boxes

Sort the scores from highest to lowest

You now need to add the two method calls into your code that will sort the

array. The lines need to go between the code that reads the data into the list

and before the code that displays the first three elements:

01 2 3

The array starts off in a random

order. It's “unsorted."

scores = []

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

scores.append(float(score))

result_f.close()

print("The top scores were:")

print(scores[0])

print(scores[1])

print(scores[2])

Here is the existing program. Add in the missing code that sorts

the data, highest-to-lowest.

Put the extra

code here.

140 Chapter 4

all sorted out

box

01 2 3

scores = []

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

scores.append(float(score))

result_f.close()

scores.sort()

scores.reverse()

print("The top scores were:")

print(scores[0])

print(scores[1])

print(scores[2])

At this point in the code, the array is

in memory but it's not in the order you

need. It's unsorted.

These two method calls will sort

the data into the required order.

Now that the array is sorted, the first

three elements contain the high scores.

After the call to sort() and

reverse(), the array is sorted

in the order you need.

Here is the existing program. You were to add in the missing code

that sorts the data, highest-to-lowest.

Geek Bits

It was very simple to sort an array of data using just two lines of code. But it turns out you can do even

better than that if you use an option with the sort() method. Instead of using these two lines:

scores.sort()

scores.reverse()

you could have used just one, which gives the same result: scores.sort(reverse = True)

data in files and arrays

Test Drive

Update your program in IDLE and run it. Look what happens:

The program works great!

The scores are in descending order, and the program is really not much longer

than when it found just one score. Using arrays allows us to solve a much

harder problem with very little extra code. And instead of using lots of

complex logic and lots of variables, you have straightforward logic and only

one variable: the scores array.

Surf-A-Thon

1. 9.12

2. 8.65

3. 8.45

Dude, those are

some sweet scores.

The additional code ensures the

results are in the order you need.

Looks much better!

scores without names

And the winner is...?

It’s time for the award ceremony.

The prizes are lined up and the scores are on the scoreboard.

There’s just one problem.

Nobody knows which surfer got which score.

Surf-A-Thon

1. 9.12

2. 8.65

3. 8.45 1st

2nd

3rd

142 Chapter 4

You know the winning

scores, but the winnng

surfers remain a mystery...

data in files and arrays

you are here 4 143

How do you think you

can remember the names

and the scores for each

surfer in the contest?

Once you've thought

about this problem, turn

over to Chapter 5 and

see if you can resolve

this issue.

You somehow forgot the surfer names

With your rush to catch some waves before the light is gone, you forgot

about the other piece of data stored in the results.txt file: the

name of each surfer.

Without the names, you can’t possibly know which score goes with which

name, so the scoreboard is only half-complete.

The trouble is, your array stores one data item in each element, not

two. Looks like you still have your work cut out for you. There’ll be no

catching waves until this issue is resolved.

Name Score

Johnny 8.65

Juan 9.12

Joseph 8.45

Stacey 7.81

Aideen 8.05

Zack 7.21

Aaron 8.31

You forgot to

process the

names.

144 Chapter 4

programming toolbox

CHAPTER 4

Your Programming Toolbox

You’ve got Chapter 4 under your belt.

Let’s look back at what you’ve learned

in this chapter:

Programming Tools

* files - reading data stored on disk

* arrays - a collection variable that holds

multiple data items that can be accessed by

index

* sorting - arranging a collection in a specific

order

Python Tools

* open() - open a file for processing

* close() - close a file

* for - iterate over something

* string.split() - cut a string into multiple

parts

* [] - the array index operator

* array.append() - add an item to the end of

an array

* array.sort() - sort an array, lowest-to-

highest

* array.reverse() - change the order of an

array by reversing it

this is a new chapter 145

To surf one‛s data properly,

one must constantly practice

one‛s pose...

hashes and databases

Putting data

in its place

Arrays aren’t the only show in town when it comes to data.

Programming languages come with other data-arranging goodies too, and our chosen tool,

Python, is no exception. In this chapter, you’ll associate values with names using a data

structure commonly called the hash (better known as dictionary to Python-folk). And when

it comes to working with stored data, you’ll read data from an external database system

as well as from regular text-based files. All the world’s awash with data, so turn the page

and start applying your ever-expanding programming skills to some cool data-processing

tasks.

still looking for a winner

Who won the surfing contest?

In the previous chapter, you worked out the top three scores,

but they’re not much use without the names of the surfers that

achieved those scores. There will no be surfing for you until

you’ve finished the program.

Here’s the code so far:

scores = []

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

scores.append(float(score))

result_f.close()

scores.sort()

scores.reverse()

print("The top scores were:")

print(scores[0])

print(scores[1])

print(scores[2])

1st

2nd 3rd

146 Chapter 5

You still don't

know who won.

hashes and databases

The fix is easy.

Just use two arrays.

How hard is that?

Rewrite your current program to use two arrays: one to keep

track of the scores, the other to keep track of the surfer names.

you are here 4 147

148 Chapter 5

two arrays

You were to rewrite your current program to use two arrays: one

to keep track of the scores, the other to keep track of the surfer

names.

scores = []

names = []

result_f = open(“results.txt")

for line in result_f:

(name, score) = line.split()

scores.append(float(score))

names.append(name)

result_f.close()

scores.sort()

scores.reverse()

names.sort()

names.reverse()

print(“The highest scores were:")

print(names[0] + ‘ with ' + str(scores[0]))

print(names[1] + ‘ with ' + str(scores[1]))

print(names[2] + ‘ with ' + str(scores[2]))

As well as the scores

array, you now need

a names array, too.

Remember to sort the

names array.

Append the surfer's name

to the names array.

Load this!

Don't forget to download results.

txt from the Head First Programming

website before continuing.

you are here 4 149

hashes and databases

Test Drive

These results don't make sense!

Those results look a little strange. Zack is only a novice surfer but, according

to the results from your program, Zack has the highest score. It looks like the

association between the surfer names and their scores is somehow lost... and if

you think about it, this is exactly what’s happening.

The two arrays are independent of each other: one contains scores, the other

names. When the data is in the file, the surfer name and the scores are associated

with each other because they appear on the same line. However, once the split

occurs and the data is in the arrays, the association is severed. Sorting one array

has no effect on the ordering of the other. No wonder your results are a little off

the wall.

How do you fix this?

With the results.txt file saved to the same directory as your program, enter this

code into IDLE and see what happens. Remember to save your program, then press

F5 to run it.

150 Chapter 5

need a new data structure

Zack

Stacey

Juan

Joseph

Johnny

Aideen

Aaron

9.12

8.65

8.45

8.31

8.05

7.81

7.21

Associate the name with the score

Using two arrays just won’t cut it. You need some other data structure to hold

your data in such a way that the association between the surfers’ name and their

score is maintained.

You need a different data structure. But which one?

Data Structure A standard method of organizing a

collection of data items in your computer's memory.

You've already met one of the classic data structures:

the array.

Scores Names

8.65 Johnny

9.12 Juan

8.45 Joseph

7.81 Stacey

8.05 Aideen

7.21 Zack

8.31 Aaron

What you need is

something that

looks like this.

Many rows

of data

Exactly two

columns of

matched data

Here's the

array of

sorted

scores...

...and here's

the array

of sorted

names.

But the scores no longer

match the names!

you are here 4 151

hashes and databases

A variable with multiple indexed slots

for holding data

Array

Match the data structure names on the left with their descriptions on

the right. We’ve already done one for you. Which one do you think

you’ll need to use for the surfer data?

A variable that creates a chain of data

where one data item points to another

data item, which itself points to

another data item, and another, and

so on and so forth

Linked list

A variable that allows data to enter

at one end of a collection and leave

at the other end, supporting a first-in,

first-out mechanism

Queue A variable that has exactly two

columns and (potentially) many rows

of data

Hash

A variable that contains a collection

of unique data items

Set

A variable that contains data arranged

as a matrix of multiple dimensions

(but typically, only two)

Multi-dimensional array

152 Chapter 5

hash it out

A variable with multiple indexed slots

for holding data

Array

You were asked to match the data structure names on the left with

their descriptions on the right. You were also to identify which one you

thought you might need to use for the surfer data.

A variable that creates a chain of data

where one data item points to another

data item, which itself points to

another data item, and another, and

so on and so forth

Linked list

A variable that allows data to enter

at one end of a collection and leave

at the other end, supporting a first-in,

first-out mechanism

Queue A variable that has exactly two

columns and (potentially) many rows

of data

Hash

A variable that contains a collection

of unique data items

Set

A variable that contains data arranged

as a matrix of multiple dimensions

(but typically, only two)

Multi-dimensional array

Use a hash

You need to use a data structure that maintains the

association between the surfer score and the surfer name,

which is exactly what a hash gives you. There are lots of

surfers with lots of scores, and you need to maintain the

association between the two pieces of information.

Let’s take a look at how hashes work.

Geek Bits

Hashes go by different names in

different programming languages:

mapping, dictionary, associative array, and

key-value list, to name a few. In this book,

we’ll stick to using the name hash.

SOlUTion

This cuts down on the amount of typing and saves our poor fingers!

Here's the

one you

need.

Known in the Python world

as a “dictionary.”

you are here 4 153

hashes and databases

Associate a key with a value using a hash

Start with an empty hash:

scores = {}

An empty hash is assigned to a

variable called “scores".

Note the use of curly brackets here.

The keys go here.

The values go here.

You add data to an existing hash by describing the association between the key

and the value. Here’s how to associate a surfers’ name with their score:

scores[8.45] = 'Joseph'

Put the key inside the

square brackets...

...and put the value to the right of

the assignment operator.

Note how the act of assigning a

value to a key CREATES the hash

entry (assuming it's not already

there). In Python, there's no

explicit “append()" method for

hashes as there is for arrays.

A new row of data is added to

the hash. Note the association.

Scores

8.45 Joseph

Scores

154 Chapter 5

iterate hash data

Iterate hash data with for

Let’s add some additional rows of data to your hash:

scores[9.12] = 'Juan'

scores[7.21] = 'Zack'

The new rows

have been added.

Data in a hash is

maintained in a

seemingly random order,

but don't worry about

that for now.

Once you have a hash created, you can use the trusty

for loop to iterate over each of the rows:

for key in scores.keys():

print(scores[key] + ' had a score of ' + str(name_part))

The “keys()" built-in method returns an array of

the keys of the hash. There are no extra points for

guessing what the “values()" method does.

Another hash method, called items(), returns each key-value pair in turn,

and can be used with the for loop, too:

for score, surfer in scores.items():

print(surfer + ' had a score of ' + str(score))

Whichever method you use to iterate over your hash’s data is up to you,

because using items() or keys() produces the same output.

Take each of the

keys in the hash

in turn...

...and display a custom

message using the data

in each row of the hash.

When referring to a value associated with a key, use

square brackets (just like you did with array data).

Scores

8.45 Joseph

7.21 Zack

9.12 Juan

The “items()” method returns each

key-value pair.

Here's what would

display on screen

(assuming all the

data in “results.txt”

was available to the

hash).

you are here 4 155

hashes and databases

Code Magnets

Rearrange the code magnets at the bottom of the page to complete

the program shown below. Rather than using two arrays, this

program stores the results from the surfing contest in a hash:

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

result_f.close()

print("The top scores were:")

print('Surfer ' + scores[each_score] + ' scored ' + each_score)

for each_score in scores.keys():

scores = {}

scores[score] = name

156 Chapter 5

surfers with their scores

Code Magnets Solution

You were to rearrange the code magnets at the bottom of the page

to complete the program shown below. Rather than using two

arrays, this program stores the results from the surfing contest in a

hash:

result_f = open("results.txt")

for line in result_f:

(name, score) = line.split()

result_f.close()

print("The top scores were:")

print('Surfer ' + scores[each_score] + ' scored ' + each_score)

for each_score in scores.keys():

scores = {}

scores[score] = name

Display each row from the hash,

describing the association.

You need to start with an empty hash,

as opposed to an empty array.

After splitting out the name and the score, use

the value of “score" as the key of the hash and

the value of “name" as the value.

You'll find out soon why the “score"

and not the “name" is used (in this

case) as the key of the hash.

Use a for loop

to process the

contents of

the hash.

Q: Can I append data to a hash just like I did with an array?

A: Yes, and no. There’s no append() method for hashes

like the one included with every array. To add a new row of

data to an existing hash, use code similar to that used in the

solution, above. Hashes do have their own set of methods, but

append() is not one of them.

Q: Can I use anything as the key of a hash?

A: No, you can’t. The rules Python applies here can be complex

(as they can be in other programming languages, too). The best

advice we can give you is to stick with numbers and strings as

keys. Trust us: this isn’t as restrictive as it first sounds and is, by

far, the best/easiest strategy to follow.

you are here 4 157

hashes and databases

Test Drive

Take the code from the previous exercise and use it to create a new file in

IDLE. Call your program scores_hash.py. When you’re ready, press

F5 to run your program.

Cool... each score is now associated

with the correct surfer name.

Great! Your program now uses a data structure that allows

you to associate two related pieces of data together.

158 Chapter 5

sort the surfers

The data isn't sorted

Your program now associates surfers and their scores, but

it displays the data from the hash in some sort of random

ordering. You need to somehow sort the data in the hash

to find out who actually won the contest.

Python hashes don't have a sort() method...

In Python, hashes are optimized for speedy insertions and even

speedier look-ups (searches). As a consequence, the good folks that

created Python were less interested in providing a method to sort a

hash, so they didn’t bother.

...but there is a function called sorted()

Obviously, there’s a need to sort a hash so, again, the good folks that

created Python decided to provide a really smart built-in function

that has the ability to sort any data structure, and they called their

function sorted(). Here’s how to use the sorted() function

with your hash:

You'll find similar design and implementation

decisions in lots of different programming

languages. People are different... and so are

the programming languages they create.

for each_score in sorted(scores.keys(), reverse = True):

print('Surfer ' + scores[each_score] + ' scored ' + each_score)

Remember: the keys in your hash are

the scores, which are numbers, so we ask

“sorted()" to order them highest-to-

lowest using “reverse = True".

Use the “sorted()" function to sort the

keys of the “scores" hash.

That’s one small change to one line at the bottom of your program.

So, let’s go ahead and make that change. Now that you are

sorting the keys of the hash (which represent the surfer’s scores), it

should be clear why the scores were used as the key when adding

data into the hash: you need to sort the scores, not the surfer names,

so the scores need to be on the left side of the hash (because that’s

what the built-in sorted() function works with).

Do this!

Make the change to your

code to use sorted().

Surf-A-Thon

1. Joseph 8.45

2. Zack 7.21

3. Juan 9.12

These scores aren't

in the right order.

hashes and databases

Test Drive

With the one-line change applied, save your program and press F5 to run it again.

The scores are associated

with the surfer's names and

they are now sorted, too!

Fantastic! You’ve identified the top 3

surfers. Time to hit those waves, dude! Surf-A-Thon

1. Juan 9.12

2. Johnny 8.65

3. Joseph 8.45

you are here 4 159

That makes it three-

in-a-row for Juan!

160 Chapter 5

more complex surfer data

101;Johnny 'wave-boy' Jones;USA;8.32;Fish;21

102;Juan Martino;Spain;9.01;Gun;36

103;Joseph 'smitty' Smyth;USA;8.85;Cruiser;18

104;Stacey O'Neill;Ireland;8.91;Malibu;22

105;Aideen 'board babe' Wu;Japan;8.65;Fish;24

106;Zack 'bonnie-lad' MacFadden;Scotland;7.82;Thruster;26

107;Aaron Valentino;Italy;8.98;Gun;19

When data gets complex

Hot on the heels of your success with the local surfing club, you’ve

just been contacted by the Regional Surfing Association (RSA) and

they want you to write a new program for them! RSA’s offering a

brand-new, state-of-the-art, epoxy resin surf board as payment...

once the program’s working to their satisfaction, of course.

You’ve been wanting to try out an epoxy board for ages. The

trouble is, they’re sooooo expensive and you’re a poor surfer. The

waves will have to wait (yet again). But, the thoughts of surfing an

epoxy board... now that’s worth waiting for.

This is too good an

offer to turn down.

So what does RSA’s data look like?

Currently, RSA’s data is stored in a text file using a proprietary

data format. For each surfer, there’s six pieces of data recorded on

each line in the file.

Here’s what the data looks like:

RSA's data is stored in

each line, with a semicolon

separating each piece of data.

RSA has tried importing this data into their favorite spreadsheet

program, but that didn’t really work for them. RSA wants a

program that allows them to quickly find a surfers’ data based on

their Competition ID, then display the surfers’ details like this:

ID: 101

Name: Johnny 'wave-boy' Jones

Country: USA

Average: 8.32

Board type: Fish

Age: 21

Each data item

is nicely labeled.

Each data item is displayed

on its own line, which makes

it really easy to read (unlike

the packed data file).

Competition ID.

Name.

Country.

Average score.

Preferred board type.

Age.

Here are the six pieces of data.

you are here 4 161

hashes and databases

Hints: If you pass a string parameter to the split() method, the data is cut where the string parameter

occurs, as opposed to the cut occurring at a space character (which is split()'s default behavior). Also,

consider using multiple-assignment on the left side of the assignment operator when assigning more than one

name-value pairing to a hash.

Here‛s one surfer‛s data from the file, assigned to a variable called line:

line = "101;Johnny 'wave-boy' Jones;USA;8.32;Fish;21"

Grab your pencil and write some code to process this line and display it on screen like this:

ID: 101

Name: Johnny 'wave-boy' Jones

Country: USA

Average: 8.32

Board type: Fish

Age: 21

162 Chapter 5

data display

Here‛s one surfer‛s data from the file, assigned to a variable called line:

line = "101;Johnny 'wave-boy' Jones;USA;8.32;Fish;21"

You were to grab your pencil and write some code to process this line and display it on screen like this:

ID: 101

Name: Johnny 'wave-boy' Jones

Country: USA

Average: 8.32

Board type: Fish

Age: 21

line = “101;Johnny ‘wave-boy' Jones;USA;8.32;Fish;21"

s = {}

(s[‘id'], s[‘name'], s[‘country'], s[‘average'], s[‘board'], s[‘age']) = line.split(“;")

print(“ID: " + s[‘id'])

print(“Name: " + s[‘name'])

print(“Country: " + s[‘country'])

print(“Average: " + s[‘average'])

print(“Board type: " + s[‘board'])

print(“Age: " + s[‘age'])

Create an empty

hash called “s”.

Display six nicely

formatted messages

on screen.

Use multiple-

assignment to assign

the split data from

“line” to “s”.

Cut the line of data

every time the split()

method sees a semicolon.

Here's one possible solution.

you are here 4 163

hashes and databases

Test Drive

As always, you need to enter this code into IDLE before you run it. Then, save your

program and take it for a spin by pressing F5.

Hey, this data is now

pretty easy to read,

eh?

Your code works for one line of data. But RSA wants to be able to

display the data for any surfer, not just wave-boy’s.

164 Chapter 5

function to hash

Return a data structure from a function

There are really only two choices for how you return data from this function. Pass

back the surfer’s data either:

But which? Returning a string requires the calling code to further process the data to

extract the information it needs, which (although possible) gets messy, because the calling

code is then required to cut up the string using split(). This is something best left to

the function, because it hides the complexity of manipulating the data from the calling code.

Returning a hash allows the calling code to simply pick out the information it needs

without too much fuss and without any further processing.

Return a hash from the function to keep the calling code simple.

Processing one line of surfer data was pretty straightforward. But now you have

to work with all the lines of data in the file. Your program has to make the data

available quickly so that a request to display the details of a particular surfer can

be performed as soon as possible.

You already know enough to write a function that takes the surfer ID as a

parameter, searches the file one line at a time for a matching ID, and then returns

the found data to the caller:

As a string

As a hash

...

find_details(id)

...

Pass the surfer ID.

Search the file

for the ID

Return all the

data for the ID.

find_

details()

101;Johnny 'wave-boy'

Jones;USA;8.32;Fish;21

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hashes and databases

Code Magnets

Rearrange the code magnets to complete the function required. The following

functionality is implemented:

1. Accept a single parameter (the surfer ID).

2. Process the data file one line at a time, creating a hash from the line on each iteration.

3. Compare the parameter against the ID read from the file.

4. If the IDs match, return the hash to the caller.

5. If no match is found, return an empty hash to the caller.

def find_details(id2find):

for each_line in surfers_f:

surfers_f.close()

(s['id'], s['name'], s['country'], s['average'], s['board'],

s['age']) = each_line.split(";")

if id2find == int(s['id']):

s = {}

surfers_f = open("surfing_data.csv")

return(s)

return({})

166 Chapter 5

process the parameter

Code Magnets Solution

You were to rearrange the code magnets to complete the function required. The following

functionality is implemented:

1. Accept a single parameter (the surfer ID).

2. Process the data file one line at a time, creating a hash from the line on each iteration.

3. Compare the parameter against the ID read from the file.

4. If the IDs match, return the hash to the caller.

5. If no match is found, return an empty hash to the caller.

def find_details(id2find):

for each_line in surfers_f:

surfers_f.close()

(s['id'], s['name'], s['country'], s['average'], s['board'],

s['age']) = each_line.split(";")

if id2find == int(s['id']):

s = {}

surfers_f = open("surfing_data.csv")

return(s)

return({})

You processed the entire

file but found NO MATCH.

Close the file and return an

empty hash.

Open the file so that you

can read the data.

Use “for" to

loop through

each of the lines

in the file.

Make sure the hash starts out empty.

Cut up the line (using split()) and

assign the data to the hash (using

multiple-assignment).

Check if the ID

supplied as a parameter

is the same as the one

read from the file.

You have a match! So, close

the file then return the

current hash to the caller.

Load this!

Download surfing_data.csv from

the Head First Programming website before

continuing. Be sure to put the file in the

same directory/folder as your code.

you are here 4 167

hashes and databases

Test Drive

Ready Bake

Code Here‛s some tester code that we‛ve cooked up especially

for you. This code will let you test your function:

lookup_id = int(input("Enter the id of the surfer: "))

surfer = find_details(lookup_id)

if surfer:

print("ID: " + surfer['id'])

print("Name: " + surfer['name'])

print("Country: " + surfer['country'])

print("Average: " + surfer['average'])

print("Board type: " + surfer['board'])

print("Age: " + surfer['age'])

Do this!

Type this tester code,

together with your

function, into IDLE. Be

sure to put the function

near the top of you file,

so that this tester code

appears after it.

Make sure the tester code and your function are entered into IDLE and saved

to a file as a new program. Press F5 to test your function.

Enter an existing surfer

ID and the program

displays the details.

Enter a nonexistent

surfer ID and the

program displays

nothing (as there's

nothing to display).

Great work! Your function is working well. Wonder how the RSA

folks feel about it?

Ask the user for the

surfer ID to find.

Call “find_details()”

on the entered surfer ID.

If data was found,

display the nicely

formatted messages.

168 Chapter 5

surfing success

Here's your new board!

The RSA folks are delighted with your work.

Here's your brand

new board... start

ripping it, dude!

This is exactly what

we need...

Your program really hits the mark. The RSA folks can display

the data from their tightly packed data file in a way that makes

it easy for them to read and work with. It’s fast, too.

Your use of a hash within the function was an inspired choice.

The calling code only needs to be aware that a hash is being

returned from the function to work with it effectively. And, as

you’ve seen, returning a data structure (like a hash) is as easy

as returning any other variable from a function.

