Python Basics: A Practical Introduction to Python 3

PythonBasics: APracticalIntroduction

toPython3

RealPython

PythonBasics: APracticalIntroductiontoPython3

RevisedandUpdated4thEdition

DavidAmos,DanBader,JoannaJablonski,FletcherHeisler

realpython.com),2012–2020

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What Pythonistas Say About Python Basics: A Practical In-

troduction to Python 3

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I’ve looked at over 10 dierent Python tutorials/books/online courses,

and I’ve probably learned the most from Real Python!”

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need a quick refresher on usage of vital Python commands.”

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dozens of incomplete online tutorials. I snoozed through hours of bor-

ing screencasts. I gave up on countless crufty books from big-time

publishers. And then I found Real Python.

The easy-to-follow, step-by-step instructions break the big concepts

down into bite-sized chunks written in plain English. The authors

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in their explanations. I’m up and running now, but I constantly refer

to the material for guidance.”

—Jared Nielsen

“I love the book because at the end of each particular lesson there are

real world and interesting challenges. I just built a savings estimator

that actually reects my savings account – neat!”

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people on my team to help them in their everyday duties. When my

managers noticed that, I was oered a new position as a developer.

I know there is heaps of things to learn and there will be huge chal-

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looking really hard for things that could maybe be added or improved,

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with a little push from you I am learning it and I can see that it will be

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About the Authors

Real Python you’lllearnreal-worldprogrammingskillsfromacom-

munityofprofessionalPythonistasfromallaroundtheworld.

The

realpython.com website launched in 2012 and currently helps

more than three million Python developers each month with free

programmingtutorialsandin-depthlearningresources.

Everyonewhoworkedonthisbookisapractitioner withseveralyears

ofprofessionalexperienceinthesoftwareindustry. Herearethemem-

bersoftheReal Python tutorialteamwhoworkedonPython Basics:

David Amos isthecontenttechnicalleadforReal Python. Afterleav-

ingacademiain2015,Davidworkedinvarious technical positions as

a programmer and data scientist. In 2019, David joined Real Python

full time to pursue his passion for education. He lead the charge on

rewritingandupdatingthePython Basics curriculumtoPython3.

Dan Bader is the owner and editor in chief of Real Python and the

main developer of the realpython.com learning platform. Dan has

been writing code for more than twenty years and holds a master’s

degree in computer science. He’s the author of Python Tricks, a best-

sellingprogrammingbookforintermediatePythondevelopers.

Joanna Jablonski is the executive editor of Real Python. She likes

natural languages just as much as she likes programming languages.

Herloveforpuzzles,patterns,andpeskylittledetailsledhertofollow

acareer in translation. It was only a matter of time before she would

fall in love with a new language: Python! She joined Real Python in

2018andhasbeenhelpingPythonistaslevelupeversince.

Fletcher Heisler is the founder of Hunter2, where he teaches de-

velopers how to hack and secure modern web apps. As one of the

foundingmembersofReal Python,Fletcherwrote the ﬁrst version of

thePythoncurriculumthisbookisbasedonin2012.

Contents

Contents 8

Foreword 13

1 Introduction 20

1.1 WhyThisBook? . . . . . . . . . . . . . . . . . . . . 21

1.2 AboutRealPython . . . . . . . . . . . . . . . . . . 23

1.3 HowtoUseThisBook . . . . . . . . . . . . . . . . . 24

1.4 BonusMaterialandLearningResources . . . . . . . 25

2 Setting Up Python 29

2.1 ANoteonPythonVersions . . . . . . . . . . . . . . 30

2.2 Windows . . . . . . . . . . . . . . . . . . . . . . . 31

2.3 macOS . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.4 UbuntuLinux . . . . . . . . . . . . . . . . . . . . . 37