Word of your programming skills is spreading

far and wide.

Look at how quickly it

pulls up each surfer's

data.

I might actually get

some beach time this

year!

I think the guys over

at TVN might want to

take a look at this...

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hashes and databases

Meanwhile, down at the studio...

You come highly

recommended from

RSA. We have a very

similar requirement at

TVN... would you consider

selling your code to us?

Head First TVN is an up-and-coming sports

network specializing in everything and anything

to do with water. They are covering the National

Surfing Championship and want their TV

presenters to be able to use your program to

access each surfer’s details in much the same

way that RSA did. There’s just one small kink in

their plans: TVN has all their data in a database

system, not in a file.

Which part of your program is most likely to change if you have to get the

surfer data from a database, as opposed to a file?

The TVN CEO

170 Chapter 5

all in the details

The code remains the same; it's

the function that changes

Your program expects the find_details() function

to return a hash representing the surfer’s details. Rather

than the function searching the file for the data, it needs

to search the TVN database, convert what the database

provides to a hash, and then return the hash to the

calling code.

All you need to know is which database system TVN

is using and how to access it from your function.

Let’s base your code on TVN’s code.

I asked our technical

people and they told me we

use SQLite3... whatever

that is. They also gave me

some code for you to amend...

they told me you‛d know

what to do with it, which is

just as well because it‛s all

gobbledygook to me!

Ready Bake

Code Here‛s the TVN code:

import sqlite3

db = sqlite3.connect("surfersDB.sdb")

db.row_factory = sqlite3.Row

cursor = db.cursor()

cursor.execute("select * from surfers")

rows = cursor.fetchall()

for row in rows:

if row['id'] == 104:

print("ID is " + str(row['id']))

print("Name is " + row['name'])

print("Board-type is " + row['board'])

cursor.close()

Import the standard

SQLite3 library

and connect to the

data in the database

file (which you can

download from this

book's website).

Grab all the surfer

data from the

database, assigning

the data to a variable

called “rows”.

Process each of

the rows...

...looking for a

surfer who has an

ID of 104.

Print out

some of the

data (if we

have a match).

Tidy up after yourself (always a good idea).

It is possible to improve the efficiency and power of this code if you know a

little bit about SQL. We are deliberately avoiding improving TVN's SQL. However,

we strongly recommend “Head First SQL” to those who want to learn more.

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hashes and databases

Rewrite your function to retrieve the data it needs from the TVN

database. Use the ready bake code as provided by TVN’s technical

people as a basis for your code. Bear in mind that your program

expects the data in the hash returned from the

find_details() function to be a collection of strings.

172 Chapter 5

grab the data

You were to rewrite your function to retrieve the data it needs

from the TVN database. You were to use the ready bake code as

provided by TVN’s technical people. You were to bear in mind

that your program expects the data in the hash returned from the

find_details() function to be a collection of strings.

import sqlite3

def find_details(id2find):

db = sqlite3.connect(“surfersDB.sdb")

db.row_factory = sqlite3.Row

cursor = db.cursor()

cursor.execute(“select * from surfers")

rows = cursor.fetchall()

for row in rows:

if row[‘id'] == id2find:

s = {}

s[‘id'] = str(row[‘id'])

s[‘name'] = row[‘name']

s[‘country'] = row[‘country']

s[‘average'] = str(row[‘average'])

s[‘board'] = row[‘board']

s[‘age'] = str(row[‘age'])

cursor.close()

return(s)

cursor.close()

return({})

Grab all the surfer

data from the

database, as opposed

to the file.

When a match is found...

...build the hash one

key-value pair at a

time.

Return the hash

to the calling

code (as before).

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hashes and databases

Test Drive

Change your program in IDLE to use the new version of the function (which now talks

to TVN’s database). Save your program under a new name and press F5 to run it.

The program displays similar

output to before... only, this

time, the data is coming from

a database, NOT a file!

Now, as well as working with data in files, you can extract data from a database

system. Your program fully integrates with TVN’s SQLite3 technology.

Send off that code to TVN, and sit back and wait for the

money to roll in...

Load this!

Download surfersDB.sdb from the

Head First Programming website before

running this test drive.

174 Chapter 5

data in the spotlight

My guys are happy

and so am I, Who

do I make the check

out to?

TVN's data is on the money!

With the surfing data now displayed directly from the TVN

database, the sports writers no longer need to worry about all

those tattered and torn competition entry forms. The data

is right there on screen when they need it. They couldn’t be

happier. Your program has made their day.

The TVN sports

writers know how to

celebrate.

you are here 4 175

hashes and databases

CHAPTER 5

Your Programming Toolbox

You’ve got Chapter 5 under your belt.

Let’s look back at what you’ve learned

in this chapter:

Programming Tools

* hash - a data structure that

associates a name with a value

* s[‘age'] - retrieve the value

associated with the ‘age' name in a

hash called ‘s'

* returning a data structure from a

function

* database system - a technology,

like SQLite3, that can store large

quantities of data in a very efficient

way

Python Tools

* {} - an empty hash

* s[‘wind'] = “off shore" - sets that value

associated with “wind” in the “s” hash to the

value “off shore”

* s.keys() - provide a list of keys for the

hash called ‘s'

* s.items() - provide a list of keys AND values

for the hash called ‘s'

* line.split(“,") - split the string contained

within the ‘line' variable at every occurrence of

a comma

* sorted() - a built-in function that can sort

most data structures

this is a new chapter 177

I know he said he knew a better

way to do things, but this wasn‛t

what I had in mind.

modular programming

Keeping things straight

The code that you write will make its way into many programs.

And, although sharing is good, you need to be careful. One programmer might take your

code and use it in an unexpected way, while another might change it without even letting

you know. You might want to use one function in all your programs and, over time, that

function’s code might change to suit your needs. Smart programmers take advantage of

modular programming techniques to keep their workload manageable. Let’s find out how

in the pages that follow...

178 Chapter 6

technology upgrade

Head First Health Club is upgrading

some systems

Head First Health Club has a new CEO, and he loves new

technology. He was shocked when he saw how old the sales

systems were.

Old and busted

The old cash registers

really need to be replaced.

A point-of-sale (POS) system is just a computer program that

works like a cash register. The boss doesn’t want to replace all

of the cash registers immediately. He wants to begin with a trial

system in the coffee bar.

Let’s see what the coffee bar POS needs to do.

Some of our technology

is really ancient. I want to

start by replacing the cash

registers with new POS

systems. Can you help?

The new hotness

The new system will run

on a PC and accept

credit card payments.

New CEO

It would be nice if the systems

you produce were modular in

design. You'll get to that

AFTER you've spent some time

understanding what's required.

you are here 4 179

modular programming

The program needs to create a

transaction file

The POS program will record credit card sales in a file called

transactions.txt. At the end of each day, this file is sent

to the bank and any money is transferred from the credit card

account into the health club’s bank account.

The bank has sent an example version of the file for you to see

what sort of data you will need to generate:

62189672574056180000220 LATTE

75362847324839070000120 TEA

93343374684577850000150 DONUT

42657423454758780000095 GRAIN BAR

The file contains formatted data. There’s a line of text (or

record ) for each item sold. The first 16 characters of the line

are the credit card number, followed by 7 characters that

represent the sale price, excluding the decimal point (so $1.75

is written as 0000175). The rest of the record is a description

of the item being sold.

How would you create a string of characters in the correct format for the

record? Which parts of the record will be the hardest to create?

The descriptions follow.

The next 7 characters

are the price.

The first 16 characters are

the credit card number.

180 Chapter 6

string formats

Use strings to format strings

Many programs need to create strings with particular pieces of data

at particular places, so most computer languages try to make life

easier for you by letting you use string formats.

The best way to understand string formats is with an example. Open

up the Python Shell and type in this:

>>> print("There are %5d %s available" % (17, "donuts"))

There are 17 donuts available

The number has extra spaces

added at the front, to make

sure it takes up the required 5

characters.

When Python sees a string followed by a percentage (%) symbol,

it knows that it needs to consider the string as a string format.

The values that follow the % operator are going to be inserted into

the formatted string in the order specified, one value at a time.

Wherever Python finds a % inside the string, it knows to insert a

value.

%5d

When Python sees this, it will insert the value 17 as a 5-character

whole number—that’s because the “d” type specifier tells Python

to print the number as a decimal. Because 17 is not 5-characters

long, Python will left-pad it with 3 extra spaces at the front to make

it the correct length.

This symbol tells Python to insert the value “donuts” as a string.

As you are not specifying a length here, the string is inserted as is.

These are just two examples of how to format

values in a string. There are lots more.

The first value will

be inserted as a

5-character number.

The second value will be

inserted as a string.

OK, this string is

followed by a % symbol.

So... I ll need to replace

%5d with the number 17,

and %s with the string

“donuts”.

A string format

specification

A number format

specification

The string-formatting

operator

you are here 4 181

modular programming

Display a string followed by 4.000000e+00.

"%s %e" % ("Value is", 16.0 ** 0.5)

Match each format string to what it does. We’ve already done one for you.

"%7d" % (11232/3)

"%x" % 127

"%20s\n" % "Banana swirl"

"%s is $%4.2f" % ("Popsicle", 1.754)

"%s %f" % ("Value is", 16.0 ** 0.5)

"%07d" % (11232/3) As well as a string, also display a floating point

number to 2 decimal places.

Display a string followed by 4.000000.

Display the value, padded with 0s.

Display the number as a hexidecimal (base 16).

Pad the string out to 20 characters, then display a

newline character.

Display the result of the calculation, padded

with spaces.

Look back at the records you need to create for the transaction

file. Each line will need to end with a newline character. If you

have the credit card number, price, and description in variables

called credit_card, price, and description, write

down what you would use for the format string:

Be careful with the

decimal point in the price.

182 Chapter 6

format string

Display a string followed by 4.000000e+00.

"%s %e" % ("Value is", 16.0 ** 0.5)

You were to match each format string to what it does.

SOlUTion

"%7d" % (11232/3)

"%x" % 127

"%20s\n" % "Banana swirl"

"%s is $%4.2f" % ("Popsicle", 1.754)

"%s %f" % ("Value is", 16.0 ** 0.5)

"%07d" % (11232/3) As well as a string, also display a floating point

number to 2 decimal places.

Display a string followed by 4.000000.

Display the value, padded with 0s.

Display the number as a hexidecimal (base 16).

Pad the string out to 20 characters, then display a

newline character.

Display the result of the calculation, padded

with spaces.

You were to look back at the records you need to create for the

transaction file and write down what you would use for the

format string:

If you have just

one value to format,

you don't need to

surround it with

parentheses.

By default, Python will pad

using spaces.

Hexadecimal numbers

are used for things

like colors on the Web.

\n means take a NEWLINE.

This means “use 4 characters."

This means “show 2 numbers

after the decimal point."

Values can be the

result of a calculation.

Following the % with a 0

means “pad with zeroes."

“%16s%07d%16s\n" % (credit_card, price*100, description)

Credit card numbers should always

be exactly 16 characters.

You need a newline character

at the end of each line.

Don't worry if your answer doesn't

look EXACTLY like this. Try out your

answer in the Python Shell to check

that it works.

Multiplying by 100 and

displaying it as a whole number

effectively removes the

decimal point from the price.

you are here 4 183

modular programming

Code Magnets

Now that you know how to format the records in the

transactions.txt file, it is time to write the rest of the coffee

bar POS program. Complete the code below by arranging the code

magnets in the correct place:

credit_card = input("Credit card number: ")

items[choice - 1]

running = False

choice=int(input("Choose an option: "))

print(str(option) + ". " + choice)

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%s%07d%s\n" % (credit_card, price * 100, description))

file.close()

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.0, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

option = option + 1

print(str(option) + ". Quit")

if choice == option:

else:

save_transaction(prices[choice - 1], credit_card, )

The “a" means you are always

going to APPEND records

to the end of the file.

This is the format string you just created.

The loop will keep running while

the “running” variable has the

value True. To end the loop, set

“running” to False.

184 Chapter 6

new pos program

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%s%07d%s\n" % (credit_card, price * 100, description))

file.close()

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.0, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

option = option + 1

print(str(option) + ". Quit")

if choice == option:

else:

save_transaction(prices[choice - 1], credit_card, )

Code Magnets Solution

Now that we know how to format the records in the

transactions.txt file, it is time to write the rest of the coffee

bar POS program. You were to complete the code below by arranging

the code magnets:

credit_card = input("Credit card number: ")

items[choice - 1]

running = False

choice = int(input("Choose an option: "))

print(str(option) + ". " + choice)

The user enters a menu

option number to make a sale.

This will be True if the user selects the LAST

option on the menu, which is “Quit.”

This is the array of menu options.

This is the matching array of menu prices.

This code displays the

program's menu.

modular programming

Test Drive

The machine has been set up in the coffee bar just in time for the

boss to come in for his morning latte, so let’s run the code and see

what he thinks. Run the program in IDLE:

The program displays a list of items for sale and when you choose

one and provide a credit card number, it adds the sale into a file

called transactions.txt. This is the same file that the POS

system in the gym uses.

That looks pretty good.

We‛ll try it for a couple of

weeks here in the coffee

bar and see how it goes.

63827462387648320000220 LATTE

63827462387648320000180 FILTER

The boss agrees to try out the system in the coffee bar, and he even

agrees to extend the trial and let your friend create another POS

for the gym, based on your code.

Things are going really well. If they continue like

this, you will win the contract to replace all of the

systems at the health club!

The sales are recorded in

the transactions.txt file.

The newline character

makes sure the sales are

recorded on separate lines.

(You can't see then in the

file, but they are there.)

you are here 4 185

186 Chapter 6

real-world format strings

Head First: Format String, it’s a pleasure to meet

you.

Format String: The pleasure is all mine.

Head First: You’re not just used in Python, are

you?

Format String: Oh no. I’m really used in lots of

programming languages, like C, C#, and Java. You

see me crop up all over the place.

Head First: Why is that?

Format String: Well, I don’t like to blow my own

bugle, but I am kind of useful. Everywhere you need

to generate formatted text, I can make life a lot easier

for you.

Head First: Can you give us some examples?

Format String: Oh sure. If you need to send data

to another system, you’ll probably need me. If you

want to display an error message in a specific format,

I’m good at that, too.

Head First: I hate to say this, it seems so rude, but

isn’t it possible to do everything you do with just

normal string operations?

Format String: Please, I don’t take offence. Yes,

you can do pretty much everything I do by writing

code and creating strings manually. But I don’t think

that’s a great idea.

Head First: Why’s that?

Format String: Two reasons: first, you might have

to create a lot of code, and second, I’m a little more

dynamic.

Head First: What do you mean?

Format String: Well, I’m just data. Lots of times,

programmers prefer to use data instead of chunks

of code, because it means they can store me away

as configuration. So I get used a lot for things like

internationalization.

Head First: Internationalization?

Format String: Yeah, say someone wants to display

a message that someone has the top score. They

could write the message in a file like “%d is the top

score!”. If they write a French version of the game,

they just have to amend me to “%d est les points

supérieurs!”

Head First: No code change necessary.

Format String: Exactly.

Head First: String Format, thanks for your time.

The Format String Exposed

This week’s interview:

Why do appearances matter?

Q: Those format strings look a little

weird. How do I find out more about

them?

A: A good reference book (we present

our favorite in the appendix) will contain all

you need to know and there’s also lots of

material on-line in the official Python docs.

Q: Are there special rules for what’s

acceptable as a credit card number?

A: Yes, there are. But let’s not get

bogged down in those type of details at this

stage. Concentrate on the

save_transaction() function

and how the code interacts with it.

Q: Why does the code generate the

menu in such a complex way?

A: If you study the menu generation

code, you will see that when you add more

items and prices, the rest of the code

doesn’t need to change to support the new

items.The menu is generated automatically.

you are here 4 187

modular programming

A late night email ruins your day

A few days after the demo, you get a late night email from the

friend who wrote the second program (based on your code):

Something really strange has happened. Even though your code used

to work, it has suddenly started to go wrong. Meanwhile, your friend’s

program, which is really just a modified copy of your program, is

working perfectly.

Looks like you better get to the health club bright and

early tomorrow morning and see what’s happened.

Hi,

Sorry to contact you so late at night, but there‛s been a *major

problem* with the new POS systems. I created the program for

the register in the gym, based on your code, plus a few other

amendments, and it works great! :-) But the problem isn‛t with my

code; it‛s with your POS program running in the coffee bar. The

boss just heard back from his new bank manager saying there‛s

been some sort of data corruption in the transactions.txt file. I

don‛t know what the problem is, but the boss seems mighty

upset and he wants you to go in first thing in the morning and

sort things out.

See you in the morning!

188 Chapter 6

pricey donut

$50,000... for a donut?!

When you arrive at the health club you find out exactly what’s

happened. The entire day’s sales have been rejected by the

bank for two reasons:

The credit card numbers are all fake/invalid.

The bank was really worried about this one because they think someone

must have hacked into the system to generate the messed-up credit card

numbers.

The prices are ridiculous.

One of the recorded sales was for a donut that cost over $50,000!

And what makes it worse, this

was the first time we sent the

transaction file to our new bank! We

only switched over the day before

yesterday so that I could secure a

loan for the new weight room!

This looks like a really serious

problem. Let’s take a look at

the file that the bank rejected.

you are here 4 189

modular programming

Only the sales from your program

were rejected

The transactions.txt file that was sent to the bank contains all

of the day’s sales from both your POS program in the coffee bar and

your friend’s POS program in the gym. This is a section of the file:

50791428746281510000150 DONUT

00035005002454912715921 WORKOUT

This record is from

the gym and was

ACCEPTED by the

bank.

As expected, each record (from each of the POS programs) has been

appended to the transactions file. That bit appears to be working fine.

But, something is not quite right here...

Study the two records carefully. Is there a difference between them that

might explain why one was accepted and the other was rejected? Think

about recent events. What do you think has caused this problem?

This record is from the

coffee bar and was

REJECTED by the bank.

190 Chapter 6

change in format

The new bank uses a new format

Your friend tells you that just after taking a copy of your code, the word

came down that the health club was switching banks. Without telling you,

your friend found out the new bank format and updated his code in the gym

program.

That means that the POS program in the gym is generating records in the

new bank format.

The new bank format is:

Price / Credit Card / Description

This is the price: $35.00.

This is the credit

card number.

The final part is a

description of the sale.

0003500 5002454912715921 WORKOUT

OK, that‛s another

workout sold, so I‛ll write

Price.... then Credit Card...

then Description...

The bank accepted the transactions from the gym because

they were in the correct format.

But what about the coffee bar program?

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modular programming

Your coffee bar program still uses

the old format

Your program in the coffee bar was never updated after the health club

switched banks. It’s still doing what it always did: it’s still creating files

in the old format.

That old format wrote out the price and the credit card the other way

round, which meant when your program wrote out a record like this:

The credit card number

Price = $1.50

Description

5079142874628151 0000150 DONUT

So that‛s a donut.

Better write Credit

Card... then Price...

then Description.

The new bank read the record like this:

This is suspicious...

$50,000 for a donut....

Does not compute... Fake

credit card information!

Security! Security!!

5079142

The price... $50,791.42!

Messed up credit card number:

the bank thought it was fake.

But at least the

description's OK.

8746281510000150 DONUT

So it’s not that somebody broke into your program and changed it.

No, it’s the exact opposite. Your code never picked up the change that

was made to the gym program in order to support the new format.

What should you do to fix it? What shouldn’t you do?

192 Chapter 6

don't copy, share

Don’t just update your copy

The code in the gym program is a copy of your code in the coffee bar.

And copying code is a bad thing. Once you have two separate copies

of a piece of code, then changes need to be applied in two places.

So how do we avoid copying code?

Smart programmers write modular code

The secret is to break your programs into smaller pieces of code

called modules. What’s a module? It’s just a file containing code the

computer can run. Every Python program you’ve written so far has

been a single module.

But most programs you’ll write will probably be split across many,

many modules. And writing modular code is important because

modules can be shared between programs.

I‛ll use transaction.py

to record the sale.

If you separate out the code that saves the transactions to a file and

store it in a new module called transactions.py, that module

can be shared by both programs. If you then ever need to change

the code in transactions.py, both programs will pick up the

changes automatically.

coffee_pos.py gym_pos.py

transactions.py

This is a SHARED module.

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modular programming

So how do you create a module...?

Remember: a module is just a file containing some Python code. So, take the

code that you want to share out of the gym_pos.py file:

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

Then save this code in a file called transactions.py. You have just

created a new module.

...and how do you use it?

Once you’ve created the module, you then need to tell the programs to use it.

When you were using library code you needed to import it. You do the same

thing with your own modules. So, instead of using library code from the

Standard Python Library, you’re really just using library code that you’ve

written yourself. You can add this line to the top of each of your programs:

from transactions import *

This means “treat everything inside

the module as if it is code within

your program."

With this line, you are telling Python that you want to run the code in the

transactions.py file and this allows you to access whatever code the

module contains as if it is just part of your program.

It’s time to fix the programs.

This line needs to be added

to any program that uses the

“transactions.py” module.

This means “run the code in

the named module."

194 Chapter 6

tale of two programs

These are the two POS programs. Here is the code to the one

used in the coffee bar (that you wrote):

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%16s%07d%16s\n" % (credit_card, price * 100, description))

file.close()

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

This is the eode to the

“coffee_pos.py” program.

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modular programming

The other program is very similar (which your friend created for use in the gym):

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%07d%16s%16s\n" % (price * 100, credit_card, description))

file.close()

items = ["WORKOUT", "WEIGHTS", "BIKES"]

prices = [35.0, 10.0, 8.0]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

Using a pencil, modify the two programs so that they use the transactions.py

module. Then write what you think should go into the transactions.py module here:

This is the eode to the

“gym_pos.py” program.

196 Chapter 6

transactions module

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%16s%07d%16s\n" % (credit_card, price * 100, description))

file.close()

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

from transactions import *

These are the two POS programs. Here is the code to the one

used in the coffee bar (that you wrote):

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modular programming

def save_transaction(price, credit_card, description):

file = open("transactions.txt", "a")

file.write("%07d%16s%16s\n" % (price * 100, credit_card, description))

file.close()

items = ["WORKOUT", "WEIGHTS", "BIKES"]

prices = [35.0, 10.0, 8.0]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

Using a pencil, you were asked modify the two programs so that they use the transactions.py

module. You were then asked to write what you think should go into the transactions.py

module here:

from transactions import *

def save_transaction(price, credit_card, description):

file = open(“transactions.txt", “a")

file.write(“%07d%16s%16s\n" % (price * 100, credit_card, description))

file.close()

Make sure you use the code that

displays the PRICE first.

The other program is very similar (which your friend created for use in the gym):

198 Chapter 6

test drive

Test Drive

Once you have completed the exercise, you should have three saved

files: gym_pos.py, coffee_pos.py, and transactions.py. You

can now run the gym_pos.py and the coffee_pos.py programs:

gym_pos.py

coffee_pos.py

The two programs look like they’ve worked

correctly. But what about the transaction file?

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modular programming

The transaction file is working great, too

00035006432425412474321 WORKOUT

00001507649463856424326 DONUT

The descriptions follow. (Note

the extra padding due to the

“%16s” format specifier.)

Both of the records, created by each of the POS programs, are now

correctly formatted. That’s because both programs are sharing the

same piece of code to save the transactions.