3 Your First Python Program 42

3.1 WriteaPythonProgram . . . . . . . . . . . . . . . 43

3.2 MessThingsUp . . . . . . . . . . . . . . . . . . . . 47

3.3 CreateaVariable . . . . . . . . . . . . . . . . . . . 50

3.4 InspectValuesintheInteractiveWindow . . . . . . 55

3.5 LeaveYourself Helpful Notes . . . . . . . . . . . . . 58

3.6 SummaryandAdditionalResources . . . . . . . . . 60

4 Strings and String Methods 62

4.1 WhatIsaString? . . . . . . . . . . . . . . . . . . . 63

4.2 Concatenation,Indexing,andSlicing . . . . . . . . . 69

Contents

4.3 ManipulateStringsWithMethods . . . . . . . . . . 79

4.4 InteractWith User Input . . . . . . . . . . . . . . . 85

4.5 Challenge: Pick Apart Your User’s Input . . . . . . . 88

4.6 WorkingWith Strings and Numbers . . . . . . . . . 88

4.7 StreamlineYourPrintStatements . . . . . . . . . . 94

4.8 FindaStringinaString . . . . . . . . . . . . . . . . 96

4.9 Challenge: Turn Your User Into a L33t H4x0r . . . . 99

4.10 Summary and Additional Resources . . . . . . . . . 100

5 Numbers and Math 102

5.1 IntegersandFloating-PointNumbers . . . . . . . . 103

5.2 ArithmeticOperators and Expressions . . . . . . . . 107

5.3 Challenge: Perform Calculations on User Input . . . 115

5.4 Make Python Lie to You . . . . . . . . . . . . . . . . 116

5.5 MathFunctionsandNumberMethods . . . . . . . . 118

5.6 Print Numbers in Style . . . . . . . . . . . . . . . . 123

5.7 ComplexNumbers . . . . . . . . . . . . . . . . . . 126

5.8 SummaryandAdditionalResources . . . . . . . . . 130

6 Functions and Loops 132

6.1 WhatIsaFunction,Really? . . . . . . . . . . . . . . 133

6.2 WriteYourOwnFunctions . . . . . . . . . . . . . . 137

6.3 Challenge: ConvertTemperatures . . . . . . . . . . 146

6.4 Runin Circles . . . . . . . . . . . . . . . . . . . . . 147

6.5 Challenge: Track Your Investments . . . . . . . . . . 156

6.6 UnderstandScope in Python . . . . . . . . . . . . . 157

6.7 SummaryandAdditionalResources . . . . . . . . . 162

7 Finding and Fixing Code Bugs 164

7.1 Use the Debug Control Window . . . . . . . . . . . 165

7.2 SquashSomeBugs . . . . . . . . . . . . . . . . . . 171

7.3 Summary and Additional Resources . . . . . . . . . 179

8 Conditional Logic and Control Flow 181

8.1 CompareValues . . . . . . . . . . . . . . . . . . . . 182

8.2 AddSomeLogic . . . . . . . . . . . . . . . . . . . . 186

8.3 Control the Flow of Your Program . . . . . . . . . . 194

Contents

8.4 Challenge: FindtheFactorsofaNumber . . . . . . . 206

8.5 BreakOutofthePattern . . . . . . . . . . . . . . . 207

8.6 RecoverFromErrors . . . . . . . . . . . . . . . . . 211

8.7 Simulate Events and Calculate Probabilities . . . . . 217

8.8 Challenge: SimulateaCoinTossExperiment . . . . . 223

8.9 Challenge: SimulateanElection . . . . . . . . . . . 223

8.10 Summary and Additional Resources . . . . . . . . . 224

9 Tuples, Lists, and Dictionaries 226

9.1 TuplesAreImmutableSequences . . . . . . . . . . . 227

9.2 ListsAreMutableSequences . . . . . . . . . . . . . 237

9.3 Nesting,Copying,andSortingTuplesandLists . . . 251

9.4 Challenge: List of lists . . . . . . . . . . . . . . . . . 257

9.5 Challenge: Wax Poetic . . . . . . . . . . . . . . . . 258

9.6 StoreRelationships in Dictionaries . . . . . . . . . . 260

9.7 Challenge: CapitalCityLoop . . . . . . . . . . . . . 270

9.8 HowtoPickaDataStructure . . . . . . . . . . . . . 272

9.9 Challenge: Cats With Hats . . . . . . . . . . . . . . 273

9.10 SummaryandAdditionalResources . . . . . . . . . 274

10 Object-Oriented Programming (OOP) 276

10.1 Deﬁne a Class . . . . . . . . . . . . . . . . . . . . . 277

10.2 Instantiate an Object . . . . . . . . . . . . . . . . . 281

10.3 Inherit From Other Classes . . . . . . . . . . . . . . 287

10.4 Challenge: ModelaFarm . . . . . . . . . . . . . . . 296

10.5 SummaryandAdditionalResources . . . . . . . . . 297

11 Modules and Packages 298

11.1 WorkingWithModules . . . . . . . . . . . . . . . . 299

11.2 WorkingWithPackages . . . . . . . . . . . . . . . . 310

11.3 Summary and Additional Resources . . . . . . . . . 318

12 File Input and Output 320

12.1 FilesandtheFileSystem . . . . . . . . . . . . . . . 321

12.2 WorkingWithFilePathsinPython . . . . . . . . . . 324

12.3 CommonFileSystemOperations . . . . . . . . . . . 333

12.4 Challenge: Move All Image Files to a New Directory . 350

Contents

12.5 ReadingandWritingFiles . . . . . . . . . . . . . . 351

12.6 ReadandWriteCSVData . . . . . . . . . . . . . . . 366

12.7 Challenge: CreateaHighScoresList . . . . . . . . . 377

12.8 SummaryandAdditionalResources . . . . . . . . . 378

13 Installing Packages With pip 379

13.1 Installing Third-Party Packages With pip . . . . . . . 380

13.2 ThePitfallsofThird-PartyPackages . . . . . . . . . 390

13.3 SummaryandAdditionalResources . . . . . . . . . 392

14 Creating and Modifying PDF Files 394

14.1 ExtractingTextFromaPDF . . . . . . . . . . . . . 395

14.2 ExtractingPagesFromaPDF . . . . . . . . . . . . . 402

14.3 Challenge: PdfFileSplitter Class . . . . . . . . . . . 409

14.4 Concatenating and Merging PDFs . . . . . . . . . . 410

14.5 Rotating and Cropping PDF Pages . . . . . . . . . . 417

14.6 Encrypting and Decrypting PDFs . . . . . . . . . . . 428

14.7 Challenge: UnscrambleaPDF . . . . . . . . . . . . 433

14.8 CreatingaPDFFileFromScratch . . . . . . . . . . 433

14.9 Summary and Additional Resources . . . . . . . . . 440

15 Working With Databases 442

15.1 An Introduction to SQLite . . . . . . . . . . . . . . 443

15.2 LibrariesforWorkingWithOtherSQLDatabases . . 455

15.3 Summaryand Additional Resources . . . . . . . . . 456

16 Interacting With the Web 458

16.1 Scrape and Parse TextFromWebsites . . . . . . . . 459

16.2 Usean HTMLParsertoScrapeWebsites . . . . . . . 469

16.3 InteractWithHTMLForms . . . . . . . . . . . . . . 475

16.4 Interact With Websites in Real Time . . . . . . . . . 481

16.5 Summary and Additional Resources . . . . . . . . . 485

17 Scientic Computing and Graphing 487

17.1 UseNumPyforMatrixManipulation . . . . . . . . . 488

17.2 UseMatplotlibforPlottingGraphs . . . . . . . . . . 499

17.3 Summary and Additional Resources . . . . . . . . . 522

Contents

18 Graphical User Interfaces 523

18.1 AddGUIElementsWithEasyGUI . . . . . . . . . . 524

18.2 ExampleApp: PDFPageRotator . . . . . . . . . . . 536

18.3 Challenge: PDFPageExtractionApplication . . . . . 543

18.4 IntroductiontoTkinter . . . . . . . . . . . . . . . . 544

18.5 WorkingWithWidgets . . . . . . . . . . . . . . . . 548

18.6 ControllingLayoutWithGeometryManagers . . . . 573

18.7 Making Your Applications Interactive . . . . . . . . 592

18.8 ExampleApp: TemperatureConverter . . . . . . . . 602

18.9 ExampleApp: Text Editor . . . . . . . . . . . . . . 607

18.10Challenge: Return of the Poet . . . . . . . . . . . . . 616

18.11 SummaryandAdditionalResources . . . . . . . . . 618

19 Final Thoughts and Next Steps 620

19.1 Free Weekly Tips for Python Developers . . . . . . . 622

19.2 PythonTricks: The Book . . . . . . . . . . . . . . . 622

19.3 RealPythonVideoCourseLibrary . . . . . . . . . . 623

19.4 Acknowledgements . . . . . . . . . . . . . . . . . . 624

Foreword

Hello, and welcome to Python Basics: A Practical Introduction to

Python 3. Ihope you’re ready to learn why so many professional and

hobbyist developers are drawn to Python and how you can begin

usingitonyourownprojects,smallandlarge,rightaway.

This book is targeted at beginners who either know a little program-

mingbutnotthePythonlanguageandecosystemorarestartingfresh

withnoprogrammingexperiencewhatsoever.

Ifyoudon’thaveacomputersciencedegree,don’tworry. David,Dan,

Joanna, and Fletcher will guide you through the important comput-

ingconceptswhileteaching you thePythonbasicsand,justas impor-

tantly,skippingtheunnecessarydetailsatﬁrst.

Python Is a Full-Spectrum Language

When learning a new programming language, you don’t yet have the

experiencetojudgehowwellitwillserveyouinthelongrun. Ifyou’re

considering learning Python, let me assure you that this is a good

choice. OnekeyreasonisthatPythonisafull-spectrum language.

WhatdoImeanbythis? Somelanguagesareverygoodforbeginners.

They hold your hand and make programming super easy. We can go

totheextremeandlookatvisuallanguagessuchasScratch.

In Scratch, you get blocks that represent programming concepts like

variables,loops,methodcalls,andsoon,andyoudraganddropthem

on a visual surface. Scratch may be easy to get started with for sim-

Contents

ple programs, but you cannot build professional applications with it.

Name one Fortune 500 company that powers its core business logic

withScratch.

Comeupempty? Metoo,becausethatwouldbeinsanity.

Other languages are incredibly powerful for expert developers. The

most popular one in this category is likely C++ and its close relative,

C. Whichever web browser you used today was likely written in C or

C++. Youroperatingsystemrunningthatbrowserwasverylikelyalso

builtwithC/C++. Yourfavoriteﬁrst-personshooterorstrategyvideo

game? Younailedit: C/C++.

You can do amazing things with these languages, but they are wholly

unwelcomingtonewcomerslookingforagentleintroduction.

You might not have read a lot of C++ code. It can almost make your

eyesburn. Here’sanexample,arealalbeitcomplexone:

template <typename T>

_Defer<void(*(PID<T>, void (T::*)(void)))

(const PID<T>&, void (T::*)(void))>

defer(const PID<T>& pid, void (T::*method)(void))

{

void (*dispatch)(const PID<T>&, void (T::*)(void)) =

&process::template dispatch<T>;

return std::tr1::bind(dispatch, pid, method);

}

Please,justno.

Both Scratch and C++ are decidedly not what I would call full-

spectrum languages. With Scratch, it’s easy to start, but you have to

switchtoa“real”languagetobuildrealapplications. Conversely,you

can build real apps with C++, but there’s no gentle on-ramp. You

dive headﬁrst into all the complexity of the language, which exists to

supporttheserichapplications.

Contents

Python, on the other hand, is special. It is a full-spectrum language.

Weoften judge the simplicity ofa language based on the Hello, World

test. Thatis,whatsyntaxandactionsarenecessarytogetthelanguage

tooutputHello, World totheuser? InPython,itcouldn’tbesimpler:

print("Hello, World")

That’sit! However,Iﬁndthisanunsatisfyingtest.

TheHello, World testisusefulbutreallynotenoughtoshowthepower

or complexity of a language. Let’s try another example. Not every-

thinghereneedstomaketotalsense—justfollowalongtogettheZen

ofit. Thebookcoverstheseconceptsandmoreasyougothrough. The

next example is certainly something you could write as you get near

theendofthebook.

Here’s the new test: What would it take to write a program that ac-

cessesanexternalwebsite,downloadsthecontenttoyourappinmem-

ory, then displays a subsection of that content to the user? Let’s try

thatexperiment using Python 3 withthe help of the requests package

(whichneedstobeinstalled—moreonthatinchapter12):

import requests

resp

= requests.get("http://olympus.realpython.org")

html = resp.text

print(html[

86:132])

Incredibly, that’s it! When run, the program outputs something like

this:

<h2>Please log in to access Mount Olympus:</h2>

This is the easy, getting-started side of the Python spectrum. A few

triviallinescanunleashincrediblepower. BecausePythonhasaccess

tosomanypowerfulbutwell-packagedlibraries,suchasrequests,it’s

oftendescribedashaving batteries included.

Contents

Sothereyouhaveasimpleyetpowerful starter example. On the real-

world side of things, many incredible applications have been written

inPythonaswell.

YouTube,theworld’smostpopularvideo streamingsite,iswritten in

Pythonandprocessesmorethanamillionrequestspersecond. Insta-

gramisanotherexampleof aPythonapplication. Closertohome, we

evenhaverealpython.comandmysites,suchastalkpython.fm.

This full-spectrum aspect of Python means that you can start with

the basics and adopt more advanced features as your application de-

mandsgrow.

Python Is Popular

You might have heard that Python is popular. It may seem that it

doesn’t really matter how popular a language is so long as you can

buildtheappyouwanttobuildwithit.

But, for better or worse, the popularity of a programming language

is a strong indicator of the quality of libraries you’ll have available as

wellthenumberofjobopeningsyou’llﬁnd. Inshort,youshouldtend

to gravitate toward more popular technologies as there will be more

choicesandintegrationsavailable.

So, is Python actually that popular? Yes it is. You’ll ﬁnd a lot of

hype and hyperbole, but there are plenty of stats backing this claim.

Let’s look at some analytics presented by stackoverflow.com, a popu-

larquestion-and-answersiteforprogrammers.

StackOverflowrunsasitecalledStackOverflowTrendswhereyoucan

lookatthetrendsforvarioustechnologiesbytag. Whenyoucompare

Contents

Pythontotheotherlikelycandidatesyoucouldpicktolearnprogram-

ming,you’llseeoneisunliketheothers:

Youcanexplorethischartandcreatesimilarchartstothisoneoverat

insights.stackoverflow.com/trends.

Notice the incredible growth of Python compared to the flat or even

downwardtrendofthe other usual candidates! Ifyou’rebettingyour

future on the success of a given technology, which one would you

choosefromthislist?

That’s just one chart—what does it really tell us? Well, let’s look at

another. Stack Overflow does a yearlysurvey of developers. It’s com-

prehensive and very well done. You can ﬁnd the full 2020 results at

insights.stackoverflow.com/survey/2020.

From that writeup, I’d like to call your attention to a section titled

“

Most Loved, Dreaded, and Wanted Languages.” In the “Most

Wanted”section,you’ll ﬁnd data on the shareof“developers who are

not developing with the language or technology but have expressed

interestindevelopingwithit.”

Contents

Again,inthegraphbelow,you’llseethatPythonistoppingthecharts

andiswellaboveevensecondplace:

If you agree with me that the relative popularity of a programming

languagematters,thenPythonisclearlyagoodchoice.

We Don’t Need You to Be a Computer Scientist

One other point that I want to emphasize as you start your Python

learning journey is that we don’t need you to be a computer scientist.

If that’s your goal, then great. Learning Python is a powerful step

in that direction. But the invitation to learn programming is often

framed as “We have all these developer jobs going unﬁlled! We need

softwaredevelopers!”

That may or may not be true. But, more importantly, programming

(evenalittleprogramming)canbeapersonalsuperpowerforyou.