When you open up the transactions.txt file, you see this

inside:

Phew! I just heard from the

bank that transactions from

both of the POS systems went

through smoothly. That‛s a big

relief. Good job fixing it!

Looks like you saved the day.

The first 7 characters

are the price.

The next 16 characters are

the credit card number.

Q: So modules are sort of like

containers for functions, right?

A: It’s true that most modules are used

to store a collection of related functions.

However, it is perfectly acceptable to

put any code in a module, which is then

executed whenever the module is imported

into your program.

Q: So when I use import, it’s as if I

typed the code in the module directly

into my program?

A: Yes, that’s a good way to think about

it. Using a shared module saves you from

having to type (or cut’n’paste) all that code

yourself. Just import it and it’s there.

Q: Do I have to use modules?

A: No, but the benefit of putting

shareable code into a module starts to

pay off the second you use that module

in another program. Sharing code with

modules is good programming practice.

200 Chapter 6

price break

The health club has a new requirement

The health club boss has a grand plan to get more customers into the

health club.

Ooh, I think we need to discount the

prices in the coffee bar for the next month.

There‛s nothing like relaxing with a latte after

a tough workout, especially if the price is right.

Our customers like to treat themselves, so I

want to make this easy for them.

The boss has a great new idea.

The boss wants to cut 10% off all the prices in the

coffee bar. If it’s successful, he may want to do the

same thing in other places, such as the gym.

Instead of just amending the code in the

coffee_pos.py file, you need to create a new

module called promotion.py that will calculate

a discounted price.

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modular programming

You need to change the coffee bar POS program to apply the 10%

discount to everything that’s sold. You have three tasks.

def discount(price):

Complete the code in the above function so that it returns 90% of the price it is

given.

This is the latest version of the coffee_pos.py module. Modify it so that it

uses the new module to cut the price of everything that’s sold.

from transactions import *

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

Start by creating a new module called promotion.py containing

one function:

202 Chapter 6

discount applied

def discount(price):

from transactions import *

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

save_transaction(prices[choice - 1], credit_card, items[choice - 1])

Complete the code in the above function so that it returns 90% of the price it is

given.

This is the latest version of the coffee_pos.py module. Modify it so that it

uses the new module to cut the price of everything that’s sold.

return 0.9 * price

Multiplying the price by 0.9

will give you a 10% discount..

from promotion import *

You need to import the code

from the “promotion.py” module.

new_price=discount(prices[choice - 1])

new_price

Your code should

call the “discount()”

function.

“new_price” is the discounted value of the price.

You needed to change the coffee bar POS program to apply the 10%

discount to everything that’s sold. You had three tasks.

Start by creating a new module called promotion.py containing

one function:

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modular programming

Test Drive

So what happens if you fire up coffee_pos.py in IDLE and buy a $2 latte?

It looks like it’s working on the screen. What about in the transactions.txt file? Will

the latte still cost $2.20?

00001983489203918924782 LATTE

No, the latte was discounted by 10% to $1.98, which is exactly what you

want to record in the transactions file.

It’s time to demo your code to the boss.

The actual price to

charge is here.

204 Chapter 6

another discount

That‛s fantastic!

You made the change

so quickly, just in time

for the doors to open. It

does handle both kinds of

discount, right?

That’s great news. Although you’ve heard about this extra discount

late in the day, at least most of the work’s already been done for you.

You just need to use the Python module the Starbuzz CEO attached

to his email, and your program will be set up to apply both discounts.

Let’s take a look at the Starbuzz code.

Great to hear from you!

Yes, of course, you can join the Starbuzz discount scheme! A lot of

people across the world are now working on systems for Starbuzz,

so I think I can help your coders out. Please find attached a

copy of the official Starbuzz Discount Module (tm). It's a Python

module that will calculate an additional 5% discount for every

customer who presents a Starbuzz Discount Card.

If we ever change the way the discount scheme works in the

future, we can send you an updated module and your systems

will get updated without you having to do any work at all!

Be well and keep drinking the coffee!

Your friend,

Starbuzz CEO

Both kinds of discount?

It seems that there was something that the boss forgot to tell

you. As well as deciding to cut the health club’s own prices, he

also got in touch with his old friend, the CEO of Starbuzz,

and arranged for a special discount for everyone who shows

the cashier a Starbuzz Discount Card. This is the email he

received back:

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modular programming

# Official Starbuzz Discount Module

# Copyright(c) Starbuzz Corporation

# This function calculates a 5% discount on a price

def discount(price):

return 0.95 * price

The Starbuzz code

The attachment from Starbuzz was a file called starbuzz.py.

When you open it, you see this:

This function returns a price

that's 5% lower than the price it

was given.

The first few lines begin with # characters; these are comments.

Comments are just notes added by a programmer that are intended

to be read by other programmers. Python will ignore them, because

comments are not code.

After the comments comes the Starbuzz discount() function. It’s

just like the discount function you wrote, except instead of returning a

10% discount, it returns a 5% discount.

Your code will have to use both discounts:

It will apply a 10% discount to everything.

And if somebody presents a Starbuzz Discount Card, it will also have to

apply the 5% Starbuzz discount.

You need to change the code so that it uses both of the discount()

functions. Can you see a problem? What is it?

Lines that start with

# are comments; Python

will ignore them.

This is the

discount function,

as provided by

Starbuzz.

206 Chapter 6

identity confusion

The two discount functions have the

same name

Here is the promotion.py module you just created:

And here is the starbuzz.py module:

Both of the modules define a function called discount(). So what

happens when you try to use them? If Python sees a line of code like

this:

new_price = discount(1.75)

which function will it call? The promotion discount? The Starbuzz

discount? Both? Neither???

This is one of the problems of using shared code. Sometimes, there’s

a function in one module that has the same name as a function in

another module. When this happens, the last function imported is the

one used, which has the effect of overloading any existing function

that has the same name. This can result in to hard-to-find bugs.

So what do you do?

You need to somehow qualify your function names.

# Official Starbuzz Discount Module

# Copyright(c) Starbuzz Corporation

# This function calculates a 5% discount on a price

def discount(price):

return 0.95 * price

def discount(price):

return 0.9 * price

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modular programming

Fully Qualified Names (FQNs) prevent

your programs from getting confused

Imagine if you lived in a world where people had first names only:

Lots of people share the same first name. But people also have

surnames. If you use a first name with a surname, things are a lot

less confusing.

And it’s the same thing with code. If you have two modules

containing functions with the same name, the computer will get

confused. But if you fully qualify the function name, by prefixing

it with the module name, the computer will know exactly what

you mean:

promotion.discount(1.75)

Oh, I need to

apply the 10% discount

from promotion.py?

That‛s not a problem,

since you‛re using a

FQN...

If you are going to use Fully Qualified Names (FQNs) from a

module, then you will also need to change the way you import the

code:

from promotion import *

import promotion

This will import the code from promotion.py

but to use it, you will need to add “promotion."

to the start of the function name.

Now you can fix the code to use both discounts.

coffee_pos.py

Hi, it‛s Michael.

Say, are you free

on Friday night?

Michael

208 Chapter 6

smarter pos

These are the two discount modules:

promotion.py

starbuzz.py

Here is the latest version of coffee_pos.py

from transactions import *

from promotion import *

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

new_price = discount(prices[choice - 1])

save_transaction(new_price, credit_card, items[choice - 1])

Write a new version of coffee_pos.py that, after choosing a menu option, will ask if the

customer has a Starbuzz Discount Card. If the answer is “Y”, apply both the Starbuzz and the

promotion discount. Otherwise, just apply the promotion discount.

# Official Starbuzz Discount Module

# Copyright(c) Starbuzz Corporation

# This function calculates a 5% discount on a price

def discount(price):

return 0.95 * price

def discount(price):

return 0.9 * price

you are here 4 209

modular programming

Write your code

here.

210 Chapter 6

getting a promotion

These are the two discount modules:

promotion.py

starbuzz.py

# Official Starbuzz Discount Module

# Copyright(c) Starbuzz Corporation

# This function calculates a 5% discount on a price

def discount(price):

return 0.95 * price

def discount(price):

return 0.9 * price

Here is the latest version of coffee_pos.py:

from transactions import *

from promotion import *

items = ["DONUT", "LATTE", "FILTER", "MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + ". " + choice)

option = option + 1

print(str(option) + ". Quit")

choice = int(input("Choose an option: "))

if choice == option:

running = False

else:

credit_card = input("Credit card number: ")

new_price = discount(prices[choice - 1])

save_transaction(new_price, credit_card, items[choice - 1])

You were asked to write a new version of coffee_pos.py that, after choosing an menu option,

will ask if the customer has a Starbuzz Discount Card. If the answer is “Y”, apply both the Starbuzz

and the promotion discount. Otherwise, just apply the promotion discount.

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modular programming

from transactions import *

import promotion

import starbuzz

items = [“DONUT", “LATTE", “FILTER", “MUFFIN"]

prices = [1.50, 2.20, 1.80, 1.20]

running = True

while running:

option = 1

for choice in items:

print(str(option) + “. " + choice)

option = option + 1

print(str(option) + “. Quit")

choice = int(input(“Choose an option: "))

if choice == option:

running = False

else:

credit_card = input(“Credit card number: ")

price = promotion.discount(prices[choice - 1])

if input(“Starbuzz card? ") == “Y":

price = starbuzz.discount(price)

save_transaction(price, credit_card, items[choice - 1])

You need to use this kind of

import for “promotion.py”

and “starbuzz.py”, because

you are going to qualify the

function names with the

module names.

By importing the

transactions module like this,

you can call the functions

without the module name.

If someone has a

Starbuzz Discount

Card, you need to

apply the second

Starbuzz discount.

212 Chapter 6

test drive

Test Drive

Let’s try running the new program and buy a $1.20 muffin without

a discount card.

00001085413765853576543 MUFFIN

But what if you try to buy a $2.20 latte using a

Starbuzz card?

00001885413765835766543 LATTE

The code works! With the Starbuzz card, it applies

two discounts. Without a Starbuzz card, your code

just applies one.

It's a muffin.

It cost $1.08 = 90% of $1.20.

It's a latte.

90% of $2.20 = $1.98.

5% Starbuzz discount

of that gives $1.88!

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modular programming

The discounts get the customers flooding in

Once word gets out that there are double-discounts available in

the coffee bar, the health club gets packed with customers.

We‛ve never been so busy! The weight

room, the gym, the steam baths. The

whole place is full of customers! I like

the way you write code. I want you to

upgrade every system in the place!

Thanks to writing modular code, you got the contract to

replace all the health club systems. Modules help you write

stable, maintainable code. The more code you write in modules,

the greater the chance that you’ll be able to reuse it elsewhere.

Breaking your code into modules turns good

programs into great programs.

Q: I don’t get it; why do I need to use

a FQN again?

A: You use a Fully Qualified Name

(or FQN) when you need to distinguish

between two functions from different

modules that have the same name.

With the health club systems, the

discount() function existed within

your module and within the one supplied by

Starbuzz. In order to keep things straight,

you had to use a FQN.

Q: So if I hadn’t used a FQN, the

wrong discount is applied to the

purchase?

A: Yes, most probably.

Q: But if I hadn’t used a FQN, how

would I know which discount was used?

A: Well... that's the problem. You

wouldn’t. It’s hard to predict what would

happen, because it all depends on which

order the code imports its modules.

Q: So, all I need to do is keep an eye

on the order when I import modules and

everything will be OK?

A: No, that’s not what we recommend.

Don't rely on what might happen. Use a

FQN so that you are always in control.

214 Chapter 6

programming toolbox

CHAPTER 6

Your Programming Toolbox

You’ve got Chapter 6 under your

belt. Let’s look back at what

you’ve learned in this chapter:

Programming Tools

* String formats let you use format specifiers to

format strings.

* String specifiers let you define the type, the

length, and the padding used in a format.

* Breaking code into separate files is called

modular programming.

* Modular code is easier to share between

programs.

* Writing modular code is like writing your own

libraries.

* Fully Qualified Names (FQNs) are function

names that include module names.

Python Tools

* Use “from ... import *" to

run code module functions

without module names.

* Use “import ..." if you need

to qualify your function names

with a module name using an

FQN (which is the recommended

approach in the Python

community).

this is a new chapter 215

You see, sometimes it‛s just

not enough to build it. It has

to look nice, too.

building a graphical user interface

Going all gooey

Your coding skills are great and getting better all the time.

It’s just a shame your programs are not that nice to look at. Displaying prompts and

messages on a text-based console is all well and good, but it’s so 1970s, isn’t it? Add

some green text on a black background and your retro look will be complete. There has

to be a better way to communicate with your users than the console, and there is: using

a graphical user interface or GUI (pronounced “gooey”). Sounds cool, but complex, and

it can be. But, don’t fret; learning a trick or two will have your code all graphical in no time.

Let’s get all gooey (sorry, GUI) in this chapter.

216 Chapter 7

a host of requests

Head First TVN now

produces game shows

It’s more than just sports at Head First TVN, as the station has

entered the lucrative world of live game show broadcasting.

Their flagship show, Who Wants to Win a Swivel Chair, is attracting

viewing figures in the millions... not bad for a station that operates

on a shoestring budget.

You’ve been approached by their stressed-out (but incredibly

smooth) game show host to help with a program he needs. TVN

was so impressed with your work last time that they are offering

two free, front-row tickets to the Grand Final as payment for your

work.

I can‛t keep track

of the score and ask the

questions and sound the

buzzer... all by myself...

can you help?

You’ve had a few conversations with the host and determined a

list of program requirements:

1. The host wants to be prompted after a question has been asked

to press either 1 for a correct answer or 2 for a wrong answer.

2. Based on the key pressed, an appropriate sound effect needs to

play.

3. The program needs to remember how many answers were

correct and how many were wrong.

4. The host will end the quiz by pressing 0. Then the program

will display the number of right, wrong, and asked questions.

Let’s flesh out what’s required in pseudo-code.

TVN’s game

show host.

you are here 4 217

building a graphical user interface

Use the space provided below to write the pseudo-code for the

program you’ve been asked to write for TVN:

Write your

pseudo-code

here.

218 Chapter 7

game show pseudo-code

You were asked to use the space provided below to write the

pseudo-code for the program you’ve been asked to write for TVN:

number_asked = 0

number_correct = 0

number_wrong = 0

ask the host to press 1 for correct, 2 for incorrect, or 0 to end

while the host response is not 0

if host response was 1

add 1 to number_asked

add 1 to number_correct

play a sound effect

if host response was 2

add 1 to number_asked

add 1 to number_wrong

play a sound effect

ask the host to press 1 for correct, 2 for incorrect, or 0 to end

display the values of number_asked, number_correct and

number_wrong on screen

You need to remember how

many questions were asked,

how many were correct,

and how many were wrong.

Don’t worry if your answer

doesn’t look EXACTLY like

this. There are a few ways

of writing the code.

This will run if the question

was answered CORRECTLY.

This will run

if the question

was answered

INCORRECTLY.

Begin by asking the

host to make a choice.

Ask the host at the

end of each loop what

he wants to do next.

Finally, display

the scores.

you are here 4 219

building a graphical user interface

Frank: I think sound is going to be a problem.

Jim: Sounds easy to me...

Joe & Frank: <groan>.

Jim: Sorry, couldn’t resist that one. Seriously, though, how hard can

it be to play a sound from a program?

Joe: Playing a sound is not the problem; getting it to work on

multiple platforms can be. For instance, what works on Windows

might not work on Mac OS X or Linux.

Jim: That’s not a problem. I only use Windows, so I’ll be OK.

Frank: Good for you, but the rest of us want to play, too, and we

don’t want to have to... um... eh... downgrade to Windows.

Jim: Typical: have a swipe at Windows when something doesn’t

work on your non-Windows computer.

Joe: Cool it, guys. We need to stop bickering and come up with a

solution that lets us play sounds and works on Windows, Mac OS X,

and Linux. And it has to work with Python, too.

Jim: You mean Python doesn’t support sound as standard?!?

Frank: No, not really. In fact, very few programming languages

support sound in a cross-platform way. This isn’t just a Python

problem.

Jim: So... we’re outta luck then. Does that mean it’s time to go

home?

Joe: Not so fast, Jim! I’m pretty sure pygame can help here.

Jim: So... I can’t go home early, but I can play games?

Frank: Seriously, Jim, I think Joe’s right. We can use pygame to

play our sounds in Python.

Jim: And it’ll work on Windows, Mac OS X, and Linux?

Joe: Yes, I’m pretty sure it will. Granted, pygame’s a set of gaming

libraries for Python, but all we need to use is the bit that handles

playing sounds.

Jim: Sounds great. I can’t wait to see it in action.

Frank: Didn’t you mean “hear it in action”?

Jim & Joe: <groan>.

See... sound‛s

the problem.

You actually can‛t

“see” sound... but, don‛t

worry, it‛ll be grand.

Jim

Frank

Joe

220 Chapter 7

get in the pygame

pygame is cross platform

Before continuing with the rest of this chapter, you need to take

five to download and then install the pygame technology onto

your computer. The pygame technology is an example of what’s

known as a third-party library: that’s extra functionality that can be

added into your Python environment, but which isn’t part of the

standard library.

As installing pygame tends to be a very platform-specific

thing, we’ve uploaded a set of instructions onto the Head First

Programming website for you to follow.

STOP! Don’t proceed with the rest of this chapter

until you’ve installed pygame for Python 3 on your

computer.

you are here 4 221

building a graphical user interface

Ready Bake

Code Take a look at this handy pygame program, which shows how to play

four sounds one after another:

import pygame.mixer

sounds = pygame.mixer

sounds.init()

def wait_finish(channel):

while channel.get_busy():

pass

s = sounds.Sound("heartbeat.wav")

wait_finish(s.play())

s2 = sounds.Sound("buzz.wav")

wait_finish(s2.play())

s3 = sounds.Sound("ohno.wav")

wait_finish(s3.play())

s4 = sounds.Sound("carhorn.wav")

wait_finish(s4.play())

Just like with other

libraries, you import the

library you want to use.

Identify and play

each of the sounds.

The value returned from

the “play()" method gets

passed to “wait_finish()".

The “wait_finish()"

function loops until the

channel's “get_busy()"

method returns False.

“pass" is a Python

construct that

does nothing.

Create a “pygame.mixer” object

and initialize the sound system.

The “get_busy()”

method checks to

see if the sound

is still playing.

Load in the

sound file you

want to play.

To run this program on your computer, you obviously need pygame installed and you

need to download the sound files for this chapter from the Head First Programming

website. Be sure to put the sound files in the same directory/folder as your program.

Load this!

222 Chapter 7

test drive

Test Drive

You’ve successfully downloaded/installed pygame and grabbed a copy of

the Head First Programming sound files for this chapter. Now, test the pygame

program in IDLE to see if things are working correctly:

I hear my cheesy

sound effects...

you gotta love that

car horn!

Oh no!

Beeeep!

Buzz!

Obviously, we can't

show you sound in

this book but trust

us, this program

works as billed. In

fact, DON’T trust

us: just be sure to

run the program

yourself.

Sounds like pygame is up and running!

Thump!

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building a graphical user interface

Q: So pygame is a library created

by some programmer other than the

Python folks?

A: Yes, it’s what’s called a third-party

library. It wasn’t created by you or the

people that bring you Python. Some other

programmer(s), a third party, created

pygame.

Q: And it’s just given away for free?

A: Yes. Which is very nice of them, isn’t

it?

Q: Are there other libraries like

pygame that can be added into my

Python environment?

A: Yes, there are lots. To see the current

list, follow the Package Index link from

the main Python website. Python refers to

third-party libraries as “packages” and, as

you’ll see, there are packages available for

every conceivable purpose, not just playing

sounds or developing games (as is the

case with pygame).

Q: The sounds I downloaded are

WAV files. I know that WAV is one of the

standards for encoding sound, but is it

the best format to use?

A: That depends on who you ask! We

are using WAV files because they are used

in lots of places and are well-supported on

most operating systems. There are lots of

file formats for sound and many of them

claim to be “better” than WAV, but for what

we are doing here, WAV is perfect.

Q: What’s the deal with the wait_

finish() function in the Ready Bake

Code? I just don’t get why it’s there.

A: It’s a function that waits for the sound

to finish playing before continuing with the

rest of the program.

Q: What?! Surely the sound just

plays?

A: Playing with pygame in this way,

although fun, masks a problem that can

surface when working with sound (in any

programming language). It turns out that,

when asked to play a sound, the main chip

inside your computer (the CPU) doesn’t

even bother trying. Instead, there’s another,

smaller chip in your computer that is

specifically designed to play sounds and

it is to this chip that your main chip hands

the sound file to and says: “play this for

me.” The main chip then goes back to

running your code, sees another request

to play a sound, doesn’t bother, hands

the new sound file off to the sound chip,

and repeats until your program ends. The

sound chip—and this is the important

part—is designed to operate in parallel

with your main chip. While your main chip

is doing something else, the sound chip is

busy playing any sounds it has been asked

to play. And—here’s the rub—if the sound

chip has been asked to play more than one

sound, it attempts to play each sound at

the same time.

Q: So the wait_finish() function is

like an artificial delay after each sound?

A: No, not really a delay, more like a

pause designed to let the sound effect play

fully before trying to play anything else.

The wait_finish() function forces

your sound chip to finish with one sound

before starting another. What happens is

that when a sound is played, the play()

method passes back the channel (or track)

number that the sound is playing on. You

can then use the channel number to ask

pygame to wait for the channel to finish

playing a sound before continuing, which

is what the code in the Test Drive on the

previous page does.

Q: And if I don’t use wait_finish(),

what happens then?

A: All the sounds attempt to play at the

same time and it sounds like a jumble of

sounds as opposed to one sound playing,

then another, then another, and so on.

224 Chapter 7

beyond pseudo-code

Now that you know how to generate a sound using pygame, it's time to write the code for

TVN’s program. Base your program on the pseudo-code you created earlier.

you are here 4 225

building a graphical user interface

226 Chapter 7

on with the show

Now that you know how to generate a sound using pygame, it's time to write the code for

TVN’s program. You were to base your program on the pseudo-code you created earlier.

import pygame.mixer

def wait_finish(channel):

while channel.get_busy():

pass

sounds = pygame.mixer

sounds.init()

correct_s = sounds.Sound(“correct.wav")

wrong_s = sounds.Sound(“wrong.wav")

prompt = “Press 1 for Correct, 2 for Wrong, or 0 to Quit: "

number_asked = 0

number_correct = 0

number_wrong = 0

You need to import

pygame’s “mixer” module

in order to play sounds.

Reuse the

“wait_finish()”

function from

earlier.

Create a mixer object

and initialize the

pygame sound system.

Load each of the

required sounds into its

own variable.

Make sure the counts that

you’ll maintain are set to a

reasonable starting value.

It would be OK to move these

three lines of code to the top

of the program, just so long as

they have starting values before

the while loop starts.

This is what you'll

ask the question

master each time.

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building a graphical user interface

`choice = input(prompt)

while choice != ‘0':

if choice == ‘1':

number_asked = number_asked + 1

number_correct = number_correct + 1

wait_finish(correct_s.play())

if choice == ‘2':

number_asked = number_asked + 1

number_wrong = number_wrong + 1

wait_finish(wrong_s.play())

choice = input(prompt)

print(“You asked " + str(number_asked) + “ questions.")

print(str(number_correct) + “ were correctly answered.")

print(str(number_wrong) + “ were answered incorrectly.")

If the answer is

correct, increment the

counters and then play

the appropriate sound.

Prompt the host.

If the answer is

incorrect, increment

the counters and play

the sound effect.

At the end of the program,

display a summary of the

counter values.

While the game

hasn't ended....

228 Chapter 7

test drive

Test Drive

Type your code into IDLE and save it under the name gameshow.py.

With the Head First Programming sounds stored in the same directory as your

program, press F5 to give it a spin.

We can’t show

these sounds

either, so go

ahead and run

the program to

hear it working

for yourself.

That looks and sounds

great... and I can‛t

wait to try it out in

the semifinal!

you are here 4 229

building a graphical user interface

Head First: Hello, pygame. Nice of you to join us.

pygame: Hello. Thank you so much for giving me

this opportunity to discuss my situation with you.

Head First: What situation is that?

pygame: Well, you know, just because I’m used for

gaming, everyone expects me to be fun all the time.

Life and soul of the party... never stopping to smell

the roses... always on the go. <sigh> It’s all too much

fun, really.

Head First: Too much fun? Really?!?

pygame: Well... yes. Not a lot of people know this,

but my life as pygame is hard. Not only do I have

to help out regular folks with their programming

problems, but there’s all those gamers, too. Some of

those guys never sleep... it’s just play, play, play, play,

play, play... I’m simply exhausted.

Head First: Oh, sorry to hear that. But, don’t you

feel good that all those programmers out there in the

Python community are using you?

pygame: I guess so.

Head First: You’ve made a lot of programmers’

lives easier. There’s lots of great code written that

would not have been written if it weren’t for you.

pygame: Yeah, right. I do all the heavy lifting while

everyone else is off doing other things.

Head First: Ah, come on, your life’s not that bad,

is it?

pygame: <sighs>

Head First: Surely you know what people are

saying about you?

pygame: Now they’re talking about me, too? How

awful... <sobs>

Head First: Yeah, there’s lots of talk, but it’s all

good. The Python programming community loves you,

pygame.

pygame: They do? <sobs even more>

Head First: Yes. You are well-tested, well-written,

and your documentation is first rate. Your support

for all the major releases of Python is right on the

money, too, and you work on Mac OS X, Windows,

and Linux.

pygame: All I’ve ever tried to do is keep everyone

happy.

Head First: And you do. We’ve heard so many

great things about you that we are recommending

you to all our friends.

pygame: Do they play games? I’m good at that, you

know.

Head First: Yes, some of them do. But others just

talk about the great things their applications can now

do thanks to you, pygame.

pygame: I guess things aren’t quite so bad after all?

Head First: Not at all. We’re big fans!

pygame: Why, thanks. That’s awesome. Do you

have time for a quick game? There’s this new

Dungeons and Dragons that I’ve been dying to try...

pygame Exposed

This week’s interview:

Is it fun being pygame?

230 Chapter 7

dated interface

0... 2... 1... 9... blast off!

1, 2, and 0...?!? Pressing

keys? Really? That‛s so

1985...

Your program’s looking pretty dated.

The program works, but it’s not going to win a Visual

Design Award any time soon. And its use of key presses

makes it a little hard to use, too.

So, its looks could be improved and its usability could be

better.

It sounds like you need a graphical user interface, or GUI.

Most people pronounce GUI as “gooey."

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building a graphical user interface

You need to design the look of your GUI for TVN. Draw what you

think your GUI should look like in the space provided below.

Hint: Take some time to think about other GUI programs that you

have used. Think about a common interface element that you

could use (and draw) here.

232 Chapter 7

interface facelift

You needed to design the look of the GUI for TVN. You were asked

to draw what you think your GUI should look like in the space

provided below.

Correct

Answer Wrong

Answer

You need two buttons for

each of your program's

events.

Press this button

when the answer

is correct.

Press this button

when the answer

is wrong.

The host will quit the program

by closing the window

you are here 4 233

building a graphical user interface

Ah... GUIs. All those lovely event

handlers, mouse clicks, widgets, frames,

scroll bars, double clicks and—my

personal favorite—the mouseover.

Frank: Since when have you been an expert on GUIs?

Jim: Isn’t every Windows user?

Joe: Well, of course, everyone knows how to use a GUI, but we are

talking about creating a GUI in code.

Jim: Ah... oh... um... eh... now, where shall we start?

Frank: It turns out that writing code for a GUI application is...

well... just like writing any other code. If you know how to program,

you know how to create a GUI. It’s just a matter of selecting the

correct GUI library, learning how to use it, then writing the code.

Joe: So we’ll head off to the Python Package Index and grab us some

GUI libraries, eh?

Frank: Not so fast. Python comes with a GUI library as standard,

called tkinter.

Jim: tk-what?

Frank: tkinter. The “tk” bit refers to the fact that Python’s standard

GUI library is built on top of the very popular Tk technology. The

“inter” bit is short for “interface.”

Jim: So we’re going to build a GUI interface in Python running on

Tk using tkinter?

Frank: Yes, we are. That’s not too confusing, is it?

Joe & Jim: Well... not if you say so.

Frank: The big thing with creating GUIs is understanding the event

loop.

Joe: Ah, that’s just looping code that reacts when certain things

happen, isn’t it? It’s just like the while loop in the non-GUI version

of TVN’s program. In that code, that loop is an event loop, isn’t it?

Frank: It sure is. Although the GUI event loop tends to be extra

capable and can do lots more than the simple while loop.

Joe: That sounds complex. Is it?

Frank: No, not really. It just takes a little getting used to.

Jim: But, it’s all just code, isn’t it?

Frank: Yes, Python code using the tkinter library.

Joe: OK. Let’s get to it, since we already know how to program...

Jim

Frank

Joe

234 Chapter 7

tkinter event loop

tkinter gives you the event loop for free

In order to process events efficiently, GUIs employ an event loop. Event

loops watch and wait for events, calling a piece of code each time an

event occurs. If you think about the current TVN Game Show program,

it already has a very basic event loop that waits for the host to press 1, 2,

or 0. The program then calls some code before waiting again for another

key-press event from the host. To implement this in code, you used a while

loop:

while choice != '0':

if choice == '1':

number_asked = number_asked + 1

number_correct = number_correct + 1

In tkinker, you don’t need to write a while loop like you did for your

non-GUI program. In tkinter, call the mainloop() method instead:

from tkinter import *

app = Tk()

app.title("Your tkinter application")

app.geometry('450x100+200+100')

app.mainloop()

Provide window

coordinates and

size values.

Click on the close

box to terminate

this application.

To add a button to your application, use code like this, being sure to put

these two lines of code before the call to mainloop():

b1 = Button(app, text = "Click me!", width = 10)

b1.pack()

The pack() method links

the newly created button

to the existing window.

Add a button to

the window and give

it some text and a

width value.

These five lines of

Python/tkinter code

produce this GUI.

Import everything from

the tkinter module.

Create a tkinter application

window called “app”.

Start the tkinter event loop.

Give the

window a

name.

The button’s been

added to the GUI.

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building a graphical user interface

tkinter is packed with options

The pack() method lets you position the button in the application

window. If you provide a value for the side parameter to pack(),

you can control where in the window the button appears. Here are the

legal values for side:

pack(side = ‛left’)

Position the button on the left side of the window.

pack(side = ‛right’)

Position the button on the right side of the window.

pack(side = ‛top’)

Position the button at the top of the window.

pack(side = ‛bottom’)

Position the button at the bottom of the window.

It is also possible to add some padding around buttons (to make them

look nicer in your window):

pack(padx = 10, pady = 10)

Position the button with 10 pixels padding on all four sides.

Based on what you now know about tkinter windows and

buttons, write the code to display the GUI that you need for the

TVN program:

The value of “side”

controls where the

button is packed.

236 Chapter 7

gooey display

Based on what you now know about tkinter windows and

buttons, you were to write the code to display the GUI that you

need for the TVN program:

from tkinter import *

app = Tk()

app.title(“TVN Game Show")

app.geometry(‘300x100+200+100')

b1 = Button(app, text = “Correct!", width = 10)

b1.pack(side = ‘left', padx = 10, pady = 10)

b2 = Button(app, text = “Wrong!", width = 10)

b2.pack(side = ‘right', padx = 10, pady = 10)

app.mainloop()

Create the window as

in the earlier example,

but change the window

title and geometry

values.

Create a button for

the “Correct" event.

Create another button

for the “Wrong" event.

Pack one button

on the left, the

other on the right,

and give them some

padding.

Start the event loop.

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building a graphical user interface

Test Drive

Let’s take your first GUI program for a test drive. With your tkinter code

entered into IDLE, save it as tvn.pyw and press F5 to see how it looks:

Looking good, eh?

There's your GUI window...

...and there are

your two buttons.

That’s one nice, professional-looking GUI! What do the

people at TVN think?

There’s a convention in the Python

world that suggests naming tkinter

programs with a “.pyw” extension,

as opposed to the usual “.py”. This

helps your operating system run your

tkinter programs properly, especially on

Windows.

Your code in IDLE

238 Chapter 7

beauty without brains

The GUI works, but doesn’t do anything

Nice interface, but it doesn‛t work.

When I click on a button, nothing

happens... I don‛t hear anything. What

happened to my cool sound effects?

The graphical user interface might

be ready, but the program is not

complete.

Q: So all tkinter gives me is the ability to draw the GUI?

A: Well, yes, but there’s quite a lot of functionality wrapped up in

that small number of lines of tkinter code.

Q: That pack() method looks a little weird... how does it

know where to put things?

A: The pack() method adopts a best-guess approach when

it comes to packing your GUI widgets within your GUI application

window. This usually works out, and when it doesn’t, pack()’s

parameters give you some control over the situation.

Q: That’s all that left, right, top, and bottom

stuff, isn’t it?

A: Yes, as well as the padx and pady parameters. They

help with widget positioning, too, by putting additonal space (or

padding) around your buttons.

Q: OK, I get that, but how come nothing happens when I

click on my buttons?

A: Ah, funny you should ask that...

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building a graphical user interface

Connect code to your button events

When you click on a button, the tkinter event loop captures the event and

looks for something to do with it. The trouble is, as your program stands, you

have not detailed what that something to do is. Your buttons have no code

associated with them, so the events occur but go unnoticed. To connect code to

buttons, put the code you want to run in its own function and then name the

function in the button code by providing a command parameter:

def button_click():

print("I've just been clicked!")

Create a function to contain

the code that runs when the

event occurs.

b = Button(app, text = "Click on me!", width = 15, command = button_click)

b.pack(padx = 10, pady = 10)

Identify the function to run when

the button is clicked.

...the event is

responded to...

...and the message

appears on screen.

The button is clicked...

Click!

240 Chapter 7

making connections

The code from the nongraphical version of the TVN program is on

this and the facing page. Take your pencil and mark the parts of

the code that you would extract and turn into functions so that

you can then connect the functions to the buttons in your GUI.

Mark the other parts of this program that also need to be added

to the GUI version.

Note: Don’t worry about prompting the host to ask a question in

the GUI. But do worry about maintaining a count of the number

of questions answered correctly and incorrectly. (The total count

is not important, either.)

How many functions do you think you need? Write their names here:

The nature of the

interface provided by the

GUI means that some of

the program’s requirements

have changed.

import pygame.mixer

def wait_finish(channel):

while channel.get_busy():

pass

sounds = pygame.mixer

sounds.init()

correct_s = sounds.Sound("correct.wav")

wrong_s = sounds.Sound("wrong.wav")

prompt = "Press 1 for Correct, 2 for Wrong, or 0 to Quit: "

number_asked = 0

number_correct = 0

number_wrong = 0

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building a graphical user interface

choice = input(prompt)

while choice != '0':

if choice == '1':

number_asked = number_asked + 1

number_correct = number_correct + 1

wait_finish(correct_s.play())

if choice == '2':

number_asked = number_asked + 1

number_wrong = number_wrong + 1

wait_finish(wrong_s.play())

choice = input(prompt)

print("You asked " + str(number_asked) + " questions.")

print(str(number_correct) + " were correctly answered.")

print(str(number_wrong) + " were answered incorrectly.")

With the code you need identified, take the time to update

your GUI application with the new functions and whatever

other code you’ve extracted from the non-GUI program.

Produce a new program that is a combination of your existing

GUI code and the extracted code from this program.

242 Chapter 7

page goal header

You were to take your pencil and mark the parts of the code on

the previous (and facing) page to identify the code you would

extract and turn into functions so that you can then connect the

functions to the buttons in your GUI. You were also to mark the

other parts of this program that also need to be added to the GUI

version:

You were to think about how many functions you might need. You

were to write their names here:

play_correct_sound()

play_wrong_sound()

...and another function

to play a sound when the

answer is wrong.

You need a function to

play a sound when the

answer is correct...

import pygame.mixer

def wait_finish(channel):

while channel.get_busy():

pass

sounds = pygame.mixer

sounds.init()

correct_s = sounds.Sound("correct.wav")

wrong_s = sounds.Sound("wrong.wav")

prompt = "Press 1 for Correct, 2 for Wrong, or 0 to Quit: "

number_asked = 0

number_correct = 0

number_wrong = 0

The GUI program still

needs to use pygame.

You still need to maintain

these counters.

you are here 4 243

building a graphical user interface

choice = input(prompt)

while choice != '0':

if choice == '1':

number_asked = number_asked + 1

number_correct = number_correct + 1

wait_finish(correct_s.play())

if choice == '2':

number asked = number_asked + 1

number_wrong = number_wrong + 1

wait_finish(wrong_s.play())

choice = input(prompt)

print("You asked " + str(number_asked) + " questions.")

print(str(number_correct) + " were correctly answered.")

print(str(number_wrong) + " were answered incorrectly.")

Turn this code into the

“play_correct_sound()”

function.

Turn this code into the

“play_wrong_sound()”

function.

Displaying the

summary remains a

requirement, too.

With the code you need identified, you were to take the time

to update your GUI application with the new functions and

whatever other code you’ve extracted from the non-GUI

program.

You were asked to produce a new program that is a

combination of your existing GUI code and the extracted

code from this program.

Turn the page for the

updated code solution...

244 Chapter 7

button press rehearsal

The GUI program’s now ready for

a screentest

Here’s what your GUI program should look like now:

from tkinter import *

import pygame.mixer

sounds = pygame.mixer

sounds.init()

correct_s = sounds.Sound("correct.wav")

wrong_s = sounds.Sound("wrong.wav")

number_correct = 0

number_wrong = 0

def play_correct_sound():

global number_correct

number_correct = number_correct + 1

correct_s.play()

def play_wrong_sound():

global number_wrong

number_wrong = number_wrong + 1

wrong_s.play()

app = Tk()

app.title("TVN Game Show")

app.geometry('300x100+200+100')

b1 = Button(app, text = "Correct!", width = 10, command = play_correct_sound)

b1.pack(side = 'left', padx = 10, pady = 10)

b2 = Button(app, text = "Wrong!", width = 10, command = play_wrong_sound)

b2.pack(side = 'right', padx = 10, pady = 10)

app.mainloop()

print(str(number_correct) + " were correctly answered.")

print(str(number_wrong) + " were answered incorrectly.")

The buttons are now

connected to event-

handling functions.

Python’s “global”

keyword lets

you adjust the

value associated

with a variable

created outside

the function.

you are here 4 245

building a graphical user interface

Test Drive

With the code you need extracted from the nongraphical application and

added to your GUI program, press F5 in IDLE to see (and hear) if things are

working any better now:

It not only looks

good, but now it

sounds good, too!

Every time you click on a button, the appropriate sound

effect is heard. Great work!

Q: So “event handlers” in tkinter are just functions?

A: Yes, as we said earlier in this chapter: it’s all just code. And

by putting the code you want to run in a function, it’s easy to

reference it using the command parameter associated with each

button. Your user clicks the button to run the code in your function.

Q: This actually isn’t too hard. I always thought building a

GUI was only for advanced programmers?

A: Well... that certainly used to be the case, but things have

changed (for the better). Technologies like tkinter allow every

programmer to build great-looking GUIs without too much fuss. It’s

a case of tkinter concentrating on the GUI, while you concentrate

on your code.

Q: And is it the case that, if I want to add other things to my

GUI program, it’s done in a similar way?

A: Yes, all you have to do is write the code.

Q: And I connect my code up to my other things using

something like the command parameter that works with

buttons?

A: Yes, that’s all there is to it. The mechanism for the other

interface elements (or widgets, for short) might be a little different,

but the concept is the same. Once you can work with one, the rest

are a lot easier to get your head around.

Ting! Splat!

246 Chapter 7

missing results

But TVN is still not happy

The sounds work great,

the GUI looks fantastic...

but where are my results?

I can‛t find them!

The results appeared in the Python Shell,

not in the GUI, so the host missed seeing

them. When you point this out to him, he’s

less than impressed and makes it clear that

he expects the results to appear in the GUI.

You need some way to display

messages in the GUI.

The results are

right there in the

Python Shell. But

this is NOT what

the host wants.

you are here 4 247

A widget that allows you to select one item

from a large list

Drop-down list

The interface elements that you add to a GUI are known as widgets.

You’ve met one already: the button. There are lots of others. Look at

the names of some other widgets below and see if you can match them

with the correct description. We’ve already done the first one for you.

A widget that displays a string message in

a window

Label

A widget that provides a way for large and

small amounts of text to be entered

Text box

A list of command options that is attached

to the top of a window

Menu The combination of a drop-down list and

a text box

Combo box

A separate window that pops up to request

additional information from the user

Dialog box

Which widget do you think you need to use in your program?

Write your

answer here.

building a graphical user interface

248 Chapter 7

label it

SOlUTion

A widget that allows you to select one item

from a large list

Drop down list

The interface elements that you add to a GUI are known as widgets.

You’ve met one already: the button. There are lots of others. You were

to look at the names of some other widgets below and see if you could

match them with the correct description.

A widget that displays a string message in

a window

Label

A widget that provides a way for large and

small amounts of text to be entered

Text box

A list of command options that is attached

to the top of a window

Menu The combination of a drop-down list and

a text box

Combo box

A separate window that pops up to request

additional information from the user

Dialog box

You were asked to identify which widget you would use in your program.

Use the “Label" widget.

You need to add a label to your GUI in

order to display the results.

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building a graphical user interface

Label it

When it comes to adding a label to your GUI, use the tkinter Label widget.

You create it in code not unlike the way you create a button. Here’s the code

to add a label to an existing GUI application. The label simply displays a

string:

l = Label(app, text='When you are ready, click on the buttons!', height = 3)

l.pack()

Create a new label, attach

it to the main window, give

it some text, and adjust the

label's height.

Don't forget to pack() the widget.

Another variation replaces the text parameter with textvariable. If

you assign a special tkinter variable to this parameter, the label will change

whenever the value of the variable changes, automatically:

num_good = IntVar()

num_good.set(0)

l1 = Label(app, textvariable = num_good)

l1.pack(side = 'left')

...

num_good.set(100)

Create an “IntVar”.

Put the label code before

the code for the buttons

and the GUI will then look

like this.

Associate

the “IntVar”

with the

label.

Use the “set()” method to adjust the

value of the “IntVar”, and the GUI

updates, as if by magic.

250 Chapter 7

add labels

Based on what you now know about adding a label to a GUI,

rework your GUI code so that it uses two labels. One should

display the number of correct answers and the other should

display the number of wrong answers. We’ve left plenty of room

for you to write in all the code that your program now needs.

you are here 4 251

building a graphical user interface

252 Chapter 7

label results

Based on what you now know about adding a label to a GUI, you

were asked to rework your GUI code so that it uses two labels.

One should display the number of correct answers and the other

should display the number of wrong answers. We left plenty

of room for you to write in all the code that your program now

needs.

from tkinter import *

import pygame.mixer

def play_correct_sound():

num_good.set(num_good.get() + 1)

correct_s.play()

def play_wrong_sound():

num_bad.set(num_bad.get() + 1)

wrong_s.play()

app = Tk()

app.title(“TVN Game Show")

app.geometry(‘300x110+200+100')

sounds = pygame.mixer

sounds.init()

Start by importing the

library code you need.

Create the two

event handlers

that set the

IntVar and play

the appropriate

sound.

Create the GUI

application window.

Initialize the

sound system.

you are here 4 253

building a graphical user interface

correct_s = sounds.Sound(“correct.wav”)

wrong_s = sounds.Sound(“wrong.wav”)

num_good = IntVar()

num_good.set(0)

num_bad = IntVar()

num_bad.set(0)

lab = Label(app, text=‘When you are ready, click on the buttons!', height = 3)

lab.pack()

lab1 = Label(app, textvariable = num_good)

lab1.pack(side = ‘left')

lab2 = Label(app, textvariable = num_bad)

lab2.pack(side = ‘right')

b1 = Button(app, text = “Correct!", width = 10, command = play_correct_sound)

b1.pack(side = ‘left', padx = 10, pady = 10)

b2 = Button(app, text = “Wrong!", width = 10, command = play_wrong_sound)

b2.pack(side = ‘right', padx = 10, pady = 10)

app.mainloop()

Load in the

required sound

effects.

Create two

IntVars: one to

count the number

of correct answers

and another to

count the number

of wrong answers.

Display a friendly

message that tells the

host what to do.

Be sure to PACK your

widgets.

Create two

labels to hold

each counter

and connect the

labels to the

relevant IntVars.

Create each of

the buttons and

connect them to

their relevant

event handler.

Start

tkinter’s main

event loop.

254 Chapter 7

test drive

Test Drive

With your newly amended code entered into IDLE, press F5 to see and

hear the GUI in all its glory:

Sweet... the fully working GUI.

The labels are displaying

running totals with each

mouse click (event).

That‛s just perfect! The new

version of the program works with

my touchscreen and is so easy to use!

I love it! Oh, and before I forget,

here are your two front-row tickets

for the Grand Final. See you then!

There it is: the host's

winning TV smile!

Admit One

GRAND FINAL

Restricted Access

Admit One

GRAND FINAL

Restricted Access

One final decision: who are you

going to take with you?

you are here 4 255

building a graphical user interface

CHAPTER 7

Your Programming Toolbox

You’ve got Chapter 7 under your

belt. Let’s look back at what

you’ve learned in this chapter:

Programming Tools

* Using a third-party programming

library

* Playing with a sound library

* Event loops - respond to events

when they occur

* Event handler - code that executes

when a certain event occurs

* GUI - a graphical user interface

(looks nice, doesn’t it?)

* Widget - a GUI interface element

Python Tools

* pygame - a professional-level set of

gaming libraries that support sound

* pass - a piece of code that does nothing

* break - exit from a loop

* tkinter - a standard library for

creating GUIs

* Tk() - a blank GUI app

* Button() - a tkinter button widget

* Label() - a tkinter label widget

* IntVar() - a tkinter integer variable

that can update the GUI “as if by magic”

this is a new chapter 257

So, you see, if we replace all our

command-line data entry systems with

GUIs, we can increase efficiency enough

for me to afford these totally blinged-out

alloys for my Hummer.

guis and data

Data entry widgets

GUIs don’t just process events. They also handle data.

Almost all GUI applications need to read user data, and choosing the right widgets can

change your interface from data entry hell to user heaven. Widgets can accept plain

text, or just present a menu of options. There are lots of different widgets out there, which

means there are lots of choices, too. And, of course, making the right choice can make all

the difference. It’s time to take your GUI program to the next level.