To illustrate this idea, suppose you are a biologist. Should you drop

out of biology and get a job as a front-end web developer? Probably

not. But skills such as the one I opened this foreword with, using re-

queststogetdatafromtheWeb,canbeincrediblypowerfulforyouas

abiologist.

Rather than manually exporting and scraping data from the Web or

from spreadsheets, you can use Python to scrape thousands of data

sources or spreadsheets in the time it takes you to do just one man-

Contents

ually. Python skills can take your biology power and amplify it well

beyondyourcolleagues’tomakeityoursuperpower.

Dan and Real Python

Finally,letmeleaveyouwithacommentonyourauthors. DanBader

and the other Real Python authors work day in and day out to bring

clear and powerful explanations of Python concepts to all of us via

realpython.com.

TheyhaveauniqueviewintothePythonecosystemandarekeyedinto

whatbeginnersneedtoknow.

I’m conﬁdent leaving you in their hands on this Python journey. Go

forth and learn this amazing language using this great book. Most

importantly,remembertohavefun!

—Michael Kennedy, Founder of Talk Python (

@mkennedy)

Chapter 1

Introduction

Welcome to Real Python’s Python Basics book, fully updated for

Python 3.9! In this book, you’ll learn real-world Python program-

mingtechniques,illustratedwithusefulandinterestingexamples.

Whether you’re a new programmer or a professional software devel-

oper looking to dive into a new language, this book will teach you all

the practical Python that you need to get started on projects of your

own.

No matter what your ultimate goals may be, if you work with a com-

puterat all, then you’ll soon be ﬁnding endless ways to improve your

life by automating tasks and solving problems through Python pro-

gramsthatyoucreate.

But what’s so great about Python as a programming language? For

one, Python is open source freeware, meaning you can download it

forfreeanduseitforanypurpose,commercialornot.

Python also has an amazing community that has built a number of

useful tools that you can use in your own programs. Need to work

withPDFdocuments? There’sacomprehensivetoolforthat. Wantto

collectdatafromwebpages? Noneedtostartfromscratch!

1.1. Why This Book?

Python was built to be easier to use than other programming lan-

guages. It’susuallymucheasiertoreadPythoncodeand muchfaster

towritecodeinPythonthaninotherlanguages.

Forinstance,here’ssomebasiccodewritteninC,anothercommonly

usedprogramminglanguage:

#include <stdio.h>

int main(void)

{

printf("Hello, World\n");

}

Alltheprogramdoesisshowthetext Hello, World onthescreen. That

wasalotofworktooutputonephrase! Here’sthesameprogramwrit-

teninPython:

print("Hello, World")

That’s pretty simple, right? The Python code is faster to write and

easiertoread. Weﬁndthatitlooksfriendlierandmoreapproachable,

too!

At the same time, Python has all the functionality of other languages

andmore. Youmightbesurprisedbyhowmanyprofessionalproducts

are built on Python code: Instagram, YouTube, Reddit, Spotify, to

namejustafew.

Pythonisnotonlyafriendlyandfunlanguagetolearn,butitalsopow-

ersthetechnologybehindmultipleworld-classcompaniesandoﬀers

fantasticcareeropportunitiesforanyprogrammerwhomastersit.

1.1 Why This Book?

Let’s face it: there’s an overwhelming amount of information about

Python on the Internet. But many beginners studying on their own

havetroubleﬁguringoutwhat tolearnand in what order tolearnit.

1.1. Why This Book?

Youmaybeasking yourself, What should I learn about Python inthe

beginning to get a strong foundation? If so, then this book is for you,

no matter if you’re a complete beginner or if you’ve already dabbled

inPythonorotherlanguages.

Python Basics is written in plain English and breaks down the core

concepts that you really need to know into bite-sized chunks. This

meansyou’lllearnenoughtobedangerouswithPython,fast.

Insteadofjustgoingthrougha boring list of languagefeatures,you’ll

see exactly how the diﬀerent building blocks ﬁt together and what’s

involvedinbuildingrealapplicationsandscriptswithPython.

Stepbystep,you’llmasterfundamentalPythonconceptsthatwillhelp

yougetstartedonyourjourneytowardlearningPython.

Many programming books try to cover every last possible variation

of every command, which makes it easy for readers to get lost in the

details. Thisapproachisgreatifyou’relookingforareferencemanual,

butit’sahorrible way to learn a programming language. Not only do

you spend most of your time cramming things into your head that

you’llneveruse,butyoualsodon’thaveanyfun!

Thisbookisbuiltonthe80/20principle,whichsuggeststhatyoucan

learnmostofwhatyouneedtoknowbyfocusingonafewcrucialcon-

cepts. We’llcover thecommandsandtechniquesusedinthevastma-

jority of cases and focus on how to program real-world solutions to

everydayproblems.

Thisway,weguaranteethatyouwill:

• Learnusefulprogrammingtechniquesquickly

• Spendlesstimestrugglingwithunimportantcomplications

• FindmorepracticalusesforPythoninyourownlife

• Havemorefunintheprocess

1.2. About Real Python

Onceyou’ve mastered the material in this book, you will have gained

astrongenoughfoundationthatventuringoutonyourownintomore

advancedterritorywillbeabreeze.

What you’ll learn here is based on the ﬁrst part of the original Real

Python Course initially released in 2012. Over the years, this Python

curriculum has been battle-tested by thousands of Pythonistas, data

scientists, and developers working for companies big and small, in-

cludingAmazon,RedHat,andMicrosoft.

ForPython Basics, we’ve thoroughly expanded, reﬁned, and updated

thematerialsoyoucanbuildyourPythonskillsquicklyandeﬃciently.

1.2 About Real Python

At Real Python, you’ll learn real-world programming skills from a

communityofprofessionalPythonistasfromallaroundtheworld.

The

realpython.com website launched in 2012 and currently helps

more than three million Python developers each month with books,

programmingtutorials,andotherin-depthlearningresources.

Everyonewhoworked onthisbookisa Python practitioner recruited

from the Real Python team with several years of professional experi-

enceinthesoftwareindustry.

Here’swhereyoucanﬁndReal Python ontheWeb:

•

realpython.com

• @realpythononTwitter

• TheReal Python Newsletter

• TheReal Python Podcast

1.3. How to Use This Book

1.3 How to Use This Book

The ﬁrst half of this book is a quick but thorough overview of all the

Python fundamentals. You don’t need any prior experience with pro-

grammingtogetstarted. Thesecondhalfisfocusedonﬁndingpracti-

calsolutionstointeresting,real-worldcodingproblems.

Ifyou’reabeginner,thenwerecommendthatyougothroughtheﬁrst

halfofthisbookfrombeginningtoend. Thesecondhalfcoverstopics

that don’t overlap as much, so you can jump around more easily, but

thechaptersdoincreaseindiﬃcultyasyougoalong.

Ifyou’reamoreexperiencedprogrammer,thenyoumayﬁndyourself

headingtoward the second part of the book right away. But don’t ne-

glectgetting a strong foundation in the basicsﬁrst, and besure to ﬁll

inanyknowledgegapsalongtheway.

Mostsections within a chapter are followed by review exercises to

helpyoumakesurethatyou’vemasteredallthetopicscovered. There

arealsoanumberof code challenges,whicharemoreinvolvedand

usuallyrequireyoutotietogetherseveraldiﬀerentconceptsfrompre-

viouschapters.

Thepracticeﬁlesthataccompanythisbookalsoincludefullsolutions

to the challenges as well as some of the trickier exercises. But to get

themostoutofthematerial,youshouldtryyourbesttosolvethechal-

lengeproblemsonyourownbeforelookingattheexamplesolutions.

Ifyou’recompletelynewto programming, then you maywanttosup-

plement the ﬁrst few chapters with additional practice. We recom-

mend working through the entry-level tutorials available for free at

realpython.comtomakesureyou’reonsolidfooting.

If you have any questions or feedback about the book, you’re always

welcometo

contactusdirectly.

1.4. Bonus Material and Learning Resources

Learning by Doing

This book is all about learning by doing, so be sure to actually type

in the code snippets you encounter in the book. For best results, we

recommendthatyouavoidcopyingandpastingthecodeexamples.

You’ll learn the concepts better and pick up the syntax faster if you

typeouteachline ofcodeyourself. Plus,ifyou screwup—whichisto-

tallynormalandhappenstoalldevelopersonadailybasis—thesimple

actofcorrectingtyposwillhelpyoulearnhowtodebugyourcode.

Trytocompletethereviewexercisesandcodechallengesonyourown

before getting help from outside resources. With enough practice,

you’llmasterthismaterial—andhavefunalongtheway!

How Long Will It Take to Finish This Book?

If you’re already familiar with a programming language, then you

couldﬁnishthisbookinaslittleasthirty-ﬁvetofortyhours. Ifyou’re

newtoprogramming,thenyoumayneedtospenduptoonehundred

hoursormore.

Takeyourtimeanddon’tfeellikeyouhavetorush. Programmingisa

super-rewardingbutcomplexskilltolearn. GoodluckonyourPython

journey. We’rerootingforyou!

1.4 Bonus Material and Learning

Resources

This book comes with a number of free bonus resources and down-

loads that you can access online at the link below. We’re also main-

taininganerratalistwithcorrectionsthere:

realpython.com/python-basics/resources

1.4. Bonus Material and Learning Resources

Interactive Quizzes

Mostchaptersinthisbookcomewithafreeonlinequiztocheckyour

learningprogress. Youcanaccessthequizzesusingthelinksprovided

at the end of the chapter. The quizzes are hosted on the Real Python

websiteandcanbeviewedonyourphoneorcomputer.

Eachquiztakesyouthroughaseriesofquestionsrelated toaparticu-

lar chapter in the book. Some of them are multiple choice, some will

ask you to type in an answer, and some will require you to write ac-

tual Python code. As you make your way through each quiz, it will

keepscoreofwhichquestionsyouansweredcorrectly.

At the end of the quiz, you’ll receive a grade based on your result. If

youdon’tscore100percentonyourﬁrsttry,don’tfret! Thesequizzes

aremeanttochallengeyou. It’s expected that you’ll go through them

severaltimes,improvingyourscorewitheachrun.

Exercises Code Repository

This book has an accompanying

code repository on the Web contain-

ingexample source code as wellas the answers to exercisesand code

challenges. The repository is broken up by chapter, so you can check

your code against the solutions provided by us after you ﬁnish each

chapter. Here’sthelink:

realpython.com/python-basics/exercises

Note

ThecodefoundinthisbookhasbeentestedwithPython3.9on

Windows,macOS,andLinux.

1.4. Bonus Material and Learning Resources

Example Code License

TheexamplePythonscriptsassociatedwiththisbookarelicensedun-

der a

Creative Commons Public Domain (CC0) License. This means

thatyou’rewelcometouseanyportionofthecodeforany purposein

yourownprograms.

Formatting Conventions

Codeblockswillbeusedtopresentexamplecode:

# This is Python code:

print("Hello, World")

TerminalcommandsfollowtheUnixformat:

$ # This is a terminal command:

$ python hello-world.py

(Thedollarsignsarenotpartofthecommand.)

Monospace text willbeusedtodenoteaﬁlename: hello-world.py.

Bold text will be used to denote a new or important term.

Keyboardshortcutswillbeformattedasfollows:

Ctrl

Menushortcutswillbeformattedasfollows:

File New File

Notesandimportantinformationwillbehighlightedasfollows:

Note

This is a note ﬁlled in with placeholder text. The quick brown

fox jumps over the lazy dog. The quick brown Python slithers

overthelazyhog.

1.4. Bonus Material and Learning Resources

Feedback and Errata

We welcome ideas, suggestions, feedback, and the occasional rant.

Did you ﬁnd a topic confusing? Did you ﬁnd an error in the text or

code? Didweleaveoutatopicthatyou’dlovetoknowmoreabout?

We’re always looking to improve our teaching materials. Whatever

thereason,pleasesendinyourfeedbackatthelinkbelow:

realpython.com/python-basics/feedback

Chapter 2

Setting Up Python

This book is about programming computers with Python. You could

readthis book from cover to cover without ever touching a keyboard,

butyou’dmissoutonthefunpart—coding!

To get the most out of this book, you need a computer with Python

installedonitandawaytocreate,edit,andsavePythoncodeﬁles.

In this chapter, you’ll learn how to:

• InstallthelatestversionofPython3onyourcomputer

• Open IDLE, Python’s built-in Integrated Development and

LearningEnvironment

Let’sgetstarted!

2.1. A Note on Python Versions

2.1 A Note on Python Versions

Many operating systems, including macOS and Linux, come with

Python preinstalled. The version of Python that comes with your

operatingsystemiscalledthesystem Python.

ThesystemPythonisusedbyyouroperatingsystemandisusuallyout

ofdate. It’s essentialthatyouhavethemostrecentversionof Python

so that you can successfully follow along with the examples in this

book.

Important

DonotattempttouninstallthesystemPython!

YoucanhavemultipleversionsofPythoninstalledonyourcomputer.

In this chapter, you’ll install the latest version of Python 3 alongside

anysystemPythonthatmayalreadyexistonyourmachine.

Note

Even if you already have Python 3.9 installed, it’s still a good

idea to skim this chapter to double-check that your environ-

mentissetupforfollowingalongwiththisbook.

This chapter is split into three sections: Windows, macOS, and

UbuntuLinux. Findthesectionforyour operating system andfollow

thestepstogetsetup,thenskipaheadtothenextchapter.

Ifyouhaveadiﬀerentoperatingsystem,thencheckoutReal Python’s

“Python 3 Installation & Setup Guide” to see if your OS is covered.

Readersontabletsandmobiledevicescanrefertothe“OnlinePython

Interpreters”

sectionforsomebrowser-basedoptions.

2.2. Windows

2.2 Windows

FollowthesestepstoinstallPython3andopenIDLEonWindows.

Important

Thecode in this book is tested only against Python installedas

describedinthissection.

Beaware that if you have installed Python through some other

means,suchasAnacondaPython,youmayencounterproblems

whenrunningsomeofthecodeexamples.

Install Python

Windows doesn’t typically come with a system Python. Fortunately,

installation involves little more than downloading and running the

Pythoninstallerfromthe

Python.orgwebsite.

Step 1: Download the Python 3 Installer

OpenawebbrowserandnavigatetothefollowingURL:

https://www.python.org/downloads/windows/

Click Latest Python 3 Release - Python 3.x.x located beneath the

“Python Releases for Windows” heading near the top of the page. As

ofthiswriting,thelatestversionwasPython3.9.

Then scroll to the bottom and click Windows x86-64 executable in-

staller tostartthedownload.

Note

If your system has a 32-bit processor, then you should choose

the32-bitinstaller. Ifyouaren’tsureifyourcomputeris32-bit

or64-bit,stickwiththe64-bitinstallermentionedabove.

2.2. Windows

Step 2: Run the Installer

Open your Downloads folder in Windows Explorer and double-click

the ﬁle to run the installer. A dialog that looks like the following one

willappear:

It’sokayifthePythonversionyou see is greater than 3.9.0 as long as

theversionisnotlessthan3.

Important

MakesureyouselecttheboxthatsaysAdd Python 3.x to PATH.

If you install Python without selecting this box, then you can

runtheinstalleragainandselectit.

Click

Install Now

toinstallPython3. Waitfortheinstallationtoﬁnish,

thencontinuetoopenIDLE.

2.2. Windows

Open IDLE

YoucanopenIDLEintwosteps:

1. ClicktheStartmenuandlocatethePython3.9folder.

2. OpenthefolderandselectIDLE (Python 3.9).

IDLEopensaPython shell inanewwindow. ThePythonshellisan

interactive environment that allows you to type in Python code and

executeitimmediately. It’sagreatwaytogetstartedwithPython!

Note

While you’re free to use a code editor other than IDLE if you

prefer, note that some chapters, especially chapter 7, “Finding

andFixingCodeBugs,”docontainmaterialspeciﬁctoIDLE.

ThePythonshellwindowlookslikethis:

At the top of the window, you can see the version of Python that is

runningandsomeinformationabouttheoperatingsystem. Ifyousee

a version less than 3.9, then you may need to revisit the installation

instructionsintheprevioussection.

2.3. macOS

The >>> symbol that you see is called a prompt. Whenever you see

this, it means that Python is waiting for you to give it some instruc-

tions.

Interactive Quiz

This chapter comes with a free online quiz to check your learn-

ingprogress. Youcanaccessthequizusingyourphoneorcom-

puteratthefollowingwebaddress:

realpython.com/quizzes/pybasics-setup

NowthatyouhavePythoninstalled,let’sgetstraightintowritingyour

ﬁrstPythonprogram! Goaheadandmoveontochapter3.

2.3 macOS

FollowthesestepstoinstallPython3andopenIDLEonmacOS.

Important

Thecode in this book is tested only against Python installedas

describedinthissection.

Beaware that if you have installed Python through some other

means,suchasAnacondaPython,youmayencounterproblems

whenrunningsomeofthecodeexamples.

Install Python

ToinstallthelatestversionofPython3onmacOS,downloadandrun

theoﬃcialinstallerfromthe

Python.orgwebsite.

Step 1: Download the Python 3 Installer

OpenawebbrowserandnavigatetothefollowingURL:

https://www.python.org/downloads/mac-osx/

2.3. macOS

Click Latest Python 3 Release - Python 3.x.x located beneath the

“PythonReleases for Mac OSX” heading near thetop of the page. As

ofthiswriting,thelatestversionwasPython3.9.

Thenscrolltothebottomofthepageandclick macOS 64-bit installer

tostartthedownload.

Step 2: Run the Installer

OpenFinderanddouble-clickthedownloadedﬁletoruntheinstaller.

Adialogboxthatlookslikethefollowingwillappear:

Press

Continue

afewtimesuntilyouareaskedtoagreetothesoftware

licenseagreement. Thenclick

Agree

You’llbeshownawindowthattellsyouwherePythonwillbeinstalled

andhowmuchspaceitwilltake. Youmostlikelydon’twanttochange

the default location, so go ahead and click

Install

to start the installa-

tion.

2.3. macOS

When the installer is ﬁnished copying ﬁles, click

to close the

installerwindow.

Open IDLE

YoucanopenIDLEinthreesteps:

1. OpenFinderandclickApplications.

2. Double-clickthePython3.9folder.

3. Double-clicktheIDLEicon.

IDLEopensaPython shell inanewwindow. ThePythonshellisan

interactive environment that allows you to type in Python code and

executeitimmediately. It’sagreatwaytogetstartedwithPython!

Note

While you’re free to use a code editor other than IDLE if you

prefer, note that some chapters, especially chapter 7, “Finding

andFixingCodeBugs,”docontainmaterialspeciﬁctoIDLE.

ThePythonshellwindowlookslikethis:

2.4. Ubuntu Linux

At the top of the window, you can see the version of Python that is

runningandsomeinformationabouttheoperatingsystem. Ifyousee

a version less than 3.9, then you may need to revisit the installation

instructionsintheprevioussection.

The >>> symbol that you see is called a prompt. Whenever you see

this, it means that Python is waiting for you to give it some instruc-

tions.

Interactive Quiz

This chapter comes with a free online quiz to check your learn-

ingprogress. Youcanaccessthequizusingyourphoneorcom-

puteratthefollowingwebaddress:

realpython.com/quizzes/pybasics-setup

NowthatyouhavePythoninstalled,let’sgetstraightintowritingyour

ﬁrstPythonprogram! Goaheadandmoveontochapter3.

2.4 Ubuntu Linux

Follow these steps to install Python 3 and open IDLE on Ubuntu

Linux.

Important

Thecode in this book is tested only against Python installedas

describedinthissection.