258 Chapter

special delivery

Head-Ex needs a new delivery system

Head-Ex Deliveries is a small delivery company that’s looking to expand.

They know that delivery companies rely on their computer systems, so they

want to have a whole new system to book deliveries around the country.

The system needs to be simple to use, so they want to use a graphical user

interface (GUI). They want to give you the job of creating the system and

even have a sweetener to make it worth your while.

I‛ll put you on a bonus.

The more we deliver,

the more you get paid!

HEAD-EX

Head-Ex employee

you are here 4 259

guis and data

They’ve already designed the interface

Head-Ex has been thinking about expanding their business for a

while and they already have a design for the interface to the new

delivery system. This is what it needs to look like:

deliveries.txt

So how will it work? It’s pretty simple, actually. The user enters the

details for a new delivery, including the description of the contents,

the address it’s heading to, and the name of the Head-Ex depot it

will be dispatched from. When the user clicks the Save button, the

details are saved into a file called deliveries.txt.

What’s the difference between this GUI and the ones you created before?

Depot:

Description:

Address:

Save

There are a bunch

of data-entry

fields.

There is a

single button.

This is where the

delivery details

need to be saved.

260 Chapter 8

text entry

Depot:

Description:

Address:

Save

Read data from the GUI

Think about the GUIs you’ve created so far. Those apps ran code in response

to events generated by users clicking buttons. So what’s different here? Well, in

addition to generating events, your users will also be creating data.

You need to do two things:

You need to create an interface with places to enter text.

The Head-Ex design shows exactly what the interface will need to look

like:

You need some way of accessing that information.

The data will need to be stored away in a file, so you need some way of

asking the interface what text has been entered. That way, when

someone clicks on the Save button, you’ll be able to write the data to

the deliveries.txt file, like this:

Depot:

Seattle, WA

Description:

Books

Address:

1 Main Street

Anytown

You will need to APPEND this

information to the file whenever

someone presses “Save."

The format of the file

doesn't matter, as long as

the depot staff can read it.

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guis and data

The Entry and Text widgets let you enter

text data into your GUI

If you look at the interface design, you’ll see that there are two different types

of text field you will need to handle: short text and longer, multi-line text

fields. To deal with these two needs, Python’s tkinter library has two different

widgets for each data-entry type:

A TEXT FIELD is just a

box on the screen that you

can use to enter text.

Entry widget: for entering single lines of text

The Entry widget is what you’ll use for most of the text fields. You can

create an Entry widget like this:

my_small_field = Entry(app)

Remember that you

always need to import the

tkinter library first.

Text widget: for longer, multi-line text

Because not all text data fits on a single line, tkinter has the Text widget.

Think of email messages, Wikipedia pages, and word processing documents.

For large multi-line data, you need something other than Entry. You need

Text:

my_large_field = Text(app)

You can enter large pieces of

textual data in here.

The Entry and Text fields should be enough to create the Head-Ex

interface. But it’s not enough to simply create the interface. You also need to

control the data inside it.

One single line of text

Multiple lines of text

We just checked...

we're pretty sure

Python is on first.

262 Chapter 8

control text fields

Read and write data to text fields

When someone enters text into an Entry widget, Python stores that text

somewhere in memory as a string. If you want to read the string, simply call

the widget’s get() method:

my_entry_field.get()

This will return the string “ice cream”.

But what if you want to change the contents of the widget, can you do that

too? Yes, you can: add text to the widget using the insert() method. It’s a

little more involved than just reading the text, because you need to say where

you want the text inserted:

my_entry_field.insert(0, "banana ")

This is the INDEX of

the insertion point.

You need to specify the index of the insertion point. Indexes in Entry fields

work just like indexes in strings: they begin at zero. So if you insert text at index

0, the new text will appear at the front of all the text already in the field. In the

same way, the delete() method lets you remove text from the field. You

might want to delete the entire contents of the field, so tkinter gives you a

handy END symbol that lets you delete all the text like this:

All the text has gone.

my_entry_field.delete(0, END)

This will delete the entire contents.

0 is the index of the first

character in the field.

END is a special value

that represents the last

character in the field.

The get(), insert(), and delete() methods give you complete

control over the contents of your Entry field.

But what about Text fields?

Poof!

Entry fields are indexed from 0.

01 2 3456

The final character

in the field is

indexed by END.

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guis and data

Large Text fields are harder to handle

So Text fields should work the same way, right? Well, actually, they don’t.

The designers of tkinter figured people might want to access particular lines

and columns in Text fields. This means that while Entry fields use a single

number to index the contents of the field, Text fields use a string, in the form

row.column:

my_large_field.get("1.0", END)

Unlike Entry() fields, you

can't just use get() to

get the entire contents.

This will return the entire

contents of the field.

Be careful how

you number

rows and

columns in

Text() fields.

Rows begin at 1, but columns

begin at 0.

Once you understand how Text indexes work, you’ll see that you can insert

and delete text from them in a very similar way to Entry fields:

my_large_field.delete("1.0", END)

This will clear the field.

my_large_field.insert("1.0", "Some text")

This will insert the text at

the start of the field.

Now that you know how to create text fields and

control the text they contain, you’re ready to

build the Head-Ex application.

This means from the ROW=1

and COLUMN=0, which is the

first character in the field.

This will return

ALL of the text

in the Text field.

Column 0.

Row 1.

This first character has index “1.0"

264 Chapter 8

create the interface

from tkinter import *

app = Tk()

app.title('Head-Ex Deliveries')

Label(app, text = "Depot:").pack()

depot = Entry(app)

depot.pack()

Button(app, text = "Save", command = save_data).pack()

Code Magnets

Complete the code to create the interface. Think carefully about the

widgets you will need for each of the fields.

Label(app, text = "Address:")

address

description =

.pack()

Entry(app)

.pack()

address =

.pack()

Text(app)

.pack()

app.mainloop()

Label(app, text = "Description:")

description

Create the GUI.

You don't need

to keep track of

the labels, so no

need to assign

them to variables.

Recall that “pack()” adds

the widgets to the window.

Calling “pack()” without options

means you leave it to tkinter

to decide how best to lay

things out on the GUI.

You'll want to keep track

of the data entry fields, so

assign them to variables.

This means the button

will call the save_data()

function when it's clicked.

you are here 4 265

guis and data

In addition to the GUI code, you need to write the function that

will save the data from the GUI to the deliveries.txt file.

The Save button on the interface is going to call a function called save_data(). The function will

append the data from the GUI to the end of the deliveries.txt file formatted like this:

Then it will need to clear the fields on the form to make them ready for the next record to be entered.

The function will have to appear in the program before the GUI code. Write the code for the function

here:

Depot:

Seattle, WA

Description:

Books

Address:

1 Main Street

Anytown

Don't forget: it FIRST

needs to save the data.

THEN it needs to clear

the fields.

266 Chapter 8

interface created

from tkinter import *

app = Tk()

app.title('Head-Ex Deliveries')

Label(app, text = "Depot:").pack()

depot = Entry(app)

depot.pack()

Button(app, text = "Save", command = save_data).pack()

Label(app, text = "Address:")

address

description =

.pack()

Entry(app)

.pack()

address =

.pack()

Text(app)

.pack()

app.mainloop()

Label(app, text = "Description:")

description

Code Magnets Solution

You were to complete the code to create the interface. You were to

think carefully about the widgets you will need for each of the fields.

Address

is a larger

Text field.

The “save_data()” code needs to

be inserted before the GUI code

so that the button can see it.

Create two

fields: one is

an Entry field

and the other

is a Text field. Don't forget to pack

those widgets.

you are here 4 267

guis and data

The Save button on the interface is going to call a function called

save_data(). The function will append the data from the GUI

to the end of the deliveries.txt file formatted like this:

Then it will need to clear the fields on the form ready for the next record to be entered. The function will

have to appear in the program before the GUI code. You were to write the code for the function here:

Depot:

Seattle, WA

Description:

Books

Address:

1 Main Street

Anytown

def save_data():

fileD = open(“deliveries.txt", “a")

fileD.write(“Depot:\n")

fileD.write(“%s\n" % depot.get())

fileD.write(“Description:\n")

fileD.write(“%s\n" % description.get())

fileD.write(“Address:\n")

fileD.write(“%s\n" % address.get(“1.0", END))

depot.delete(0, END)

description.delete(0, END)

address.delete(“1.0", END)

Your code may look a

little different. Append the text to the end of the file,

just as you did when we wrote the POS

programs for the Health Club.

get() returns the

contents of an Entry

field.

get(“1.0", END)

returns the

contents of a Text

field.

This means “1st row, 0th column." Remember

that rows start at 1 and columns from 0.

Don't forget to clear

the fields after saving

the data.

268 Chapter 8

test drive

Test Drive

The deliveries program is ready, so it is time for the demo. With the code

entered into IDLE, press F5 to fire it up and you should see this:

You can enter data into each of the fields, even the multi-

line address. When you click Save, the fields all get cleared.

When you open up the deliveries.txt file, you see that

the data you entered into the GUI has been saved.

Depot:

Seattle, WA

Description:

Books

Address:

1 Main Street

Anytown

Congratulations! You’ve written your

first GUI data-entry application.

Cool! That‛s exactly

what we need. I‛ll

tell every office to

start using it.

The data you

enter into the

text fields gets

written to the

file.

Your GUI is

looking good.

Click!

you are here 4 269

guis and data

Entry fields are used for single lines

of text.

Text fields are used to handle multi-

line text.

Read the contents of fields with the

get() method.

Add text using the insert() method.

Remove text with the delete()

method.

Entry fields are indexed with a

number starting at 0.

Text fields are indexed with a string,

starting at “1.0”.

Q: Is it possible to put more than one line of text into an

Entry box widget?

A: Yes, you probably could, but if you need to do this, you are

much better off using a Text box, because they are designed to

handle more than one line of text.

Q: I notice that we are calling the pack() method as part of

the label creation code, whereas before we assigned the label

to a variable then called pack() on the new variable. Which

packing technique should I use and does it really matter?

A: No, it does not really matter which technique you use to call

pack(). If it makes sense to pack your widgets as they are

created, include the call to pack() as part of the creation code.

If it doesn’t make sense, assign the widget to a variable and do

your packing whenever you need to. If you look at other examples

of tkinter code on the Web, you’ll see that other programmers use

both techniques.

Q: Why can’t we just assign a value to an Entry box using

the assignment operator (=)? Why do we have to use the

insert() method?

A: The Entry box is a widget object, not a Python variable,

so using the assignment operator does not make sense here.

When working with objects, you need to use the application

programming interface (API) provided by and included with the

object, which in this case is the insert() method.

Q: Why do the rows in a Text box start counting from one

as opposed to zero (like everything else in Python)?

A:Beats the hell out of us. No idea.

Q: So, just to be clear, Who is on first?

A: No. Who is on second. Python is on first.

270 Chapter 8

the other cambridge

One of the Head-Ex deliveries went astray

For the first few days, the system worked great. Deliveries were correctly

booked and goods were shipped. But then, something odd happened at the

British delivery depot.

I‛m terribly sorry, old chap,

but I think these boxes

were meant for the other

Cambridge. Cup of tea?

One of the deliveries went seriously astray. A consignment of college

football jerseys was sent to Cambridge in England, instead of the Cambridge

in Massachusetts. But the system is still working fine, isn’t it? So what could

possibly have gone wrong?

There’s no time for tea. Let’s get to the bottom of this.

But it was lovely

to be asked...

you are here 4 271

guis and data

Users can enter anything in the fields

The system is doing exactly what it was designed to do: it lets people enter

the details for depots, descriptions, and addresses. The trouble is, even though

there are only a few depots, the Entry text fields lets the user type pretty

much anything. There are no controls on what gets entered into the GUI.

This is AMBIGUOUS. It

is not totally clear which

Cambridge is meant.

It wasn’t obvious whether the delivery was intended for the Head-Ex depot

in Cambridge, MA, or the Head-Ex depot in Cambridge, England. You need

some way to prevent users from entering ambiguous data in the depot field.

Think about the GUIs you’ve used in the past. How could you

restrict the values that someone could enter into a GUI?

272 Chapter 8

radio buttons

Depot:

Description:

Address:

Save

Cambridge, MA

Cambridge, UK

Seattle, WA

Radio buttons force users to choose a

valid depot

Text fields aren’t the only data-entry game in town. If you look at any

GUI program, you will find a whole host of different widgets being used:

sliders, buttons, check boxes. Why are there so many different types of

widget? Are they just there to make the interface more interesting to use?

The reason there are so many widgets is to allow the programmer to

efficiently manage and control the kind of data that people can enter.

If you want to allow a small number of values for a field, you can

use the radio button widget. A radio button works just like the AM/FM

selection buttons on your radio: press AM and the FM button pops up.

Press FM and the reverse happens.

Radio buttons in a GUI program work in the same way: if you select

one of the buttons, the other buttons are automatically deselected. That

way, you can choose only one from a small group of options.

Sounds OK.

Can I see a

prototype?

Click AM and the FM button pops up.

Click FM and the AM button pops up.

So if you replace the depot Entry field in the Head-Ex

program with a set of radio buttons, you prevent users from

entering ambiguous depot names. That way, they will be able

to choose one, and only one, depot.

Radio buttons

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guis and data

Creating radio buttons in tkinter

You need to create three radio buttons in the interface, one for each of the

depots. This is how you might do that in tkinter:

You need to remove the depot Entry widget and replace it with three

Radiobutton widgets, one for each of the valid depots. The text given to

each widget will be the text that appears alongside the radio button in the

interface.

What about reading which radio button has been selected? For now, you just

need to create a prototype of the interface, so there’s no need to change any

of the code that saves records. That’s something we can deal with later.

Let’s demo the new interface to the guys at Head-Ex.

Label(app, text = "Depot:").pack()

depot Entry(app)

depot pack()

Radiobutton(app, text = "Cambridge, MA").pack()

Radiobutton(app, text = "Cambridge, UK").pack()

Radiobutton(app, text = "Seattle, WA").pack()

Label(app, text = "Description:").pack()

This is the text

that will appear

next to the radio

button.

You don't need the

Entry field anymore.

274 Chapter 8

test drive

Test Drive

It’s time for the demo of the new version of the interface. When the guys

from Head-Ex arrive, you fire up the new program in IDLE:

I don‛t understand... is

this delivery being sent

to all of the depots? Why

are they all selected?

There’s clearly something wrong with the prototype. There is only

supposed to be one radio button selected at any one time, and yet the

program shows all of the radio buttons selected together.

What happened? You need to look in a little more

detail at how radio buttons actually work.

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guis and data

The radio buttons should work together

When you modified the code, you added the three radio buttons like this:

Radiobutton(app, text = "Cambridge, MA").pack()

Radiobutton(app, text = "Cambridge, UK").pack()

Radiobutton(app, text = "Seattle, WA").pack()

That code added three new radio buttons to the interface, but it created

them as three independent widgets. This means that each of the radio buttons is

working separately, with no knowledge of the other two.

Think about the way you want radio buttons to work. Is there something

that they all need to share? What is it?

But the whole point of radio buttons is that they work together.

When you select one radio button, you expect all of the other radio

buttons to be deselected, just like the buttons on the radio.

GUI programs often need to synchronize different widgets

together. You do something to one widget, which results in

something else happening to another widget.

So how might you get your radio buttons to

cooperate?

Look, I don‛t care

what you guys do, I‛m

gonna stay selected.

Yeah,

me too.

Huh, and me.

When the AM button is

selected, the FM button

should be deselected.

276 Chapter 8

model update

The radio buttons can share a model

The text fields you originally created each stored a single data item. For every

widget on the screen, there was a single piece of data. But that’s not true

for your radio buttons. The three radio buttons will be used by the user to

record just one thing: the depot that a delivery is sent to. The three radio

buttons needs to share a single piece of data. And that piece of data is called the

model.

I‛ve just been clicked,

so I ll SET the model

to my value.

So if the Seattle radio button is selected, it will update the model with

a new value.

You haven’t set values on the radio buttons yet; you’ve only set text

descriptions. You could set the values to be whatever you want, but it is

simplest to set them to the same thing used as the description of the field:

RadioButton(app, text = "Cambridge, MA", value = "Cambridge, MA")

This VALUE is the one that

will be used in the model.

The text is the description

that appears on the screen.

You can make the text different

from the value, but let's leave

them the same here.

So what happens after the model is updated?

This is the model

hidden away in your

computer's memory.

I‛ll keep my eye on

the model until I‛m

clicked...

Click me, so I can

update the model...

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guis and data

The system tells the other widgets when

the model changes

The tkinter library code will keep a track of which widgets are using which

models and, whenever a model changes, tkinter will let the widgets know

about it. So if we select the Seattle, WA radio button, it will update

the model, and the other radio buttons that share the model will deselect

themselves.

Hey, I‛m set

to “Seattle,

WA” now.

Well, that‛s not my

value, so I'd better

deselect myself.

Not mine

either.

Deselect.

There’s a special name for the way this code works:

Model View Controller

The model is the data stored.

The view is just a fancy name for the widget.

And the controller is the code in tkinter that lets all of the views know

when the model has changed.

So much for the MVC theory. Time to fix the code.

Seattle, WA

278 Chapter 8

tkinter models

So how do you use models in tkinter?

Imagine you wanted to add delivery options to the program. You could use

radio buttons and do something like this:

Radiobutton(app, text = "First Class", value = "First Class").pack()

Radiobutton(app, text = "Next Business Day", value = "Next Business Day").pack()

It's important to EXPLICITLY give

each button a VALUE.

You then need to create a model for the radio buttons to share. In tkinter,

models are called control variables, and control variables that store text

are called StringVars:

service = StringVar()

service.set(None)

Radiobutton(app, text = "First Class", value = "First Class",

variable = service).pack()

Radiobutton(app, text = "Next Business Day", value = "Next Business Day",

variable = service).pack()

This code will now give us a pair of buttons that work together. If you

select one, the other will automatically become deselected:

Click the SECOND option and

the FIRST will be deselected.

And if you ever need to read or change the model value in the code, you just

need to call the StringVar's get() or set() methods:

>>> print(service.get())

"Next Business Day"

>>> service.set("First Class")

This returns the current

value of the model.

This sets the model object

back to “First Class", which will

automatically select the correct

radio button on the screen.

A StringVar is just like the IntVar from

Chapter 7, except that it holds a string value.

This sets the StringVar to the special value

“None" which means “No value."

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guis and data

Label(app, text = "Depot:").pack()

depot.set(None)

Radiobutton(app, , , ).pack()

Pool Puzzle

This is the section of the program that

creates the depot radio buttons. See

if you can complete it using the

fragments of code from the pool.

Be warned: you might not need

all of the pieces...

Note: each thing from

the pool can be used

only once!

depot=

depot

depot depot

StringVar()

text=

value=

variable=

variable= variable=

text

"Cambridge, MA"

"Cambridge, UK"

"Seattle, WA" "Seattle, WA"

"Cambridge"

What piece of code would you use to make sure all of the radio

buttons are cleared after the record is saved?

280 Chapter 8

depot buttons

Pool Puzzle Solution

This is the section of the program that

creates the depot radio buttons.

You were asked to see if you could

complete it using the fragments of

code from the pool. Not all of the

pieces were needed.

Label(app, text = "Depot:").pack()

depot = StringVar()

depot.set(None)

Radiobutton(app, variable = depot, text = "Cambridge, MA", value = "Cambridge, MA").pack()

Radiobutton(app, variable = depot, text = "Cambridge, UK", value = "Cambridge, UK").pack()

Radiobutton(app, variable = depot, text = "Seattle, WA", value = "Seattle, WA").pack()

Note: each thing from

the pool can be used

only once!

text "Cambridge"

What piece of code would you use to make sure all of the radio

buttons are cleared after the record is saved?

depot.set(None)

Because none of the radio

buttons have this value, none

of them will be selected.

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guis and data

Test Drive

Now it’s time for another demo of your program. When you first fire up the

program, you see that there are initially no depots selected.

That’s good. Now what happens if you select the first option, then change to

the third?

If you select “Seattle,

WA”, you automatically

deselect “Cambridge,

MA”.

If you make a selection, the other radio buttons automatically deselect

themselves, which is exactly what you need.

The radio buttons are working correctly.

Phew. Now the users will

always send the packages

to real depots. You did a

great job. Thanks!

282 Chapter 8

many more depots

Head-Ex's business is expanding

With the new systems in place, business is better than ever at Head-Ex. They

have new trucks, more employees, and an increasing number of offices.

We‛re opening

depots

everywhere!

But with this success comes a new problem...

Head-Ex is opening depots

all over the world.

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guis and data

There are too many depots on the GUI

The coders at Head-Ex have been amending your program to add new

depots as they were opened. But there’s a problem. Now there are so many

depots that they can’t all fit on the screen.

Something needs to be done to the GUI. But what?

What kind of widget would you use instead of a radio button to fix this?

284 Chapter 8

multiple options

An OptionMenu lets you have as many

options as needed

An OptionMenu or drop-down listbox is a widget that lets you restrict the

number of options a user can choose, just like a group of radio buttons. But

it has a couple of important differences.

First, it takes up a lot less space than a functionally equivalent group of radio

buttons, about the same amount of space as an Entry text field. Second—

and this is the really important characteristic—when you click it, an option

menu can display a large list of options.

If the Head-Ex coders use an option menu, they will be able to increase

the number of depots available, but they won’t have to increase the size or

complexity of their GUI.

So what needs to be changed in the code if you want to swap the radio

buttons for an option menu?

It’s not what changes, but what stays the same.

Depot:

Save

The user could

select a depot from

an option menu.

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guis and data

The model stays the same

Think for a moment about the model you are using with the radio buttons.

It represents the name of the chosen depot, which you want to keep for the

option menu. If the model stays the same, then your code, which currently

looks like this:

depot = StringVar()

depot.set(None)

Radiobutton(app, variable = depot, text = "Cambridge, MA", value = "Cambridge, MA").pack()

Radiobutton(app, variable = depot, text = "Cambridge, UK", value = "Cambridge, UK").pack()

Radiobutton(app, variable = depot, text = "Seattle, WA", value = "Seattle, WA").pack()

can be replaced with code like this:

depot = StringVar()

depot.set(None)

OptionMenu(app, depot, "Cambridge, MA", "Cambridge, UK", "Seattle, WA").pack()

This is the same

model as before.

The second parameter must

be the model and it doesn't

need the “variable =" bit.

A list of all the options that

appear in the widget

But wait... you don't have to list all the values like that

It looks like a lot of work to put all of the options in the actual function call to

OptionMenu(), doesn’t it? After all, there’s a large list of depots.

Thankfully, Python comes to the rescue. If you have the options already

stored in a list:

depots = ["Cambridge, MA", "Cambridge, UK", "Seattle, WA"]

you can pass the entire list instead of separate values like this:

OptionMenu(app, depot, *depots).pack()

This * means “Take the rest of

the parameters from this list

and insert them here."

Now let’s put the pieces together.

Cambridge, MA

286 Chapter 8

adding options

This is a version of the program that uses radio buttons.