Beaware that if you have installed Python through some other

means,suchasAnacondaPython,youmayencounterproblems

whenrunningsomeofthecodeexamples.

2.4. Ubuntu Linux

Install Python

There’s a good chance that your Ubuntu distribution already has

Python installed, but it probably won’t be the latest version, and it

maybePython2insteadofPython3.

Toﬁndoutwhatversion(s)youhave,openaterminalwindowandtry

thefollowingcommands:

$ python --version

$ python3 --version

One or more of these commands should respond with a version, as

below:

$ python3 --version

Python 3.9.0

Your version number may vary. If the version shown is Python 2.x

or a version of Python 3 that is less than 3.9, then you want to in-

stall the latest version. How you install Python on Ubuntu depends

onwhich version of Ubuntu you’re running. Youcan determine your

localUbuntuversionbyrunningthefollowingcommand:

$ lsb_release -a

No LSB modules are available.

Distributor ID: Ubuntu

Description: Ubuntu 18.04.1 LTS

Release: 18.04

Codename: bionic

LookattheversionnumbernexttoRelease intheconsoleoutput,and

followthecorrespondinginstructionsbelow.

2.4. Ubuntu Linux

Ubuntu 18.04 or Greater

Ubuntu version 18.04 does not come with Python 3.9 by default, but

it is in the Universe repository. You can install it with the following

commandsintheTerminalapplication:

$ sudo apt-get update

sudo apt-get install python3.9 idle-python3.9 python3-pip

Note that because the Universe repository is usually behind the

Pythonreleaseschedule,youmaynotgetthelatestversionofPython

3.9. However,anyversionofPython3.9willworkforthisbook.

Ubuntu 17 and Lower

For Ubuntu versions 17 and lower, Python 3.9 is not in the Universe

repository. YouneedtogetitfromaPersonalPackageArchive(PPA).

To install Python from the

deadsnakes PPA, run the following com-

mandsintheTerminalapplication:

$ sudo add-apt-repository ppa:deadsnakes/ppa

$ sudo apt-get update

sudo apt-get install python3.9 idle-python3.9 python3-pip

YoucancheckthatthecorrectversionofPythonwasinstalledbyrun-

ningpython3 --version. Ifyouseeaversionnumberlessthan3.9,then

you may need to type python3.9 --version. Now you can open IDLE

andgetreadytowriteyourﬁrstPythonprogram.

Open IDLE

YoucanopenIDLEfromthecommandlinebytypingthefollowing:

$ idle-python3.9

2.4. Ubuntu Linux

On some Linux installations, you can open IDLE with the following

shortenedcommand:

$ idle3

IDLEopensaPython shell inanewwindow. ThePythonshellisan

interactive environment that allows you to type in Python code and

executeitimmediately. It’sagreatwaytogetstartedwithPython!

Note

While you’re free to use a code editor other than IDLE if you

prefer, note that some chapters, especially chapter 7, “Finding

andFixingCodeBugs,”docontainmaterialspeciﬁctoIDLE.

ThePythonshellwindowlookslikethis:

At the top of the window, you can see the version of Python that is

runningandsomeinformationabouttheoperatingsystem. Ifyousee

a version less than 3.9, then you may need to revisit the installation

instructionsintheprevioussection.

2.4. Ubuntu Linux

Important

IfyouopenIDLEwiththeidle3 commandandseeaversionless

than3.9displayedinthePythonshellwindow,thenyou’llneed

toopenIDLEwiththeidle-python3.9 command.

The >>> symbol that you see in the IDLE window is called a prompt.

Wheneveryouseethis,itmeansthatPythoniswaitingforyoutogive

itsomeinstructions.

Interactive Quiz

This chapter comes with a free online quiz to check your learn-

ingprogress. Youcanaccessthequizusingyourphoneorcom-

puteratthefollowingwebaddress:

realpython.com/quizzes/pybasics-setup

NowthatyouhavePythoninstalled,let’sgetstraightintowritingyour

ﬁrstPythonprogram! Goaheadandmoveontochapter3.

Chapter 3

Your First Python Program

NowthatyouhavethelatestversionofPythoninstalledonyourcom-

puter,it’stimetostartcoding!

In this chapter, you will:

• WriteyourﬁrstPythonprogram

• Learnwhathappenswhenyourunaprogramwithanerror

• Learnhowtodeclareavariableandinspectitsvalue

• Learnhowtowritecomments

ReadytobeginyourPythonjourney? Let’sgo!

3.1. Write a Python Program

3.1 Write a Python Program

Ifyoudon’talreadyhaveIDLEopen,thengoaheadandopenit. There

aretwomainwindowsthatyou’llworkwithinIDLE:theinteractive

window, which is the one that opens when you start IDLE, and the

editor window.

Youcantypecodeintoboththeinteractivewindowandtheeditorwin-

dow. Thediﬀerencebetweenthetwowindows is in how theyexecute

code. In this section, you’ll learnhowto execute Python code in both

windows.

The Interactive Window

IDLE’s interactive window contains a Python shell, which is a tex-

tualuserinterfaceusedtointeractwiththePythonlanguage. Youcan

typea bitofPythoncodeintotheinteractivewindowandpress

Enter

toimmediatelyseetheresults. Hencethenameinteractive window.

The interactive window opens automatically when you start IDLE.

You’ll see the following text, with some minor diﬀerences depending

onyoursetup,displayedatthetopofthewindow:

Python 3.9.0 (tags/v3.9.0:1b293b6)

[MSC v.1916 32 bit (Intel)] on win32

Type "help", "copyright", "credits" or "license" for more information.

>>>

This text shows the version of Python that IDLE is running. You

can also see information about your operating system and some

commands you can use to get help and view information about

Python.

The >>> symbol in the last line is called the prompt. This is where

you’lltypeinyourcode.

3.1. Write a Python Program

Goaheadandtype1 + 1 atthepromptandpress

Enter

>>> 1 + 1

>>>

Pythonevaluatestheexpression,displaystheresult(2),thendisplays

anotherprompt. Everytimeyourunsomecodeintheinteractivewin-

dow,anewpromptappearsdirectlybelowtheresult.

ExecutingPythonintheinteractivewindowcanbedescribedasaloop

withthreesteps:

1. Pythonreads thecodeenteredattheprompt.

2. Pythonevaluates thecode.

3. Pythonprints theresultandwaitsformoreinput.

This loop is commonly referred to as a read-evaluate-print loop and

isabbreviatedasREPL.Pythonprogrammerssometimesrefertothe

PythonshellasthePythonREPL,orjust“theREPL”forshort.

Let’s try something a little more interesting than adding numbers. A

riteofpassageforeveryprogrammer iswritingaprogramthatprints

thephrase“Hello,World”onthescreen.

Atthepromptintheinteractivewindow,typethewordprint followed

byasetofparentheseswiththetext"Hello, World" inside:

>>> print("Hello, World")

Hello, World

3.1. Write a Python Program

A function iscodethat performs some task and can be invokedbya

name. Theabovecodeinvokes,orcalls,theprint() functionwiththe

text"Hello, World" asinput.

TheparenthesestellPython tocalltheprint() function. They also en-

closeeverythingthatgetssenttothefunctionasinput. Thequotation

marks indicate that "Hello, World" really is text and not something

else.

Note

IDLE highlights parts of your code in diﬀerent colors as you

typetomakeiteasierforyoutoidentifythediﬀerentparts.

Bydefault,functionsarehighlightedinpurpleandtextishigh-

lightedingreen.

The interactive window executes a single line of code at a time. This

isusefulfortryingoutsmallcodeexamplesandexploringthePython

language, but it has a major limitation: you have to enter your code

onelineatatime!

Alternatively,youcansavePythoncodeinatextﬁleandexecuteallof

thecodeintheﬁletorunanentireprogram.

The Editor Window

You’ll write your Python ﬁles using IDLE’s editor window. You can

opentheeditorwindowbyselecting

File New File

fromthemenuat

thetopoftheinteractivewindow.

Theinteractivewindowstaysopenwhenyouopentheeditorwindow.

Itdisplaystheoutputgeneratedbycodeintheeditorwindow,soyou’ll

wanttoarrangethetwowindowssothatyoucanseethembothatthe

sametime.

3.1. Write a Python Program

Intheeditorwindow,typeinthesamecodeyouusedtoprint"Hello,

World" intheinteractivewindow:

print("Hello, World")

IDLE highlights code typed into the editor window just like in the in-

teractivewindow.

Important

WhenyouwritecodeinaPythonﬁle,youdon’tneedtoinclude

the>>> prompt.

Before you run your program, you need to save it. Select

File Save

fromthemenuandsavetheﬁleashello_world.py.

Note

Onsomesystems,thedefaultdirectoryforsavingﬁlesinIDLE

is the Python installation directory. Do not save your ﬁles to

thisdirectory. Instead,savethemtoyourdesktoportoafolder

inyouruser’shomedirectory.

The .py extension indicates that a ﬁle contains Python code. In fact,

saving your ﬁle with any other extension removes the code highlight-

ing. IDLEonlyhighlightsPythoncodewhenit’sstoredina.py ﬁle.

Running Python Programs in the Editor Window

To run your program, select

Run Run Module

from the menu in the

editorwindow.

Note

Pressing

alsorunsaprogramfromtheeditorwindow.

Programoutputalwaysappearsintheinteractivewindow.

3.2. Mess Things Up

Every time you run code from a ﬁle, you’ll see something like the fol-

lowingoutputintheinteractivewindow:

>>> =================== RESTART ===================

IDLErestartsthePythoninterpreter,whichisthecomputerprogram

thatactuallyexecutesyourcode,everytimeyourunaﬁle. Thismakes

surethatprogramsareexecutedthesamewayeachtime.

Opening Python Files in the Editor Window

To open an existing ﬁle in IDLE, select

File Open

from the menu,

then select the ﬁle you want to open. IDLE opens every ﬁle in a new

editorwindow,soyoucanhaveseveralﬁlesopenatthesametime.

You can also open a ﬁle from a ﬁle manager, such as Windows

Explorer or macOS Finder. Right-click the ﬁle icon and select

Edit with IDLE

toopentheﬁleinIDLE’seditorwindow.

Double-clicking on a .py ﬁle from a ﬁle manager executes the pro-

gram. However, this usually runs the ﬁle with the system Python,

and the program window disappears immediately after the program

terminates—oftenbeforeyoucanevenseeanyoutput.

For now, the best way to run your Python programs is to open them

inIDLE’seditorwindowandrunthemfromthere.

3.2 Mess Things Up

Everybody makes mistakes—especially while programming! In case

you haven’t made any mistakes yet, let’s get a head start and mess

somethinguponpurposetoseewhathappens.

Mistakes in programs are called errors. You’ll experience two main

typesoferrors: syntaxerrorsandruntimeerrors.

3.2. Mess Things Up

Syntax Errors

A syntax error occurswhen you write code thatisn’t allowed in the

Pythonlanguage.

Let’s create a syntax error by removing the last quotation mark from

thecodeinthehello_world.py ﬁlethatyoucreatedinthelastsection:

print("Hello, World)

Save the ﬁle and press

to run it. The code won’t run! IDLE dis-

playsanalertboxwiththefollowingmessage:

EOL while scanning string literal.

Therearetwotermsinthismessagethatmaybeunfamiliar:

1. A string literal is text enclosed in quotation marks. "Hello,

World" isastringliteral.

2. EOL standsforendof line.

So, the message tells you that Python got to the end of a line while

readingastringliteral. String literalsmustbeterminatedwitha quo-

tationmarkbeforetheendofaline.

IDLEhighlightsthelinecontainingprint("Hello, World) inredtohelp

you quickly ﬁnd the line of code with the syntax error. Without the

second quotation mark, everything after the ﬁrst quotation mark—

includingtheclosingparenthesis—ispartofastringliteral.

Runtime Errors

IDLE catches syntax errors before a program starts running. In con-

trast,runtime errors only occur while a program is running.

To generate a runtime error, remove both quotation marks in the

hello_world.py ﬁle:

3.2. Mess Things Up

print(Hello, World)

Didyounoticehowthetextcolorchangedtoblackwhenyouremoved

the quotation marks? IDLE no longer recognizes Hello, World as text.

Whatdoyouthinkwillhappenwhenyouruntheprogram? Press

toﬁndout!

Thefollowingtextdisplaysinredintheinteractivewindow:

Traceback (most recent call last):

File "/home/hello_world.py", line 1, in <module>

print(Hello, World)

NameError: name 'Hello' is not defined

Whenever an error occurs, Python stops executing the program and

displaysseverallinesoftextcalledatraceback. Thetracebackshows

usefulinformationabouttheerror.

Tracebacksarebestreadfromthebottomup:

• The last line of the traceback tells you the name of the error and

the error message. In this case, a NameError occurred because the

nameHello isnotdeﬁnedanywhere.

• Thesecondtolastlineshowsyouthecodethatproducedtheerror.

There’s only one line of code in hello_world.py, so it’s not hard to

guess where the problem is. This information is more helpful for

largerﬁles.

• Thethirdtolastlinetellsyouthenameoftheﬁleandthelinenum-

ber so you can go to the exact spot in your code where the error

occurred.

In the next section, you’ll see how to deﬁne names for values in your

code. Before you move on, though, you can get some practice with

syntaxerrorsandruntimeerrorsbyworkingonthereviewexercises.

3.3. Create a Variable

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. WriteaprogramthatIDLEwon’trunbecauseithasasyntaxerror.

2. Writeaprogramthatcrashesonlywhileit’srunningbecauseithas

aruntimeerror.

3.3 Create a Variable

InPython,variables arenamesthatcanbeassignedavalueandthen

usedtorefertothatvaluethroughoutyourcode.

Variablesarefundamentaltoprogrammingfortworeasons:

1. Variables keep values accessible: For example, you can as-

signtheresultofsome time-consuming operation toavariableso

thatyourprogramdoesn’thavetoperformtheoperationeachtime

youneedtousetheresult.

2. Variables give values context: Thenumber28 couldmeanlots

of diﬀerent things, such as the number of students in a class, the

number of times a user has accessed a website, and so on. Giving

the value 28 a name like num_students makes the meaning of the

valueclear.

Inthissection,you’lllearnhowtousevariablesinyourcode,aswellas

some of the conventions Python programmers follow when choosing

namesforvariables.

The Assignment Operator

An operator is a symbol, such as +, that performs an operation on

one or more values. For example, the + operator takes two numbers,

one to the left of the operator and one to the right, and adds them

together.

3.3. Create a Variable

Values are assigned to variable names using a special symbol called

theassignment operator (=). The= operatortakesthevaluetothe

rightoftheoperatorandassignsittothenameontheleft.

Let’smodifythehello_world.py ﬁlefromtheprevioussectiontoassign

sometextinavariablebeforeprintingittothescreen:

>>> greeting = "Hello, World"

>>> print(greeting)

Hello, world

Ontheﬁrstline,youcreateavariablenamedgreeting andassignitthe

value"Hello, World" usingthe= operator.

print(greeting) displaystheoutputHello, World becausePythonlooks

for the name greeting, ﬁnds that it’s been assigned the value "Hello,

World", and replaces the variable name with its value before calling

thefunction.

If you hadn’t executed greeting = "Hello, World" before executing

print(greeting), then you would have seen a NameError like you did

whenyoutriedtoexecuteprint(Hello, World) intheprevioussection.

Note

Although= looksliketheequalssignfrommathematics,ithasa

diﬀerentmeaninginPython. This distinctionisimportantand

canbeasourceoffrustrationforbeginnerprogrammers.

Justremember,wheneveryouseethe= operator,whateveristo

therightofitisbeingassignedtoavariableontheleft.

Variable names are case sensitive, so a variable named greeting is

notthesameasavariablenamedGreeting. Forinstance,thefollowing

codeproducesaNameError:

3.3. Create a Variable

>>> greeting = "Hello, World"

>>> print(Greeting)

Traceback (most recent call last):

File

"<stdin>", line 1, in <module>

NameError: name 'Greeting' is not defined

If you have trouble with an example in this book, double-check that

everycharacterinyourcode—includingspaces—matchestheexample

exactly. Computers have no common sense, so being almost correct

isn’tgoodenough!

Rules for Valid Variable Names

Variable names can be as long or as short as you like, but there are a

few rules that you must follow. Variable names may contain upper-

case and lowercase letters (A–Z, a–z), digits (0–9), and underscores

(_),buttheycannotbeginwithadigit.

Forexample,eachofthefollowingisavalidPythonvariablename:

• string1

• _a1p4a

• list_of_names

The following aren’t valid variable names because they start with a

digit:

• 9lives

• 99_balloons

• 2beOrNot2Be

In addition to English letters and digits, Python variable names may

containmanydiﬀerentvalidUnicodecharacters.

Unicode is a standard for digitally representing characters used in

most of the world’s writing systems. That means variable names can

containlettersfromnon-English alphabets, suchasdecoratedletters

3.3. Create a Variable

likeéandü,andevenChinese,Japanese,andArabicsymbols.

However,noteverysystemcandisplaydecoratedcharacters, so it’s a

goodideatoavoidthemifyou’regoingtoshareyourcodewithpeople

indiﬀerentregions.

Note

You’lllearnmoreaboutUnicodeinchapter12.

You can also read about Python’s

support for Unicode in the

oﬃcialPythondocumentation.

Just because a variable name is valid doesn’t necessarily mean that

it’sagoodname.

Choosingagoodnameforavariablecanbesurprisinglydiﬃcult. For-

tunately, there are some guidelines that you can follow to help you

choosebetternames.

Descriptive Names Are Better Than Short Names

Descriptive variable names are essential, especially for complex

programs. Writing descriptive names often requires using multiple

words. Don’tbeafraidtouselongvariablenames.

Inthefollowingexample,thevalue3600 isassignedtothevariables:

s = 3600

Thenames istotallyambiguous. Usingafullwordmakesitaloteasier

tounderstandwhatthecodemeans:

seconds = 3600

3.3. Create a Variable

seconds is a bettername than s because it provides more context. But

itstilldoesn’tconveythefullmeaningofthecode. Is3600 thenumber

ofsecondsittakesforaprocesstoﬁnish,orisitthelengthofamovie?

There’snowaytotell.

Thefollowingnameleavesnodoubtaboutwhatthecodemeans:

seconds_per_hour = 3600

When you read the above code, there’s no question that 3600 is the

number of seconds in an hour. seconds_per_hour takes longer to type

than both the single letter s and the word seconds, but the payoﬀ in

clarityismassive.

Althoughnamingvariablesdescriptivelymeans usinglongervariable

names, you should avoid using excessively long names. A good rule

ofthumbistolimitvariablenamestothreeorfourwordsmaximum.

Python Variable Naming Conventions

In many programming languages, it’s common to write variable

names in mixedCase. In this system, you capitalize the ﬁrst letter

of every word except the ﬁrst and leave all other letters in lowercase.

Forexample,numStudents andlistOfNames arewritteninmixedCase.

In Python, however, it’s more common to write variable names in

lower_case_with_underscores. In this system, you leave every

letter in lowercase and separate each word with an underscore. For

instance, both num_students and list_of_names are written using the

lower_case_with_underscoressystem.

There’s no rule mandating that you write your variable names in

lower_case_with_underscores. The practice is codiﬁed, though, in a

document called

PEP 8, which is widely regarded as the oﬃcial style

guideforwritingPython.

3.4. Inspect Values in the Interactive Window

Note

PEP stands for Python Enhancement Proposal. A PEP is a de-

sign document used by the Python community to propose new

featurestothelanguage.

Following the standards outlined in PEP 8 ensures that your Python

code is readable by most Python programmers. This makes sharing

codeandcollaboratingwithotherpeopleeasierforeveryoneinvolved.