You are going to update this program so that it uses an option menu. But the options need to be read

from a text file.

from tkinter import *

def save_data():

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set(None)

description.delete(0, END)

address.delete("1.0", END)

app = Tk()

app.title('Head-Ex Deliveries')

Label(app, text = "Depot:").pack()

depot = StringVar()

depot.set(None)

Radiobutton(app, variable = depot, text = "Cambridge, MA", value = "Cambridge, MA").

pack()

Radiobutton(app, variable = depot, text = "Cambridge, UK", value = "Cambridge, UK").

pack()

Radiobutton(app, variable = depot, text = "Seattle, WA", value = "Seattle, WA").

pack()

Label(app, text = "Description:").pack()

description = Entry(app)

description.pack()

Label(app, text = "Address:").pack()

address = Text(app)

address.pack()

Button(app, text = "Save", command = save_data).pack()

app.mainloop()

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guis and data

First, you need to create a function called read_depots() that will read the lines in a text file and return them to

your code as a list.

Hint: When you read a line from the file, it might have a newline character at the end. The rstrip() string method

will remove it for you.

def read_depots(file):

Then, you need to replace this section of the code with code that generates an option menu using the data from the

read_depots() function you just created. It should use a file called depots.txt. Write the code here:

This function will be

inserted here.

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options added

This is a version of the program that uses radio buttons.

from tkinter import *

def save_data():

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set(None)

description.delete(0, END)

address.delete("1.0", END)

app = Tk()

app.title('Head-Ex Deliveries')

Label(app, text = "Depot:").pack()

depot = StringVar()

depot.set(None)

Radiobutton(app, variable = depot, text = "Cambridge, MA", value = "Cambridge, MA").

pack()

Radiobutton(app, variable = depot, text = "Cambridge, UK", value = "Cambridge, UK").

pack()

Radiobutton(app, variable = depot, text = "Seattle, WA", value = "Seattle, WA").

pack()

Label(app, text = "Description:").pack()

description = Entry(app)

description.pack()

Label(app, text = "Address:").pack()

address = Text(app)

address.pack()

Button(app, text = "Save", command = save_data).pack()

app.mainloop()

You needed to update this program so that it used an option menu. But the options

were to be read from a text file.

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guis and data

depots = []

depots_f = open(file)

for line in depots_f:

depots.append(line.rstrip())

return depots

options = read_depots(“depots.txt")

OptionMenu(app, depot, *options).pack()

First, you needed to create a function called read_depots() to read the lines in a text file and return them as a list.

def read_depots(file):

Then, you needed to replace this section of the code with code that generated an option menu using the data returned

by the read_depots() function. It needed to use a file called depots.txt. You were to write the code here:

Start with an

empty array.

Open the file.

Read from the file one

line at a time.

Append a stripped copy of

the line to the array.

Return the list to

the calling code.

Call the function, passing in the

name of the file to read the data

from.

Use the data to build the

option menu.

290 Chapter 8

test drive

Test Drive

Before you run your program, be sure to create the depots.txt file. When

you start up the program in IDLE, the interface is a lot more compact and

even easier to use:

The system is working really well now. And, due to the depots being stored in

a file, Head-Ex can change the list of depots in the file without having to amend

the code in any way.

Your GUI program builds the list of depots dynamically

and on-demand, which makes it very flexible indeed.

A nice and neat list of

depot options

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guis and data

HEAD-EX HEAD-EX

Things are going great at Head-Ex

The GUI system is easy to use, and by restricting the depots with an option

menu, the quality of the entered data is spot on. This means the Head-Ex

delivery service is more reliable, the number of customers is increasing, and

the business is growing bigger—worldwide.

Your GUI data-entry

systems have helped our

business take off. Your

first bonus check is

queued for delivery!

Using tkinter’s data models, you used the power of

model-view-controller to build a GUI data-entry system

that really rocks.

Deliveries

Check out Head-Ex's

ever-expanding fleet.

292 Chapter 8

programming toolbox

Programming Tools

* MVC - Model, View, Controller.

* Think of widgets as views.

* Use data models to keep your data

separate from your views.

* Radio buttons work together if

they share a model.

* Object API - the application

programmer interface provided by an

object.

* Populate a GUI widget dynamically

and on-demand.

Python Tools

* Entry() - used by tkinter to enter

small amounts of text - one line.

* Text() - handle multi-line text in

tkinter.

* Entry fields are indexed from 0.

* Text fields are indexed with a “row.

column" string, starting with “1.0".

* The tkinter controller - keeps the

views informed about data changes.

* StringVar() - a tkinter stringed

variable that can update the GUI “as

if by magic.”

* RadioButton() - useful for when

you want to select one item from a

group in tkinter.

* OptionMenu() - useful for when

you want to select one item from a

LARGE group of items in tkinter.

CHAPTER 8

Your Programming Toolbox

You’ve got Chapter 8 under your belt.

Let’s look back at what you’ve learned

in this chapter:

this is a new chapter 293

So when you say, “Not even

if you were the last man on

Earth,” what do you mean?

exceptions and message boxes

8 ½

Get the message?

Sometimes things just go wrong. You just need to handle it.

There will always be things beyond your control. Networks will fail. Files will disappear.

Smart coders learn how to deal with those kinds of errors and make their programs

recover gracefully. The best software keeps the user informed about the bad things that

happen and what should be done to recover. By learning how to use exceptions and

message boxes, you can take your software to the next level of reliability and quality.

294 Chapter 8 ½

the cheese stands alone

What’s that smell?

Just when it looked like things were going so well, there was

a problem in the Head-Ex storeroom.

A consignment of cheese

never got recorded and

it‛s been going bad in the

storeroom. We‛ve had to

issue gas masks...

A trainee was recording the consignment of cheese when

there was a problem that prevented the program from

recording the delivery. That meant the cheese was never

assigned to a delivery truck. So the cheese never left the

storeroom and it just sat there for a very long time. And that

meant—well, you can just imagine...

To prevent the same thing happening again,

you need to find what caused the problem.

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exceptions and message boxes

Someone changed the file permissions

It turns out the whole thing was caused when someone from Technical Support

decided to change the permissions on the deliveries.txt file, making it

read-only. When the system tried to write deliveries into the file, it failed. But

what’s worse, it failed silently:

deliveries.txt

“deliveries.txt”

was made read-

only.

When you were writing programs that ran in the Python Shell,

you could always tell when it failed: a huge, ugly error message

appeared. Why not with GUIs? Don’t they do the same?

They do, but the trouble is that the message appears in the Shell

and your user is busy looking at the nice GUI, so the ugly error

can often be missed.

When using a GUI, how do you spot errors?

Once spotted, what happens then?

Note: To reproduce this error on your PC,

you need to make your deliveries.txt

file read-only. How you do this depends

upon your operating system. If you are

unsure how to make a file read-only, check

the Web for more advice (or ask a friendly

local guru). On most systems, it involves

editing the properties of the file.

The error in the

Python Shell was

not noticed by the

trainee.

Because no error

message appeared in

the GUI, the trainee

thinks everything is OK.

296 Chapter 8 ½

making an exception

When it couldn’t write to the file,

the program threw an exception

What happens when an error occurs? Some errors are really bad: they cause

the program to crash. Other, less serious errors are known as recoverable: the

program can keep running, even though something went wrong. You can spot

these situations in your code, because most programming technologies throw

an exception when they occur.

Imagine a line of code that has a problem, such as the line that was trying

to write to the deliveries.txt file. Python will spot that the append

operation failed and, instead of running the rest of the code that follows,

Python abandons ship and skips out of the code completely. That’s what

throwing an exception means: the program doesn’t crash, but it abandons what

you were trying to do and tries to recover the situation:

But why skip past the rest of the code? Why not keep on running? Generally,

that would be a bad idea. Once a line of code has gone bad, there’s no way of

knowing if it makes sense to keep running the code that follows. For example,

if the Head-Ex code can’t open the deliveries file to append to it, it makes no

sense to continue trying to write data to the unopened file!

In order to recover, you need to start running your code

from somewhere else.

def save_data():

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

Bang!

I‛m outta here!

This line

of code

causes the

exception

to be

thrown.

All of this code

is skipped.

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exceptions and message boxes

A piece of code

that runs when

an exception is

thrown is called an

exception handler.

Catch the exception

Python spots when an exception is thrown, and you can write some code to

run when the exception occurs. This is called catching the exception. The code

that you run when there’s an error resulting in the thrown exception is called

an exception handler.

OK, it says here I can

recover by displaying

an error message, then

restarting from this line

of code...

You can CATCH

an exception.

Exception

handling

code.

Creating exception handlers can really make life easy for your users. Instead

of a flaky program that crashes or fails silently the first time something weird

happens, you can write programs that gracefully recover from errors. Exception

handlers tidy up when something goes wrong and can even let your user

know that something strange happened.

That’s what you need here: an exception handler that tells the user when

there’s a problem writing a delivery to the file.

How are exception handlers written in Python?

def save_data():

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

Bang!

298 Chapter 8 ½

try to except

Watch for exceptions with try/except

In order to recover from an error as it happens, you need to indicate the code

that might throw an exception. In Python, you do this with try and except.

All you need to do is take the piece of potentially troublesome code and add

the try and except labels:

def save_data():

try:

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

except Exception as ex:

If an exception is thrown between the try and except labels, the code

that follows the except label runs. The code that threw the exception

is abandoned. If no exception occurs, the code runs normally and the

code that comes after the except label is ignored.

Notice that the try/except labels are wrapped around all of the

function’s code. If there’s a problem opening the deliveries.txt

file, you don’t ever want to try writing to it. So, when trouble strikes,

you should adandon ship and skip to the code that tries to recover from

the error.

The code that then runs is the exception handler.

If an exception is thrown

ANYWHERE in this

section, the computer will

jump to the code HERE.

This is where the EXCEPTION

HANDLER code goes.

Inside the handler, the exception is

assigned to a variable called “ex”.

Put the exception

handler labels around all

of this code.

Note: this code

is indented

under the “try”

statement.

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Exception Magnets

Assemble the code to handle an exception in the save_data()

function. The exception handler needs to display the details of the

exception in the title bar of the window. Note: remember to indent

the code in the exception handler, in addition to the code in the

try block.

def save_data():

app.title(

ex:

"Can’t write to the file %s"

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

)

try:

except

Exception

300 Chapter 8 ½

exception handler

Exception Magnets Solution

You were asked to assemble the code to handle an exception in the

save_data() function. The exception handler needs to display

the details of the exception in the title bar of the window. You

needed to remember to indent the code in the exception handler, in

addition to the code in the try block.

def save_data():

app.title(

ex:

"Can’t write to the file %s"

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

)

try:

except

Exception

We’re using format strings to

display the exception in the title.

This is the exception

handler code.

The exception handler can have

several lines as long as they are

all indented in the same way.

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exceptions and message boxes

Test Drive

Let’s see if the code works. Make sure the deliveries.txt file is read-

only. Then run the new version of the program in IDLE and try to record a

delivery by clicking on the Save button.

deliveries.txt

Note: make sure

deliveries.txt is

set to read-only.

Sure enough, when you try to save the delivery details, the program catches the

exception and the exception handler displays the error message in the window

title.

Wonder what the people at Head-Ex will think of this?

Clicking “Save"

causes the

title bar to

change, due to

the error.

Click!

302 Chapter 8 ½

unexceptional exception handler

There’s an issue with the exception handler

You do a quick demo for the folks at Head-Ex and, even though the program

works, it’s not quite what they need.

I‛m not sure the error

message is really visible

in the title bar. If there

is an error, I really don‛t

want to miss it.

The error message is more visible than when it was appearing in the Python

Shell, but it isn’t a whole lot more visible. Sure, you’ve proved that you can

spot when an error happens and then run an exception handler in order to

do something about the error. But you really need to do something that will

interrupt the user and highlight the situation. You need something that will

force the user to acknowledge the error before he continues doing something

else.

A GUI message box might do the trick.

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exceptions and message boxes

A message box demands attention

Most of the time, GUI programs put the user in charge. If the user chooses

to click a button or edit a field, the computer lets them do just that in

whatever order and at whatever time the user chooses. But sometimes, GUI

programs need to stop the user and ask her a question, getting her to confirm

or acknowledge something. That’s where message boxes come in.

A message box is something that requires a response, which is why it’s

sometimes called a dialog box.

The simplest message box displays a message with a single OK button:

A message box always displays the message in a separate window, typically

in front of your main GUI window. And it won’t go away until you click

it, dismissing it. That’s why message boxes are the most commonly used way

of displaying errors. The user has to read and respond to the error before

continuing.

You should be sparing in how often you display message boxes, because if

users see too many of them, they are likely to click them without reading the

message. But, when used carefully, they keep your user informed and alert.

The user must click the “OK” button

to continue, indicating that she

acknowledges the message.

This is the message.

This icon shows

it's a warning.

304 Chapter 8 ½

declarative and interrogative messages

Creating message boxes in Python

All of the message box code is contained within a tkinter module called

messagebox, so the first thing to do is import the module:

import tkinter.messagebox

Then, you’re good to go. Within the messagebox module, there’s a whole

bunch of different dialogs to choose from. But all of them fall into two main

categories.

Message boxes that say stuff

To display a simple message on the screen, you might display a message box

like this:

tkinter.messagebox.showinfo("Delivery", "The cupcakes have arrived in Istanbul")

When tkinter gets to this line, it will wait for the user to

answer the question and then assign True (yes), False

(no), or None (cancel) to the response variable.

Let’s see what other message boxes are

available.

response = tkinter.messagebox.askyesnocancel("Gift?", "Gift wrap the package?")

Message boxes that ask stuff

If you need a message box that asks the users a question, you will need to

check the return value to see what they chose:

The title of the message box.

The contents of the message.

You need to click the OK

button to close the dialog.

The icon in the window shows

that this is just for information.

A value is assigned to “response” after

the user clicks one of the buttons.

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exceptions and message boxes

“OK to fire boosters?”

showinfo

These are the message boxes available in tkinter. Think carefully about

each of the following examples. We know which we’d use at Head First

Labs. Which type of box on the left would you choose for each of the

messages on the right? Connect the messages to the boxes with lines.

showwarning

showerror

askquestion

askokcancel

askyesnocancel

askretrycancel

“Your tartan clogs have arrived.”

“Seriously, I think he’s just ignoring the phone.”

“Danger, Will Robinson!”

“Do you want fries with that?”

“Dude, the printer’s busted.”

“So, you want Nutella on your bacon and jelly sandwich?”

306 Chapter 8 ½

mixed messages

“OK to fire boosters?”

showinfo

SOlUTion

showwarning

showerror

askquestion

askokcancel

askyesnocancel

askretrycancel

“Your tartan clogs have arrived.”

“Seriously, I think he’s just ignoring the phone.”

“Danger, Will Robinson!”

“Do you want fries with that?”

“Dude, the printer’s busted.”

“So, you want Nutella on your bacon and jelly sandwich?”

This is pure information. Nothing

to worry about. Except the risk of

clashing with your velvet pixie hood.

OK, so there's nothing actually

broken YET, but BE CAREFUL

Stuff's broken. You need to know.

You are going to continue, but

do you want this extra option?

Are you REALLY sure you want to

continue and do this thing? It's your

last chance to change your mind.

Do you want this additional option, or would

you like to forget about the whole thing?

It didn't work last time, but if you like, you can try again.

Did your answers match ours? They might not. Selecting which type of message box to use

depends a lot upon the particular program you're writing and how serious you think a decision is.

These are the message boxes available in tkinter. You were to think

carefully about each of the following examples, then indicate which

type of box on the left you would choose for each of the messages on

the right. You were to connect the messages to the boxes with lines.

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exceptions and message boxes

The folks at Head-Ex want your program to display this message

box if there’s a problem saving a record to the

deliveries.txt file:

Complete the missing lines in this section of your program to create the message box.

Hint: You need to include a newline in the message box text.

from tkinter import *

def save_data():

try:

fileD = open("deliveries.txt", "a")

fileD.write("Depot:\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description:\n")

fileD.write("%s\n" % description.get())

fileD.write("Address:\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

except Exception as ex:

308 Chapter 8 ½

get the message

The folks at Head-Ex want your program to display this message

box if there’s a problem saving a record to the

deliveries.txt file:

Complete the missing lines in this section of your program to create the message box.

Hint: You need to include a newline in the message box text.

from tkinter import *

def save_data():

try:

fileD=open("deliveries.txt", "a")

fileD.write("Depot\n")

fileD.write("%s\n" % depot.get())

fileD.write("Description\n")

fileD.write("%s\n" % description.get())

fileD.write("Address\n")

fileD.write("%s\n" % address.get("1.0", END))

depot.set("")

description.delete(0, END)

address.delete("1.0", END)

except Exception as ex:

import tkinter.messagebox

tkinter.messagebox.showerror(“Error!", “Can't write to the file\n %s" % ex)

You should use the showerror() function so that

the dialog box gets the correct error icon.

Remember to

import the

necessary module.

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exceptions and message boxes

Test Drive

Now what happens if you try to save a record when the deliveries.txt

file is read-only?

This is an ERROR message box.

The icon shows

the user that

something has

gone wrong.

Great! The exception handler displays an error message

with an icon that really alerts the user to the problem.

Let’s check back with Head-Ex to see if they like it.

310 Chapter 8 ½

durian delivered

That‛s exactly what we need. And not a

moment too soon. We just had a delivery

of durian, the world‛s stinkiest fruit.

There‛s no way we want to leave THAT

delivery too long in the stock room!

The error message box was exactly what

Head-Ex needed.

By catching exceptions and displaying

important information in message boxes,

you can greatly improve the experience of

your users when things go wrong.

Great work!

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exceptions and message boxes

Programming Tools

* Some errors don't crash your program - they

throw exceptions instead.

* You can run code when there's an exception -

this is called “catching the exception.”

* Code that runs because of an exception is

called an exception handler.

* GUI message boxes display information and ask

questions.

* Message boxes are also known as “dialog boxes.”

* Message boxes require the user to respond, even

if it is just to click an OK button.

CHAPTER 8.5

Your Programming Toolbox

You’ve got Chapter 8.5 under

your belt. Let’s look back at what

you’ve learned in this chapter:

Python Tools

* You can catch exceptions by using a try/except

block.

* “except Exception as ex" will assign the exception

message to a variable called “ex".

* You can display the exception error message by

formatting it as a string.

* To display message boxes, you need to import the

“tkinter.messagebox” module.

* Message boxes that display information are all

called “show...()".

* Message boxes that ask questions are all called

“ask...()".

* Message boxes return True if the response was OK,

Yes, or Retry.

* Message boxes return False if the response was No.

* Message boxes return None if the response was

Cancel.

this is a new chapter 313

Oh, how lovely.

graphical interface elements

Selecting the

right tool

It’s easy to make your programs more effective for your users.

And when it comes to GUI applications, there’s a world of difference between a working

interface and one that’s both useful and effective. Selecting the right tool for the right job

is a skill that comes with experience, and the best way to get that experience is to use

the tools available to you. In this chapter, you’ll continue to expand your GUI application

building skills. There’s a bunch of truly useful widgets waiting to be experienced. So, turn

the page and let’s get going.

Yikes! I was sure the

salesman said the box

contained an Ultimate

GUI ToolkitTM.

314 Chapter 9

dj goes digital

Time to mix it up

Your best friend is an ultra hip DJ with a problem: his vinyl

collection is now so large that he can’t carry it all around

from club to club anymore. He’s decided to digitize his entire

collection, put it on his laptop, and mix his sounds from there.

His problem is that commercial mixing software costs a fortune

and he doesn’t like any of the free alternatives. He has his

own ideas for the mixing software he wants.

What with spending all his time mixing music, he’s never

learned how to program his computer... and that’s where you

come in. If you help him to write the software he needs, he

promises to showcase your work at the upcoming World Music

Mixing Expo.

Let’s build the mixing software bit-by-bit based on the DJ’s

requirements.

I‛m dumping vinyl and

going digital. Can you help

me write my software?

I need to be able to start

and stop a track.

All of this has to go.

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graphical interface elements

Code Magnets

A couple of buttons on a GUI ought to do it. Here’s the code to a

small tkinter program that starts and stops a sound file. Rearrange

the code magnets to make the program:

from tkinter import *

import pygame.mixer

def track_start():

track.play(loops = -1)

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

sound_file = "50459_M_RED_Nephlimizer.wav"

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

def track_stop():

track.stop()

track = mixer.Sound(sound_file)

mixer = pygame.mixer

mixer.init()

app.mainloop()

316 Chapter 9

start and stop

Code Magnets Solution

A couple of buttons on a GUI ought to do it. Here’s the code to a

small tkinter program that starts and stops a sound file. You were

asked to rearrange the code magnets to make the program:

from tkinter import *

import pygame.mixer

def track_start():

track.play(loops = -1)

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

sound_file = "50459_M_RED_Nephlimizer.wav"

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

def track_stop():

track.stop()

track = mixer.Sound(sound_file)

mixer = pygame.mixer

mixer.init()

app.mainloop()

Download the sound tracks for this chapter from the Head First Programming

website. Be sure to put the sound files in the same directory/folder as your code.

Load this!

Import the required libraries.

Create the GUI application window.

Identify the DJ's track.

Start the sound system.

The “track_start()" function will respond

to the “Start" button-click event.

The “loops = -1" parameter to “play()"

repeats the track until you stop it.

The “track_stop()" function

will respond to the “Stop"

button-click event.

Load up the track sound file.

Create a button for

“Start" and “Stop," then

connect each of them to

their event handlers.

Start the GUI event loop.

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graphical interface elements

Test Drive

Enter the code from the previous page into IDLE, save it as hfmix.pyw,

and press F5 to try it out.

That‛s not a

bad start...

The first version

of the DJ's

program entered in

IDLE.

The track starts

playing with a click

here and stops playing

with a click here.

You already know how to display buttons on a GUI and

associate them with event-handling code. What’s new in this

code is the loops = -1 bit, which arranges to play the sound

repeatedly. That is, the track loops.

That was almost too easy!

318 Chapter 9

stop the music

The music just kept on playing...

In his haste to show off your program to his DJ rivals, your friend

fired up the program as is. He didn’t realize that the track you

included by default is pretty bad. In a panic, he clicked the window’s

close box before pressing Stop, and the awful track just kept right on

playing. His rivals haven’t had such a good laugh in ages...

Man, I am so

embarrassed...

Oh, dude, that

was soooo funny...

you should‛ve seen

your face!

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graphical interface elements

Not all events are generated by button clicks

Your GUI program processes lots of events, not just the events generated by your

user when, for instance, buttons are clicked.

Your operating system can send events to your GUI program, too. Some of these

events are commonly handled by the graphical programming technology you

are working with. For most of the operating system’s events, tkinter very kindly

handles them for you. When you click the close box on your GUI, this generates a

Window Manager event for your code to handle. If your code doesn’t handle a

Window Manager event, tkinter handles it for you in a default way, too.

If the default event handling isn’t what you want, you have to capture the event

before it gets to tkinter and is handled in the default way. At the moment, the click

on the close box is being handled for you by tkinter and the default behavior is to

close the window.

Let's take control of this default behavior.

Click!

Wait for another event.

Your user

generates a

button-click

event in the

GUI.

The Window Manager

generates an event

when the user clicks

on the close box.

Run the event-handling code

associated with whatever

event occurred or perform

some default action.

Click!

320 Chapter 9

the cost of interactive loveliness

Frank: What’s up now?

Joe: Well, it looks like I have to worry about lots of other GUI

events, not just my own.

Jim: Yeah. It appears the operating system and this Window

Manager thing can give the GUI application work to do.

Frank: Yes, that’s right. All that interactive loveliness comes at a

cost.

Joe: Cost?!? You mean we have to pay?