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at realpython.com/python-basics/resources

1. Usingtheinteractivewindow,displaysometextusing print().

2. Using the interactive window, assign a string literal to a variable.

Thenprintthecontentsofthevariableusingtheprint() function.

3. Repeattheﬁrsttwoexercisesusingtheeditorwindow.

3.4 Inspect Values in the Interactive

Window

TypethefollowingintoIDLE’sinteractivewindow:

>>> greeting = "Hello, World"

>>> greeting

'Hello, World'

When you press

Enter

after typing greeting a second time, Python

prints the string literal assigned to greeting even though you didn’t

usetheprint() function. Thisiscalledvariable inspection.

3.4. Inspect Values in the Interactive Window

Nowprintthestringassignedtogreeting usingtheprint() function:

>>> print(greeting)

Hello, World

Can you spot the diﬀerence between the output displayed by using

print() and the output displayed by just entering the variable name

andpressing

Enter

When you type the variable name greeting and press

Enter

, Python

prints the value assigned to the variable as it appears in your code.

Youassignedthestringliteral"Hello, World" togreeting,whichiswhy

'Hello, World' isdisplayedwithquotationmarks.

Note

String literals can be created with single or double quotation

marks in Python. At Real Python, we use double quotes wher-

everpossible,whereasIDLEoutputappearsinsinglequotesby

default.

Both "Hello, World" and 'Hello, World' mean the same thing in

Python—what’smostimportantisthatyoubeconsistentinyour

usage. You’lllearnmoreaboutstringsinchapter4.

Ontheotherhand,print() displaysamorehuman-readablerepresen-

tation of the variable’s value which, for string literals, means display-

ingthetextwithoutquotationmarks.

Sometimes,bothprintingandinspectingavariableproducethesame

output:

>>> x = 2

>>> x

>>> print(x)

3.4. Inspect Values in the Interactive Window

Here,youassignthenumber2 tox. Bothusingprint(x) andinspecting

x display output without quotation marks because 2 is a number and

not text. In most cases, though, variable inspection gives you more

usefulinformationthanprint().

Suppose you have two variables: x, which is assigned the number 2,

and y, which is assigned the string literal "2". In this case, print(x)

andprint(y) bothdisplaythesamething:

>>> x = 2

>>> y = "2"

>>> print(x)

>>> print(y)

However, inspecting x and y shows the diﬀerence between each vari-

able’svalue:

>>> x

>>> y

'2'

The key takeaway here is that print() displays a readable representa-

tionofa variable’s value, whilevariableinspection displays the value

asitappearsinthecode.

Keep in mind that variable inspection works only in the interactive

window. For example, try running the following program from the

editorwindow:

greeting = "Hello, World"

greeting

The program executes without any errors, but it doesn’t display any

output!

3.5. Leave Yourself Helpful Notes

3.5 Leave Yourself Helpful Notes

Programmers sometimes read code they wrote a while ago and won-

der,“Whatdoesthisdo?” Whenyouhaven’tlookedatcodeinawhile,

itcanbediﬃculttorememberwhyyouwroteitthewayyoudid!

To help avoid this problem, you can leave comments in your code.

Comments arelinesoftextthatdon’taﬀectthewayaprogramruns.

Theydocumentwhatcodedoesorwhytheprogrammermadecertain

decisions.

How to Write a Comment

The most common way to write a comment is to begin a new line in

your code with the # character. When you run your code, Python ig-

noreslinesstartingwith#.

Commentsthatstartonanewlinearecalledblock comments. You

can also write inline comments, which are comments that appear

onthesame line as the code they reference. Justputa # attheend of

thelineofcode,followedbythetextinyourcomment.

Here’sanexampleofaprogramwithbothkindsofcomments:

# This is a block comment.

greeting = "Hello, World"

print(greeting) # This is an inline comment.

Ofcourse, you can stilluse the # symbolinside a string. For instance,

Pythonwon’tmistakethefollowingforthestartofacomment:

>>> print("#1")

Ingeneral,it’sagoodideatokeepcommentsasshortaspossible,but

sometimesyouneedtowritemorethanreasonablyﬁtsonasingleline.

In that case, you can continue your comment on a new line that also

beginswiththe# symbol:

3.5. Leave Yourself Helpful Notes

# This is my first program.

# It prints the phrase "Hello, World"

# The comments are longer than the code!

greeting = "Hello, World"

print(greeting)

You can also use comments to comment out code while you’re test-

ing a program. Putting a # at the beginning of a line of code lets you

run your program as if that line of code didn’t exist, but it doesn’t ac-

tuallydeletethecode.

TocommentoutasectionofcodeinIDLE,highlightoneormorelines

tobecommentedandpress:

• Windows:

Alt

• macOS:

Ctrl

• Ubuntu Linux:

Ctrl

Toremovecomments,highlightthecommentedlinesandpress:

• Windows:

Alt

• macOS:

Ctrl

• Ubuntu Linux:

Ctrl

Shift

Nowlet’slookatsomecommonconventionsforcodecomments.

Conventions and Pet Peeves

Accordingto

PEP8,comments should always be written in complete

sentences with a single space between the # and the ﬁrst word of the

comment:

# This comment is formatted to PEP 8.

#this one isn't

Forinlinecomments,PEP8recommendsatleasttwospacesbetween

3.6. Summary and Additional Resources

thecodeandthe# symbol:

phrase = "Hello, World" # This comment is PEP 8 compliant.

print(phrase)# This comment isn't.

PEP 8 recommends that comments be used sparingly. A major pet

peeveamongprogrammersiscommentsthatdescribewhatisalready

obviousfromreadingthecode.

Forexample,thecommentinthefollowingcodeisunnecessary:

# Print "Hello, World"

print("Hello, World")

The comment is unnecessary because the code itself explicitly de-

scribes what’s happening. Comments are best used to clarify code

that may be diﬃcult to understand or to explain why something is

codedacertainway.

3.6 Summary and Additional Resources

In this chapter, you wrote and executed your ﬁrst Python program!

Youwroteasmallprogramthat displays the text "Hello, World" using

theprint() function.

Thenyou learnedaboutsyntax errors,whichoccurbefore IDLEex-

ecutes a program that contains invalid Python code, and runtime

errors,whichonlyoccurwhile aprogramisrunning.

You saw how to assign values to variables using the assignment

operator (=)andhowtoinspectvariablesintheinteractivewindow.

Finally,youlearnedhowtowritehelpfulcomments inyourcodefor

whenyouorsomeoneelselooksatitinthefuture.

3.6. Summary and Additional Resources

Interactive Quiz

This chapter comes with a free online quiz to check your learn-

ingprogress. Youcanaccessthequizusingyourphoneorcom-

puteratthefollowingwebaddress:

realpython.com/quizzes/pybasics-ﬁrst-program

Additional Resources

Tolearnmore,checkoutthefollowingresources:

•

“11BeginnerTipsforLearningPythonProgramming”

• “WritingCommentsinPython(Guide)”

For links and additional resources to further deepen your Python

skills, visit realpython.com/python-basics/resources

Chapter 4

Strings and String Methods

Many programmers, regardless of their specialty, deal with text on a

daily basis. For example, web developers work with text input from

web forms. Data scientists process text to extract data and perform

tasks like sentiment analysis, which can help identify and classify

opinionsinabodyoftext.

Collections of text in Python are called strings. Special functions

called string methods are used to manipulate strings. There are

stringmethodsforchangingastringfromlowercasetouppercase,re-

moving whitespace from the beginning or end of a string, replacing

partsofastringwithdiﬀerenttext,andmuchmore.

In this chapter, you’ll learn how to:

• Manipulatestringswithstringmethods

• Workwithuserinput

• Dealwithstringsofnumbers

• Formatstringsforprinting

Let’sgetstarted!

4.1. What Is a String?

4.1 What Is a String?

In chapter 3, you created the string "Hello, World" and printed it in

IDLE’s interactive window using print(). In this section, you’ll get a

deeperlookintoexactlywhatstringsareandthevariouswaysyoucan

createtheminPython.

The String Data Type

StringsareoneofthefundamentalPythondatatypes. Thetermdata

type refers to what kind of data a value represents. Strings are used

torepresenttext.

Note

ThereareseveralotherdatatypesbuiltintoPython. Forexam-

ple, you’ll learn about numerical data types in chapter 5 and

Booleandatatypesinchapter8.

Wesaythatstringsareafundamental data type becausetheycan’t

be broken down into smaller values of a diﬀerent type. Not all data

typesarefundamental. You’ll learn about compound data types, also

knownasdata structures, in chapter 9.

The string data type has a special abbreviated name in Python: str.

Youcanseethisbyusingtype(),whichisafunctionusedtodetermine

thedatatypeofagivenvalue.

TypethefollowingintoIDLE’sinteractivewindow:

>>> type("Hello, World")

The output <class 'str'> indicates that the value "Hello, World" is an

instanceofthestr datatype. Thatis,"Hello, World" isastring.

4.1. What Is a String?

Note

Fornow,youcanthinkofthewordclass asasynonymfordata

type, although it actually refers to something more speciﬁc.

You’llseejustwhataclassisinchapter10.

type() alsoworksforvaluesthathavebeenassignedtoavariable:

>>> phrase = "Hello, World"

>>> type(phrase)

Stringshavethreeimportantproperties:

1. Stringscontainindividuallettersorsymbolscalledcharacters.

2. Strings have a length, deﬁned as the number of characters the

stringcontains.

3. Characters in a string appear in a sequence, which means that

eachcharacterhasanumberedpositioninthestring.

Let’stakeacloserlookathowstringsarecreated.

String Literals

As you’ve already seen, you can create a string by surrounding some

textwithquotationmarks:

string1 = 'Hello, World'

string2 = "1234"

You can use either single quotes (string1) or double quotes (string2)

to create a string as long as you use the same type at the beginning

andendofthestring.