Frank: No, not that sort of cost. You sometimes need to write a little

extra code to interact with the Window Manager when and where

necessary... that’s what I mean by “cost.”

Joe: Phew! So... what’s a Window Manager, anyway?

Frank: It’s something built into every GUI application that handles

the management of your application’s windows. Python’s GUI,

tkinter, has a Window Manager, as do all the other GUI toolkits.

Joe: So, how do I work with the events generated by the Window

Manager?

Frank: You create a function with the code you want to run and

then connect the function to the event.

Joe: OK, I get that. But which event do I connect up to? It’s not like

the Window Manager has a button to click on, is it?

Jim: That’s a good question... Frank?

Frank: Well spotted. What happens with the Window Manager is

that there’s a set of protocol properties that you can interact with as

and when required.

Joe: Protocol what? Properties?

Jim: Yeah, you’ve lost me there, Frank.

Frank: Yes, protocol properties... they really are not as scary as they

sound. Remember: with GUIs, it’s all just code.

Jim & Joe: Where have we heard that before... ?

Frank: Here, let me show you what I mean...

You see, GUIs might look

nice and easy, but they are

actually a pretty complicated

beast under the hood. There‛s

lots of different events to

worry about.

Joe

Frank

Jim

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graphical interface elements

A message sent to your main window

when the operating system is shutting

down

WM_TAKE_FOCUS

Study the three tkinter properties presented below and see if you can

match up the properties to the correct description:

A message sent to your main window

when the close box has been clicked

WM_SAVE_YOURSELF

A message sent to your main window

when the window has been selected

after a mouse click

WM_DELETE_WINDOW

Which of these protocol messages do you think you need to capture?

Geek Bits

The tkinter library provides a mechanism to

react to an event that is associated with the GUI

window. These are known as Window Manager

protocol properties. Think of the event as a

protocol event.

322 Chapter 9

capture the event

app.protocol("WM_DELETE_WINDOW", shutdown)

A message sent to your main window

when the operating system is shutting

down

WM_TAKE_FOCUS

You were to study the three tkinter properties presented below and see

if you could match up the properties to the correct description:

SOlUTion

A message sent to your main window

when the close-box has been clicked

WM_SAVE_YOURSELF

A message sent to your main window

when the window has been selected

after a mouse click

WM_DELETE_WINDOW

You were to identify which of these protocol messages you think you

need to capture:

WM_DELETE_WINDOW

You'll have to create the

“shutdown” function.

Associate the event-

handling function

with the property.

Be sure to call

“protocol()" BEFORE

“mainloop()".

OK... here comes a click

on the close box. What‛s my

protocol? Ah, yes... it hasn‛t

been captured, so I‛ll just

execute the default behavior

and close that sucker!

Controlling the Window Manager

To capture the event before it gets to tkinter, call your app’s

protocol() method and identify the function that should be

called instead of executing the default behavior:

Click!

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graphical interface elements

Now that you know about window manager properties and how

to capture them, write the code for the shutdown() function:

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_start():

track.play(loops = -1)

def track_stop():

track.stop()

track = mixer.Sound(sound_file)

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

app.mainloop()

Here's the code so far.

Put the

“shutdown”

function here.

What needs

to go here?

324 Chapter 9

shutdown function

Now that you know about window manager properties and

how to capture them, you were asked to write the code for the

shutdown() function:

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_start():

track.play(loops = -1)

def track_stop():

track.stop()

track = mixer.Sound(sound_file)

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

app.mainloop()

def shutdown():

track.stop()

Simply arrange for the

track to stop playing

when the window closes.

app.protocol(“WM_DELETE_WINDOW", shutdown)

Call “app.protocol()”

before the call to

“app.mainloop()”.

graphical interface elements

Test Drive

With the changes made to your program in IDLE, press F5 to see how things

perform now.

The sound stops when I

click to close the window,

but now the GUI doesn‛t

disappear!

What gives? You’ve solved one problem, but created another.

When the DJ clicks on the close box, the track stops playing,

which is what you wanted. But now, the window won’t close.

This can’t be good, can it?

Click!

You'll need to quit IDLE and

the Python Shell to force the

window to close.

No matter how often you click

the close box, the window won't

go away...

you are here 4 325

Click!

326 Chapter 9

terminate with extreme prejudice

from tkinter import *

from tkinter.messagebox import askokcancel

app = Tk()

app.title("Capturing Events")

app.geometry('250x100+200+200')

def shutdown():

if askokcancel(title = 'Are you sure?', message = 'Do you really want to quit?'):

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

Capturing the protocol event isn’t enough

Your code captures the protocol event and redefines its behavior. But, what

about the behavior that used to execute by default?

Take a look at this small example program, which redefines the close-box

protocol to check with the user before actually destroying the window:

Do this!

Add the app.destroy() line of code to the end of your

shutdown() function and see if it makes any difference.

Clicking on the close box runs the

“shutdown()” function, which then

displays an AskOkCancel dialog.

If you click OK, the GUI is

destroyed (which has the effect of

closing the window).

327

graphical interface elements

Test Drive

You’ve added in the line of code that terminates (destroys) your GUI

application. Now, press F5 to see what happens.

That‛s great!

Let‛s see my

rivals scoff now...

Poof!

When you click on the

close box now, the GUI

application disappears.

Which helps explain why you

can't see it on this page

anymore!

Your GUI is not only doing what the DJ wants; it’s behaving itself, too. By

redefining the protocol event associated with the click on the close box, you

are able to stop the track... eh... in its tracks. You also ensure that the default

behavior associated with the click is performed by arranging to destroy the

GUI application.

That’s great!

This extra line of

code makes all the

difference.

328 Chapter 9

toggle time

Two buttons, or not two buttons?

That is the question...

The DJ is happy with the program so far. However, he thinks it would work

better if he had just one button instead of two. He’s convinced this would be

easier to use, because he wouldn’t have to move his mouse around the screen

quite so much.

Speed‛s important, dude.

Can‛t I just press the

button to start the track,

then press it again to stop

it?

It is possible to use a single button here but, without changing the physical

appearance of the button each time it’s clicked, a user can’t possibly know

what state the button is currently in, even though the DJ does have ears in

this instance. But, in general, using a button to switch between two states is not

regarded as best practice in GUI design.

What your DJ friend actually wants is some sort of visual toggle...

something that can be flipped between one of two states: on/off, open/close,

flip/flop, and so on. You need to use a different graphical interface element.

Is there a GUI visual toggle you can use here?

Will the next click

start or stop whatever

this button does?

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graphical interface elements

Take a look at the following window that appears when you ask a full-featured web browser to

display its preferences. Grab a pencil and draw a circle around any graphical interface elements

you haven’t worked with yet.

From the elements that you have circled, identify one that might be of use to you in your program. Which

would work best as an on/off toggle?

Write your answer here.

330 Chapter 9

check on the checkbox

Take a look at the following window that appears when you ask a full-featured web browser

to display its preferences. You were to grab a pencil and draw a circle around any graphical

interface element you haven’t worked with yet.

From the elements that you have circled, identify one that might be of use to you in your program. You

were asked to identify an element that would work best as an on/off toggle:

The checkbox is

either on or off.

The checkbox

Here's an

element we

haven't used

yet.

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graphical interface elements

The checkbox is an on/off, flip/flop toggle

The great thing about the checkbox graphical interface element is that it can

be in only one of two states, either on or off. Depending on the current state

of the checkbox, an object can be set to either 1 for “on” or 0 for “off.”

In tkinter, checkboxes are created using Checkbutton(), and they can be

associated with a tkinter IntVar, first introduced in Chapter 7. The tkinter

Checkbutton() is either on or off and sets the associated IntVar to

either 1 or 0, which is perfect for what you need.

Let’s look at using a checkbox in tkinter.

Set me, checkbox,

set me... I can be

either 1 or 0.

Whoo hoo! Set

that object to

1, baby!

Click the checkbox

to “tick” it and set

the object to “on”.

Click the checkbox to

“untick” it and set the

object to “off”.

Ah, phooey! Set

that object to 0,

which is, like, oooh.

332 Chapter 9

flip it off

flipper = IntVar()

def flip_it():

if flipper.get() == 1:

print("Cool. I'm all ON, man!")

else:

print("Phooey. I'm OFF.")

Checkbutton(app, variable = flipper,

command = flip_it,

text = "Flip it?").pack()

Working with checkboxes in tkinter

The tkinter Checkbutton needs three things: an IntVar to hold its

current value, an event-handler function to run when it’s ticked, and a

descriptive label to say what it does. Take a look at this example code:

The “flip_it()"

function is the

Checkbutton's

event handler.

The Checkbutton is

associated with the

“IntVar”, links to the

event handler, and has

a descriptive label, too.

Create an “IntVar” to hold a value that

is either 1 or 0, depending on whether

the checkbox is ticked.

Using the get() method

If you look closely at the code for the flip_it()

event handler, you’ll notice that the message displayed

on screen is controlled by whatever value is returned

from the call to flipper.get(). The get()

method is part of every IntVar object, and it lets

you easily determine, in this case, the current value

associated with the flipper variable.

But, what sets the value?

The Checkbutton automatically sets the value of

flipper as a result of the click on the checkbox.

Tick the box and the value is set to 1. Untick the box

and the value is set to 0.

Cool. I‛m all

ON, man!

Phooey.

I‛m OFF.

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graphical interface elements

Here’s your code from earlier. Use your pencil to put a line

through the code you don’t need anymore:

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_start():

track.play(loops = -1)

def track_stop():

track.stop()

def shutdown():

track.stop()

app.destroy()

track = mixer.Sound(sound_file)

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

Write the code you need to implement the checkbox here, based on the sample code from the previous

page. Give your IntVar the name track_playing. Use track_toggle as your function name,

and call the checkbox track_button:

334 Chapter 9

checkbox added

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle,

text = sound_file)

track_button.pack()

Here’s your code from earlier. You were to use your pencil to put a

line through the code you don’t need anymore:

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_start():

track play(loops = -1)

def track_stop()

track.stop()

def shutdown():

track.stop()

app.destroy()

track = mixer.Sound(sound_file)

start_button = Button(app, command = track_start, text = "Start")

start_button.pack(side = LEFT)

stop_button = Button(app, command = track_stop, text = "Stop")

stop_button.pack(side = RIGHT)

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

Write the code you need to implement the checkbox here. You were asked to give your IntVar the name

track_playing, use track_toggle as your function name, and call the checkbox track_button:

The functions that start and stop

the track are no longer needed.

You can also get rid of the two buttons,

since they aren't needed either.

The “track_toggle" function

either plays or stops the

track, based on the state of

the checkbox.

Use the name of the sound

file as the text associated

with the checkbox.

All of this code needs to

be added to your program

BEFORE the call to “app.

mainloop()".

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graphical interface elements

Test Drive

With your program amended to include the checkbox code, let’s

run the latest version of the DJ’s program in IDLE and see if the

checkbox works as advertised.

Sweet! Toggle the sound

on and off by simply

clicking on the checkbox.

Man, that‛s so easy

to use! Let‛s adjust

the volume, too, while

the sound is playing...

Creating the program bit by bit is working out. Each time

your DJ friend needs something new, you incrementally

improve the program in order to provide the functionalilty

he needs. Of course, there’s always something new.

Will this guy ever be satisfied?!?

336 Chapter 9

add a slider

Pump up the volume!

To make the program more useful, the DJ wants to be able to dynamically

and interactively control the volume of the track as it plays. Basically, as the

track plays, the DJ wants to fiddle with the volume.

The control might look like this,

a slider that can be moved from

left (down) to right (up)...

...or the control

might look like this.

Turn it up to the

right, down to the

left..

Because a slider is much easier to manipulate with a click and drag of a

mouse than a knob, the slider on a scale is used in lots of modern GUIs. It’s the

classic choice for showing a volume control.

Look closely at the slider. What do you need to model?

In the physical world, most devices

that have a volume control provide

either a large, circular knob or

a slider on a scale:

Louder!

Softer, softer,

softer...

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graphical interface elements

Look at the volume control shown above and identify four key

characteristics of the volume control.

Write your four answers here.

Model a slider on a scale

There’s quite a bit going on with a slider on a scale. It’s a simple control that

everyone’s familiar with, but the simplicity masks quite a bit of complexity.

The devil is most definitely in the details:

Here's the standard

representation of a

slider on a scale that we

all know and love.

This end of the scale

marks a LOW value.

This end of the scale

marks a HIGH value.

There's a bunch of markings on the

device that indicate where on the

scale you currently are.

There is always something

that you can grab onto

and slide from left to

right.

338 Chapter 9

sound before beauty

You were to look at the volume control on the previous page and

identify four key characteristics of the volume control.

There's a scale that goes from a low value to a high value.

The scale has a fixed set of intervals.

The volume control “slider" moves from left to right.

Moving the “slider" dynamically adjusts the volume based on its current

position on the scale.

Start with the volume

Before you start worrying about creating the appropriate GUI interface

element to actually model the slider, you first need to know how to adjust the

volume of a track.

Once you know how to adjust the volume, you can then start to worry

about linking the volume to the slider, with the current position of the slider

dictating the current volume setting.

Then you can allow your user to move the slider which has the effect of

dynamically and interactively adjusting the volume.

Sounds easy, eh?

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graphical interface elements

Use pygame to set the volume

Turns out pygame has this functionality built right into its library code via the

set_volume() method.

Take a look at this small example program:

Set the volume to a LOUD

setting.

Set the volume to

a very low setting.

Louder, dude, louder!

Turn that racket down!

import pygame.mixer

from time import sleep

mixer = pygame.mixer

mixer.init()

track = mixer.Sound("50459_M_RED_Nephlimizer.wav")

print("Play it LOUD, man!")

track.play(loops = -1)

track.set_volume(0.9)

sleep(2)

print("Softly does it ... ")

track.set_volume(0.1)

sleep(2)

track.stop()

When you set the track’s volume to a high value using set_volume(), it’s

the equivalent of cranking up the volume by moving the slider to the right. When

you set it to a low value, that’s like moving the slider to the left.

340 Chapter 9

create the slider

Use tkinter for everything else

The tkinter library has a graphical interface element called Scale that lives

to help you create a slider. Take a look at this example code and let’s see how

it works:

volume = DoubleVar()

volume_scale = Scale(app,

variable = volume,

from_ = 0.0,

to = 1.0,

resolution = 0.1,

command = change_volume,

label = "Volume",

orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Create a tkinter DoubleVar variable. Like

IntVar and StringVar, the DoubleVar variable

stores a “magic" value, this time a float.

The tkinter Scale

widget creates a

slider.

The slider can be linked

to a “magic" variable.

Specify the

resolution value,

which indicates the

interval.

Connect the

widget to an

event handler.

Provide a nice, human-

friendly label for the

slider.

Indicate whether the slider

runs across the screen

(HORIZONTAL) or up the

screen (VERTICAL).

Specify the lowest and

the HIGHEST values on

the scale.

Click and drag

the slider to

adjust the volume.

The Scale() element is your most complex tkinter widget yet. But, despite

this, it is not hard to work out what’s going on here. The graphical interface

element is linked to a tkinter DoubleVar (called variable), the lowest/

highest slider values are provided (to and from_), and an interval between

them (resolution) is specified. The event handler is assocated with an

event handler (command), a descriptive label is supplied (label), and, finally,

the orientiation of the slider is specified (orient). There’s a lot going on

here, but none of it is that hard to understand.

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graphical interface elements

Q: The Scale() code on the previous

page has a variable called from_, that is,

the word “from” together with a trailing

underscore. Was that a typo?

A: No, it wasn’t a tyop, eh, typo. The

reason for the underscore has to do

with the fact that Python uses “from” as

a reserved word. These are words that

Python uses for its own special purposes,

which means you cannot name one of your

variables after a reserved word. As using

the word “from” makes a lot of sense when

taking about a scale, the authors of tkinter

decided to tack on the underscore so that

the meaning of the variable would be clear,

while allowing for the variable name not to

clash with a reserved word.

Q: Are there other reserved words?

A: Yes, a few. And every programming

language has its own list. In Python, words

like “while”, “for”, “if”, “def”, “class”, and

“pass” are all reserved words.

Q: What happens if I use one

anyway?

A: Python will most likely complain with

a syntax error.

Q: Where can I find a list?

A: Any good Python reference book

will contain a list, and it’s also included as

part of the Python documentation that’s

installed with Python on your computer and

available on the web at the main Python

website.

Q: How do I know which graphical

interface element to use and when?

A: This is really a matter of experience.

However, a lot of platforms go to great

lengths to specify exactly when each of

the elements should be used and for what

purpose. Of them all, the Macintosh is the

leader of the pack. Apple’s engineers have

worked hard to strictly enforce consistent

usage of the Mac GUI among programmers.

Q: So, it’s a case of anything goes

with the other operating systems?

A: No. That’s not what we are saying.

The Apple folks are very strict with their

rules and regulations, and the others are

less so. But there are still standards that

you should try as much as possible to

adhere to. One of the reasons for using

a GUI is that your users will expect your

program to work in a standard way. This

makes your program immediately familiar

to new users and lets them become

productive with your program more quickly.

Q: So there are no badly designed

GUI apps?

A: No. There are plenty of howlers out

there... and they tend to be harder to use

than necessary, due to the fact that the

programmers responsible for creating them

did not conform to established interface

standards and practices. When it comes

to GUI programs, conformance is a good

thing.

Q: So does tkinter work well on all

platforms?

A: The latest version of tkinter (which

comes with Python 3) is pretty good. If

you run your tkinter program on a Mac, it

looks like a Mac OS X program, whereas

on Windows it looks like a Windows

application, and on Linux it takes on the

look and feel of the graphical environment

you happen to be using (there are a few

choices on Linux).

Q: Other than tkinter, what other

graphical toolkits does Python support,

and should I learn any of them?

A: Python supports lots of other toolkits

on lots of operating systems. For now,

tkinter is all you really need, and you

shouldn’t worry about the other choices

until you are in a situation where learning

how to use them becomes a necessity.

342 Chapter 9

control the volume

Take the pygame and tkinter code and combine it to support a volume control. Then, complete

the next version of your program.

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

Add a function here

to adjust the volume

that the track

currently plays at.

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graphical interface elements

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle,

text = sound_file)

track_button.pack(side = LEFT)

def shutdown():

track.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

Add the code

that implements

the volume

control here.

344 Chapter 9

volume controlled

def change_volume(v):

track.set_volume(volume.get())

You were to take the pygame and tkinter code and combine it to support a volume control,

then complete the next version of your program.

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

app.geometry('250x100+200+100')

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

Put the

pygame code

here.

Things are starting to

get crowded on the GUI,

so let's have tkinter

automatically decide

on the geometry for

us. Remove the “app..

geometry()” call from the

code.

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graphical interface elements

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle,

text = sound_file)

track_button.pack(side = LEFT)

def shutdown():

track.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

volume = DoubleVar()

volume.set(track.get_volume())

volume_scale = Scale(variable = volume,

from_ = 0.0,

to = 1.0,

resolution = 0.1,

command = change_volume,

label = “Volume",

orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Put the tkinter

code here.

346 Chapter 9

test drive

Test Drive

Let’s take the latest version of the program for a spin in IDLE. In addition

to turning the sound on and off with the checkbox, you should be able to

dynamically and interactively adjust the volume with the slider.

Turn it way up.

It works. Not only can you

start and stop the track,

but you can now adjust the

volume, too! And all from

the one GUI... sweet!

Flip it on and off.

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graphical interface elements

Wow! This is really

fabulous... I can now start

converting all my vinyl tracks

and, thanks to your program,

my laptop is on its way to

being my music mixing desk!

The DJ is over the moon!

Your code has shown the DJ that his idea for the Ultimate Music

Mixing program is not as far-fetched as everyone once thought. With

his idea, talent, good looks, and—above all else—modesty, together with

your coding skills, the world’s the limit for what you can accomplish

together. The World Music Mixing Expo is next month, and the DJ

can’t wait to show off your completed software.

348 Chapter 9

programming toolbox

CHAPTER 9

Your Programming Toolbox

You’ve got Chapter 9 under your belt.

Let’s look back at what you’ve learned

in this chapter:

Programming Tools

* The Window Manager manages the windows created by

your GUI.

* GUI events can be generated by the operating system, the

Window Manager, and your user.

* If you redefine some default functionality, be sure to code

the default behavior into your code (if required).

* A checkbox lets you indicate whether something is set to

ON or OFF.

* A scale/slider lets you implement a volume control.

* Be sure to look at other GUI programs when looking for

inspiration and ideas for your own programs.

* Reserved words are names that have special meaning in a

programming language and which cannot be used as variable

names.

Python Tools

* app.destroy() - used to terminate a

tkinter GUI application

* DoubleVar() - like IntVar and StringVar,

but used to hold a floating-point number

* Checkbutton() - a tkinter widget for

creating checkboxes

* Scale() - a tkinter widget for creating

scales/sliders

this is a new chapter 349

Since we learned about custom

widgets and object orientation,

we‛ve become a real class act.

custom widgets and classes

With an object in mind

Requirements can be complex, but programs don’t have to be.

By using object orientation, you can give your programs great power without writing lots

of extra code. Keep reading, and you’ll create custom widgets that do exactly what you

want and give you the power to take your programming skills to the next level.

350 Chapter 10

mixing multiple tracks

The DJ wants to play more than one track

The World Music Mixing Expo is only a few weeks away. Your DJ friend is

thrilled with the work you’ve done, but now he has a few extra requirements.

Just playing one track isn’t enough. To be useful, the program needs to be able

to mix several tracks together at the same time. Each track needs its own

graphical interface elements (or widgets).

I can fade a track

in and out, but that‛s

not enough. I need to

be able to mix several

tracks together.

How will you create the widgets for each track?

To play and control a single track, you created two widgets on the GUI: a

check button to start and stop the track and a scale to change the volume. Then

you added event-handling code to hook those widgets up to the sound

system.

To play multiple tracks together, you just need more of the same. Each track will

need its own set of widgets and its own event handlers to connect the widgets and

the track together. Then, each set of widgets needs to be added to the same

window in the GUI.

Let’s generate the widgets and event handlers.

Track.

Turn on/off.

Control the

volume.

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custom widgets and classes

Create code for each track as a function

You could just copy and paste the code several times for each track. But

duplicated code is a bad idea, because it can lead to all sorts of problems

whenever you need to change how the code for each track works. Instead of

duplicating code, you should always try to reuse it.

One way to reuse code is by creating a function that will generate the widgets

and the event handlers as needed.

If you have a function that creates the widgets and event handlers for a single

track, you could call it for each of the tracks, which would then quickly let you

build the entire interface.

But what code would you need in such a function?

A function, when called, could

create the interface for a

single track as needed.

352 Chapter 10

new function

Here is the code from the end of the previous chapter. Study it carefully, and then, in the

space on the next page, write the code for your new function (based on the code below).

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

def change_volume(v):

track.set_volume(volume.get())

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle, text = sound_file)

track_button.pack(side = LEFT)

volume = DoubleVar()

volume.set(track.get_volume())

volume_scale = Scale(variable = volume, from_ = 0.0, to = 1.0, resolution = 0.1,

command = change_volume, label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

def shutdown():

track.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

Begin by importing the

libraries you need.

Create the GUI application...

...and initialize the

sound system.

The event-handler functions

detail what happens when an

event occurs.

Define the checkbox widget.

Define the slider widget.

Handle a click on

the close box.

Start the event loop.

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custom widgets and classes

354 Chapter 10

function created

Here is the code from the end of the previous chapter. You were to study it carefully, and then,

in the space on the next page, write the code for your new function (based on the code below).

from tkinter import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

sound_file = "50459_M_RED_Nephlimizer.wav"

mixer = pygame.mixer

mixer.init()

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

def change_volume(v):

track.set_volume(volume.get())

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle, text = sound_file)

track_button.pack(side = LEFT)

volume = DoubleVar()

volume.set(track.get_volume())

volume_scale = Scale(variable = volume, from_ = 0.0, to = 1.0, resolution = 0.1,

command = change_volume, label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

def shutdown():

track.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

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custom widgets and classes

from tkinter import *

import pygame

def create_gui(app, mixer, sound_file):

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

def change_volume(v):

track.set_volume(volume.get())

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle,

text = sound_file)

track_button.pack(side = LEFT)

volume = DoubleVar()

volume.set(track.get_volume())

volume_scale = Scale(variable = volume, from_ = 0.0, to = 1.0,

resolution = 0.1, command = change_volume,

label = “Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Create a new function

that contains the GUI-

creating code from the

current program.

All of this code is part

of the function, so it

needs to be indented.

Begin by importing

the libraries you

need to use in this

module.

356 Chapter 10

function in a function

The new function contains other functions

With all the code gathered together in a new function, the code for the

create_gui() function looks like this:

from tkinter import *

import pygame

def create_gui(app, mixer, sound_file):

def track_toggle():

if track_playing.get() == 1:

track.play(loops = -1)

else:

track.stop()

def change_volume(v):

track.set_volume(volume.get())

track = mixer.Sound(sound_file)

track_playing = IntVar()

track_button = Checkbutton(app, variable = track_playing,

command = track_toggle, text = sound_file)

track_button.pack(side = LEFT)

volume = DoubleVar()

volume.set(track.get_volume())

volume_scale = Scale(variable = volume, from_ = 0.0, to = 1.0,

resolution = 0.1, command = change_volume,

label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Do you notice anything strange? The new function actually has two other

functions inside it. Python (and several languages) lets you create local

functions. A local function is just a function inside a function.

Let’s see why they’re important for the DJ’s program.

A function-in-a-

function is called

a local function.

This function is

LOCAL to the

“create_gui()”

function.

This function is

LOCAL, too.

Note: when this function is called,

it is expecting three parameters.

When this

function is

called, it starts

executing from

here.

These are the event

handlers, which are

linked to the “command”

parameter of each widget.

As always,

the calls to

“pack()” add

the widgets to

the GUI.

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custom widgets and classes

Your new function needs to create widgets

and event handlers

When you’re wiring up the widgets in the interface, you need event handlers

to respond to changes in the state of the widgets. If someone clicks the

checkbox to play the track, the track_toggle() event handler gets called

and switches the track on or off.

But if you are creating several checkoxes, you are going to need a separate event

handler for each of them.

That’s why you have local functions inside create_gui(). As well as creating

new widgets for the interface, it also uses the local functions to create new

event handlers.

Each widget

needs its own

event handler.

Now, let’s update the program to use this new function.

This is a new

Checkbutton.

This is a new Scale.

This is a new checkbox

event handler.

This is a new slider

event handler.

create_gui()

track_toggle()

change_volume()

358 Chapter 10

localize functionality

Functions inside of functions inside

of functions inside of functions...

now, that‛s what I call complexity. Of

course, everything‛s local, you know.

Frank: A function inside a function? Surely that’s not legal?

Jim: Well... it depends on the programming language.

Joe: Don’t tell me this is something that only works with Python?!?

Jim: No... there are lots of programming languages that allow you

to put a function inside another function.

Frank: Name one (other than Python)!

Jim: Pascal.

Frank: Pasc... what?!?

Jim: Look, it really doesn’t matter which programming language

supports this feature. What is important is that we can do it.

Frank: And by “we” you mean “Python.”

Jim: OK, yes. Python can do it.

Frank: That’s all I was trying to say...

Joe: So, this is cool how?

Jim: Because it lets you localize functionality and handle complexity.

Joe: What?!?

Jim: Look: if a function gets big and complex, it can help to break

the function down into a collection of smaller functions, just like we

do when programs get big. We can keep the functionality local to the

code that needs it inside the function. That way, we keep things as

simple as we can

Joe: Even if the code itself is complex?

Frank: Like the GUI-building code we are working on now?

Jim: Yes.

Frank: OK. A function of functions it is, then. I’m all for keeping it

simple, stupid. ;-)

Joe: Yes, I like KISS, too.

Jim: Yeah... their last album was really something special, wasn’t it?

Frank & Joe: Eh?!?

Jim

Frank

Joe

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custom widgets and classes

Begin by putting the create_gui() function in a separate module called sound_panel.

py. Then, write a new version of the hfmix.pyw program that uses the sound_panel.py

module:

Write your code here.

360 Chapter 10

sound panel

You were asked to begin by putting the create_gui() function in a separate module called

sound_panel.py. Then, you were to write a new version of the hfmix.pyw program that

uses the sound_panel.py module:

from tkinter import *

from sound_panel import *

import pygame.mixer

app = Tk()

app.title(“Head First Mix")

mixer = pygame.mixer

mixer.init()

create_gui(app, mixer, “50459_M_RED_Nephlimizer.wav")

create_gui(app, mixer, “49119_M_RED_HardBouncer.wav")

def shutdown():

track.stop()

app.destroy()

app.protocol(“WM_DELETE_WINDOW", shutdown)

app.mainloop()

Import all the

functions from

the new module.

By calling the new

function TWICE, you

create TWO sets of

sound controls on the

GUI.

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custom widgets and classes

Test Drive

With the code typed into IDLE, take this latest version of the DJ’s program

for a quick spin by pressing F5.

These check buttons

start and stop each

of the tracks.

These scales control

the volume.

The program has created a checkbox and a volume slider for each track.

The program called the create_gui() function twice, creating two sets

of widgets. Of course, you can call it as many times as you like and the

create_gui() function will create the two widgets for each track.

When you click on the two checkboxes, the two tracks both play at the

same time! You can start and stop each of them independently by selecting

and deselecting the checkboxes. But more than that, the volume sliders

independently change the volume of the tracks, allowing you to mix them

together.

This is a big deal. The create_gui() function is not only creating the

separate widgets and adding them to the interface, but it is also creating the

event handlers that allow each pair of widgets to control each separate track.

Here the program is controlling just two tracks, but if you added more calls

to create_gui() in the code, there’s no reason why you couldn’t get the

interface to work with as many tracks as you like.

Let’s see what the DJ thinks.

362 Chapter 10

confusing interface

The DJ is confused

The program does exactly what the DJ wants, right? Well, not quite.

How am I supposed to

know which volume scale

is for which track?

The problem is that even though the program technically works, it has

a confusing interface. All of the checkboxes are added on the left of

the interface, and all of the volume controls are on the right.

There’s nothing in the interface that indicates which volume scale

goes with which track.

Track #1 or track #2?

Track #2 or track #1?

The checkboxes are labeled with the filename of the track, but the

volume sliders aren’t. Even though each volume slider is linked to a

single track, there is nothing in the interface that tells the user which

track that is.

So... what to do? You could just add labels to each volume slider.

That would probably fix it, but adding more widgets, like labels, can

make an interface more complex. And you want your interfaces (and

your GUI code) to be simple and clean.

Fortunately, there is a way of rearranging the widgets in the interface

to make it a lot clearer.

To avoid confusion,

the GUI needs to look

something like this.

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custom widgets and classes

Group widgets together

If the interface were laid out with the checkbox for a track grouped

alongside the volume slider for the same track, it would look better.

Each track could then have a row of widgets associated with it. As long as you

know which widgets belong to which track, you can load a lot more tracks at

once without the checkboxes and sliders getting separated (and without your

users getting confused).

The program currently uses a function to add the checkboxes and sliders to

the interface one widget at a time. If you call the function several times, the

computer creates two more widgets with each call. But the widgets are not

grouped. So, how do you group widgets together in a GUI interface?

Create a new type of widget

What if you don’t just hand the computer a set of instructions? What if you

give it a brand new widget instead?

If you create a new kind of widget that groups a checkbox with a slider,

you can add your new widget to the interface and then guarantee that the

checkbox and slider stay together:

Add a checkbox

Add a slider. Add a

checkbox...

The GUI

packer

Your new widget

“glues together”

the other widgets

so they always

stay grouped.

Your new widget becomes a new building block for your GUI interface.

So, how are new widgets created? And how do they work?

364 Chapter 10

frame widgets

panel = SoundPanel(app, mixer, “50459_M_RED_Nephlimizer.wav”)

panel.pack()

panel = SoundPanel(app, mixer, “49119_M_RED_HardBouncer.wav”)

panel.pack()

A frame widget contains other widgets

Most GUI libraries (including tkinter) let you create custom widgets from a

set of other components, and tkinter includes a special kind of widget called

a frame. A frame works just like a picture frame, in that it surrounds other

things. It’s rectangular and it can contain other widgets:

In tkinter, a frame is created using Frame(). If you can work out a way to

create a new type of frame (called, say, SoundPanel) that contains the checkbox

and the slider, then you could use code something like this in your program:

This look like a great solution. However, you still have a big problem.

This code uses an entirely new type of object, a whole new kind of

widget that has never existed before. How do you tell the computer to create

something like that, which is effectively a custom GUI object?

How do you convince the computer to create a new

widget each time you call SoundPanel()?

Do this!

Even though you

haven’t created the

SoundPanel code

yet, let’s replace the

calls to create_

gui() in hfmix.

pyw with these lines of

code now. Just don’t try

to run it yet.

Create a new

SoundPanel

widget.

You can add your widget to the

GUI using the packer, just like

with all the other widgets.

These are the same parameters you

passed to the “create_gui()” method.

A frame is like...

a frame.

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custom widgets and classes

Wouldn‛t it be dreamy if there

were a way in code to create an

entirely new kind of widget. But

I know it‛s just a fantasy…

366 Chapter 10

programming class

A class is a machine for creating objects

Object oriented programming (OOP) languages (like Python) let you create

an entirely new kind of object using a class. A class is like a template that you

use to create new objects.

Think of the class like a cookie-cutter, and think of the object as the cookie

that is created based on the class. As all the cookies are created from the same

cookie cutter, they all have the same characteristics, even though they are

all individual cookies. When an individual object is created from a class, it’s

referred to as an instance of that class.

So, if you can arrange for SoundPanel() to be a class, you can create

custom widgets as required:

The CLASS creates OBJECTs, which

are INSTANCEs of the class.

You need a new

SoundPanel() class.

SoundPanel

You need code that creates a new grouped widget in the GUI every time you

make a call like this:

panel = SoundPanel(app, mixer, "49119_M_RED_HardBouncer.wav")

Let’s define a SoundPanel() class.

Use the class to create a new object.

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custom widgets and classes

A class has methods that define behavior

The SoundPanel() class creates a new kind of tkinter Frame(), and you

can specify this relationship using the code like this:

class SoundPanel(Frame):

This says that we are

defining a new CLASS.

This is the class name.

Indicate that the

new class is a type of

Frame.

The METHODs of the class go here.

The word “method” is used in OOP

instead of the word “function.”

As well as the what (it’s a frame), you also have to worry about the how, which

will define the behavior of your new widgets. To do this, you need to add

methods inside the class. To understand how this works, imagine you have

created an alarm button object from a class. The alarm button will need to

know what to do when somebody hits it:

Uh-oh, someone just

hit me. I‛d better ask

the class that made me

what I should do.

class AlarmButton(Frame):

def button_hit(self):

klaxon.hoot_loudly()

This will be a method

inside the alarm

button's class.

The method tells the button how to

behave when something happens.

You need to create some methods for the new SoundPanel()

class. Which of the following behaviors do you think you need to

create methods for? Draw a circle around each one:

Someone moves the volume slider.

The computer starts up.

Create the interface.

Someone clicks the checkbox.

You get to the end of the track.

368 Chapter 10

necessary methods

You needed to create some methods for the new SoundPanel()

class. You were asked to identify which of the following behaviors you

thought you needed to create methods for:

Someone moves the volume slider.

The computer starts up.

Create the interface.

Someone clicks the checkbox.

You get to the end of the track.

You’ve already created

event handlers that

do something like this.

What do you need

to do to create

the interface?

Q: Why is there a method to create the widget?

A: There isn’t a method to create the widget. But there is a

method to create the interface. That method will run immediately

after the widget is created.

Q: I don’t get it. What’s the difference between a widget

and an object?

A: A widget is a particular type of object. It’s an object that you

can add to a graphical user interface.

Q: So there are some objects that are not widgets?

A: Absolutely. Most objects are used behind the scenes in

programs. All of the numbers and strings you’ve used so far have

actually been objects.

Q: So you can’t always see objects on the screen then?

A: No, most objects run quietly in memory and they don’t have

any display at all.

Q: Is Python the only object oriented language?

A: Lots of languages—such as Java, C#, and Ruby—use

objects to handle complexity.

Q: So learning object orientation is a good way of getting

into other languages?

A: Yes, understanding object orientation gives you a insight into

how other languages think.

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custom widgets and classes

But how does an object call a method?

To see in more detail how the new SoundPanel widgets use the methods in

the SoundPanel class, let’s look in more detail at just one of the methods.

What happens if someone clicks on the checkbox within the widget?

Hey, someone clicked

the checkbox. The event

handler for that is called

“track_toggle”. What happens

now, SoundPanel class?

The method you need to add to your class should look familiar. This code

is almost the same as the track_toggle() event handler we created

before. The only difference is that this method is a little more selfish.

self identifies the widget calling the method

The methods in the class are going to be used for lots of objects, so the code

in the class needs some way to know which SoundPanel object it is working

with at any point in time. It does that with the self variable.

The self variable is passed to each of the methods in the class automatically

by Python and it identifies the current widget object being used. By adding

“self.” to the front of the object’s variable names in the class code, you

make sure the code is using the data that belongs to the current widget.

Let’s add some methods to the SoundPanel() class...

This is the code to the

“track_toggle()” method.

def track_toggle(self):

if self.track_playing.get() == 1:

self.track.play(loops = -1)

else:

self.track.stop()

“self” identifies the

widget calling the

method.

The method takes a

single parameter.

370 Chapter 10

class replaces function

The SoundPanel class looks a lot like the

create gui() function

If you convert the original change_volume() function to a method and

add it to the class, you end up with code that looks rather like the original

create_gui() function:

from tkinter import *

import pygame.mixer

class SoundPanel(Frame):

def track_toggle(self):

if self.track_playing.get() == 1:

self.track.play(loops = -1)

else:

self.track.stop()

def change_volume(self):

self.track.set_volume(self.volume.get())

Most of this code looks

very similar to the

“create_gui()” method,

except for all those uses

of “self”.

In fact, the new SoundPanel() class can completely replace the code in

the sound_panel.py file (as create_gui() is no longer needed).

But before you do that, there’s still a little more code to write. The class needs

to be told what to do when the brand new SoundPanel() is created. The

class needs an initializer method that knows how to create instances of the

class.

Some programming languages call these

initializer methods CONSTRUCTORs, because

they detail what happens when a new object

is created or “constructed.”

Let’s create the initializer for the SoundPanel() class.

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custom widgets and classes

Code Magnets

We’ve started to create the initializer code for you, but there are still a few

parts missing. See if you can work out where the missing code fragments

fit. Here is the code that creates a SoundPanel() object. Position the

code magnets properly to complete the method:

volume = DoubleVar()

change_volume

track_playing

self.

track = mixer.Sound(sound_file)

self.

self

self.

track_playing = IntVar()

self

self.

def __init__(self, app, mixer, sound_file):

Frame.__init__(self, app)

track_button = Checkbutton( , variable = ,

command = self.track_toggle, text = sound_file)

track_button.pack(side = LEFT)

self.volume.set(track.get_volume())

volume_scale = Scale( , variable = self.volume, from_ = 0.0, to = 1.0,

resolution = 0.1, command = ,

label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Because SoundPanel() inherits from

tkinter’s Frame(), you need to be sure

to initialize the Frame() BEFORE you

initialize the SoundPanel().

372 Chapter 10

initializer constructed

def __init__(self, app, mixer, sound_file):

Frame.__init__(self, app)

track_button = Checkbutton( , variable = ,

command = self.track_toggle, text = sound_file)

track_button.pack(side = LEFT)

self.volume.set(track.get_volume())

volume_scale = Scale( , variable = self.volume, from_ = 0.0, to = 1.0,

resolution = 0.1, command = ,

label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

Code Magnets Solution

We’ve started to create the initializer code for you, but there are still a few

parts missing. See if you can work out where the missing code fragments

fit. Here is the code that creates a SoundPanel() object. You were

asked to position the code magnets properly to complete the method:

volume = DoubleVar()

change_volume

track_playing

self.

track = mixer.Sound(sound_file)

self.

self

self.

track_playing = IntVar()

self

self.

Each SoundPanel()

object has its own track.

Each SoundPanel() object has its

own checkbox.

Each SoundPanel()

object has its own slider.

class = methods + data

The SoundPanel() class has methods that define the behavior

that it implements. In addition to the methods, the class also

has to detail the data that it holds. For the SoundPanel()

class, this data is made up from three things: the track to play,

its checkbox, and its associated slider.

That’s a double

underscore at

either side of

the word “init”.

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custom widgets and classes

Head First: Hello, Class. It’s good of you to find

the time to speak to us.

Class: I assure you the inestimable pleasure is all

mine.

Head First: So, to begin...

Class: One moment... <hums>

Head First: I’m sorry. What’s that?

Class: Apologies. Just checking my initializer. I

always do it when I create.

Head First: Ah, yes. That’s your constructor, isn’t

it? The method you use to create objects?

Class: Well, I’m aware that some people refer to it

as a constructor, but I prefer initializer. I don’t use

it to create objects, you see. I just use it to configure

them once they’ve been created.

Head First: You have a lot of methods?

Class: Oh, more than you can possibly imagine.

Head First: In the code we’ve just seen, the

SoundPanel() class, there were only three

methods, weren’t there?

Class: Oh, dear boy, there were only three methods

defined explicitly in the class. But SoundPanel()

inherited many, many more methods from its parent

class, dear old tkinter’s Frame().

Head First: Frame() has a lot of methods, too?

Class: Too many to discuss, really. There are

methods to paint components on the screen and

details of what to do if things change size. Frame

is a fearfully busy fellow. <beep beep> Excuse me.

Hello? Yes? No, you need to stop playing track four.

No, no, it’s quite all right. Goodbye.

Head First: One of your objects?

Class: Yes. They keep me very busy, but I’d miss

them if they didn’t call.

Head First: I believe when someone calls an object

method, the object always asks you to get involved?

Class: Yes. I’m in charge of the object’s behavior. I

do think it is so important to behave properly. Don’t

you?

Head First: Of course! Class, thank you.

Class: Love the tie, by the way.

The Class Exposed

This week’s interview:

Life in senior object management.

374 Chapter 10

code review

Code Review

It’s always good every once in a while to check back on the state

of your code and make sure everything’s looking spiffy. This is

what your program should look like at this point. It’s probably

worth checking to make sure everything in your code looks like

this:

from tkinter import *

from sound_panel import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

mixer = pygame.mixer

mixer.init()

def shutdown():

.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

from tkinter import *

from sound_panel import *

import pygame.mixer

app = Tk()

app.title("Head First Mix")

mixer = pygame.mixer

mixer.init()

panel = SoundPanel(app, mixer, "50459_M_RED_Nephlimizer.wav")

panel.pack()

panel = SoundPanel(app, mixer, "49119_M_RED_HardBouncer.wav")

panel.pack()

def shutdown():

track.stop()

app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)

app.mainloop()

hfmix.pyw

Did you remember to use SoundPanel() instead of create_gui()?

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custom widgets and classes

from tkinter import *

import pygame.mixer

class SoundPanel(Frame):

def __init__(self, app, mixer, sound_file):

Frame.__init__(self, app)

self.track = mixer.Sound(sound_file)

self.track_playing = IntVar()

track_button = Checkbutton(self, variable = self.track_playing,

command = self.track_toggle, text = sound_file)

track_button.pack(side = LEFT)

self.volume = DoubleVar()

self.volume.set(self.track.get_volume())

volume_scale = Scale(self, variable = self.volume, from_ = 0.0, to = 1.0,

resolution = 0.1, command = self.change_volume,

label = "Volume", orient = HORIZONTAL)

volume_scale.pack(side = RIGHT)

def track_toggle(self):

if self.track_playing.get() == 1:

self.track.play(loops = -1)

else:

self.track.stop()

def change_volume(self, v):

self.track.set_volume(self.volume.get())

sound_panel.py

An initializer method comes first. Note that this method has to be

called “__init__()” in Python in order to be called automatically

when the object is created.

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test drive

Test Drive

OK, so you’re finally ready to start up your new custom widget code. This

is what it looks like after you bring your code into IDLE and press F5:

The checkboxes and volume scales are now grouped together on the

same widget within the GUI. From now on, you will know that the

widgets on your GUI will always stay together as the SoundPanel()

class has grouped them for you.

Dude, that

interface looks

awesome! It‛s

so easy to use!

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custom widgets and classes

Q: I don’t get it. What’s this “self” thing again?

A: “self” is a variable that refers to the calling object.

Q: The calling object? What does that mean?

A: Imagine you create a new method called bleep() in

the SoundPanel class. An object called panel can use

the bleep() method as needed, and when it does, it calls the

SoundPanel class. The “self.” bit ensures that the bleep()

method associated with the panel object is called, not some

other bleep() method associated with some other object.

Q: So objects don’t actually own their own methods?

A: In some languages, such as Ruby, they can, but in most

languages, no, objects don’t own their own methods. The methods

all belong to the class that created the object.

Q: But why do I need to add “self.” to the start of the

variables?

A: Because then you are changing data inside the current

object referred to by “self.”. You will be working with the object’s

own data, and not with data that belongs to the class.

Q: Not all the variables had “self.” at the beginning. Why

is that?

A: If you look at the code, the volume_scale variable

does not begin with “self.”. That’s because the object does not

need to keep track of the volume_scale variable once the

initializer method has finished creating the object. volume_

scale is a variable that is local to the initializer.

The SoundPanel widget is a type of frame.

Objects get created by classes.

A class has methods.

The methods define the behavior of the

object.

When an object needs to know what to do,

it calls a method in the class that created it.

The methods in the class all have a self

variable.

The self variable points to the object

that called the method.

By prefixing variables with “self.”, you

can keep each object’s values separate

from each other.

378 Chapter 10

add the tracks

The DJ has an entire directory of tracks

The DJ is so impressed by how usable your program is that he wants to

try it out tonight for his complete set, prior to its official unveiling at

the World Music Mixing Expo. The DJ needs to work with a lot more than

two tracks. In fact, he has an entire directory full of loops.

I want to use all

of these loops. Can

your program add all

of these WAV files

to my interface?

Now you could just change the code to add the extra files to the interface,

but the DJ wants to be able to manage which tracks the program uses.

So you will have to find all of the WAV files in the current directory and

then add them to the interface when the program starts.

Let’s get this thing to work.

A Learner's Guide To Programming Using The Python Language

A%20Learner's%20Guide%20to%20Programming%20Using%20the%20Python%20Language%20(2009)

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