Whenever you create a string by surrounding text with quotation

marks, the string is called a string literal. The name indicates that

the string is literally written out in your code. All the strings you’ve

seenthusfararestringliterals.

4.1. What Is a String?

Note

Notevery string is a string literal. Sometimes strings areinput

by a user or read from a ﬁle. Since they’re not typed out with

quotationmarksinyourcode,they’renotstringliterals.

The quotes surrounding a string are called delimiters because they

tellPythonwhereastringbeginsandwhereitends. Whenonetypeof

quotes is used as the delimiter, the other type can be used inside the

string:

string3 = "We're #1!"

string4 = 'I said, "Put it over by the llama."'

After Python reads the ﬁrst delimiter, it considers all the characters

afteritpartof the string until it reaches a second matching delimiter.

Thisiswhyyoucan use a singlequoteinastringdelimitedbydouble

quotes,andviceversa.

If you try to use double quotes inside a string delimited by double

quotes,you’llgetanerror:

>>> text = "She said, "What time is it?""

File "<stdin>", line 1

text = "She said, "What time is it?""

SyntaxError: invalid syntax

PythonthrowsaSyntaxError becauseitthinksthestringendsafterthe

second ", and it doesn’t know how to interpret the rest of the line. If

you need to include a quotation mark that matches the delimiter in-

sideastring,thenyoucanescape thecharacterusingabackslash:

>>> text = "She said, \"What time is it?\""

>>> print(text)

She said,

"What time is it?"

4.1. What Is a String?

Note

When you work on a project, it’s a good idea to use only single

quotesoronlydoublequotestodelimiteverystring.

Keepinmindthattherereallyisn’tarightorwrongchoice! The

goal is to be consistent because consistency helps make your

codeeasiertoreadandunderstand.

Strings can contain any valid Unicode character. For example, the

string"We're #1!" containsthepoundsign(#)and"1234" containsnum-

bers. "×Pýŧħøŋ×" isalsoavalidPythonstring!

Determine the Length of a String

The number of characters contained in a string, including spaces, is

called the length of the string. For example, the string "abc" has a

lengthof 3, and the string "Don't Panic" hasalengthof 11.

Pythonhasabuilt-inlen() functionthatyoucanusetodeterminethe

lengthofastring. Toseehowitworks,typethefollowingintoIDLE’s

interactivewindow:

>>> len("abc")

Youcanalsouselen() to get thelengthofastringthat’sassignedto a

variable:

>>> letters = "abc"

>>> len(letters)

First,youassignthestring"abc" tothevariableletters. Thenyouuse

len() togetthelengthof letters,whichis3.

4.1. What Is a String?

Multiline Strings

The

PEP8styleguiderecommendsthateachlineofPythoncodecon-

tainnomorethanseventy-ninecharacters—includingspaces.

Note

PEP 8’s seventy-nine-character line length is a recommenda-

tion, not a rule. Some Python programmers prefer a slightly

longerlinelength.

In this book, we’ll strictly follow PEP 8’s recommended line

length.

Whether you follow PEP 8 or choose a longer line length, sometimes

you’llneedtocreatestringliteralswithmorecharactersthanyourcho-

senlimit.

Todealwithlongstrings,youcanbreakthemupacrossmultiplelines

into multiline strings. For example, suppose you need to ﬁt the

followingtextintoastringliteral:

This planet has—or rather had—a problem, which was

this: most of the people living on it were unhappy for

prettymuchofthetime. Many solutionsweresuggested

for this problem, but most of these were largely con-

cerned with the movements of small green pieces of

paper, which is odd because on the whole it wasn’t the

smallgreenpiecesofpaperthatwereunhappy.

—DouglasAdams,The Hitchhiker’s Guide to the Galaxy

This paragraph contains far more than seventy-nine characters, so

any line of code containing the paragraph as a string literal violates

PEP8. So,whatdoyoudo?

Thereareacoupleofwaystotacklethis. Onewayistobreakthestring

upacrossmultiplelinesandputabackslash(\)attheendofallbutthe

4.1. What Is a String?

lastline. TobePEP8compliant,thetotallengthoftheline,including

thebackslashes,mustbeseventy-ninecharactersorfewer.

Here’s how you could write the paragraph as a multiline string using

thebackslashmethod:

paragraph = "This planet has—or rather had—a problem, which was \

this: most of the people living on it were unhappy for pretty much \

of the time. Many solutions were suggested for this problem, but \

most of these were largely concerned with the movements of small \

green pieces of paper, which is odd because on the whole it wasn't \

the small green pieces of paper that were unhappy."

Notice that you don’t have to close each line with a quotation mark.

Normally, Python would get to the end of the ﬁrst line and complain

thatyoudidn’tclosethestringwithamatchingdoublequote. Witha

backslashattheend,youcankeepwritingthesamestringonthenext

line.

When you print() a multiline string that’s broken up by backslashes,

theoutputisdisplayedonasingleline:

>>> long_string = "This multiline string is \

displayed on one line"

>>> print(long_string)

This multiline string is displayed on one line

Youcanalsocreatemultilinestringsusingtriplequotes(""" or''')as

delimiters. Here’showtowritealongparagraphusingthisapproach:

paragraph = """This planet has—or rather had—a problem, which was

this: most of the people living on it were unhappy for pretty much

of the time. Many solutions were suggested for this problem, but

most of these were largely concerned with the movements of small

green pieces of paper, which is odd because on the whole it wasn't

the small green pieces of paper that were unhappy."""

4.2. Concatenation, Indexing, and Slicing

Triple-quoted strings preserve whitespace, including newlines. This

meansthatrunningprint(paragraph) woulddisplay the string on mul-

tiplelines,justas itappearsinthestring literal. Thismayormaynot

be what you want, so you’ll need to think about the desired output

beforeyouchoosehowtowriteamultilinestring.

Toseehowwhitespaceispreservedinatriple-quotedstring,typethe

followingintoIDLE’sinteractivewindow:

>>> print("""An example of a

... string that spans across multiple lines

... and also preserves whitespace."""

)

An example of a

string that spans across multiple lines

and also preserves whitespace.

Notice how the second and third lines in the output are indented in

exactlythesamewayasthestringliteral.

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Printastringthatusesdoublequotationmarksinsidethestring.

2. Printastringthatusesanapostropheinsidethestring.

3. Printastringthatspansmultiplelineswithwhitespacepreserved.

4. Printa string that iscoded on multiple lines butgets printed on a

singleline.

4.2 Concatenation, Indexing, and

Slicing

Nowthat you know whata string is and how to declarestring literals

inyourcode,let’sexploresomeofthethingsyoucandowithstrings.

4.2. Concatenation, Indexing, and Slicing

Inthissection,you’lllearnaboutthreebasicstringoperations:

1. Concatenation,whichjoinstwostringstogether

2. Indexing,whichgetsasinglecharacterfromastring

3. Slicing,whichgetsseveralcharactersfromastringatonce

Let’sdivein!

String Concatenation

Youcancombine,orconcatenate,twostringsusingthe+ operator:

>>> string1 = "abra"

>>> string2 = "cadabra"

>>> magic_string = string1 + string2

>>> magic_string

'abracadabra'

Inthisexample,thestringconcatenationoccursonthethirdline. You

concatenate string1 and string2 using +, and then you assign the re-

sulttothevariablemagic_string. Noticethatthetwostringsarejoined

withoutanywhitespacebetweenthem.

You can use string concatenation to join two related strings, such as

joiningaﬁrstnameandalastnameintoafullname:

>>> first_name = "Arthur"

>>> last_name = "Dent"

>>> full_name = first_name + " " + last_name

>>> full_name

'Arthur Dent'

Here, you use string concatenation twice on the same line. First, you

concatenate first_name with " " to ensure a space appears after the

ﬁrstnameintheﬁnalstring. Thisproducesthestring"Arthur ",which

you then concatenate with last_name to produce the full name "Arthur

Dent".

4.2. Concatenation, Indexing, and Slicing

String Indexing

Each character in a string has a numbered position called an index.

Youcanaccessthecharacteratthenthpositionbyputtingthenumber

n betweentwosquarebrackets([])immediatelyafterthestring:

>>> flavor = "fig pie"

>>> flavor[1]

'i'

flavor[1] returnsthecharacteratposition1 in"fig pie",whichisi.

Wait. Isn’tf theﬁrstcharacterof "fig pie"?

In Python—and in most other programming languages—counting al-

ways starts at zero. To get the character at the beginning of a string,

youneedtoaccessthecharacteratposition0:

>>> flavor[0]

'f'

Important

Forgetting that counting starts with zero and trying to access

theﬁrst character in a string withthe index 1 results in an o-

by-one error.

Oﬀ-by-oneerrorsareacommonsourceoffrustrationforbegin-

ningandexperiencedprogrammersalike!

The following ﬁgure shows the index for each character of the string

"fig pie":

| f | i | g | | p | i | e |

0 1 2 3 4 5 6

4.2. Concatenation, Indexing, and Slicing

If you try to access an index beyond the end of a string, then Python

raisesanIndexError:

>>> flavor[9]

Traceback (most recent call last):

File

"<pyshell#4>", line 1, in <module>

flavor[9]

IndexError: string index out of range

Thelargestindexinastringisalwaysonelessthanthestring’slength.

Since"fig pie" hasalengthofseven,thelargestindexallowedis 6.

Stringsalsosupportnegativeindices:

>>> flavor[-1]

'e'

The last character in a string has index -1, which for "fig pie" is the

lettere. Thesecondtolastcharacteri hasindex-2,andsoon.

The following ﬁgure shows the negative index for each character in

thestring"fig pie":

| f | i | g | | p | i | e |

-7 -6 -5 -4 -3 -2 -1

Justlikewithpositiveindices,PythonraisesanIndexError ifyoutryto

accessanegativeindexlessthantheindexoftheﬁrstcharacterinthe

string:

>>> flavor[-10]

Traceback (most recent call last):

File

"<pyshell#5>", line 1, in <module>

flavor[-10]

IndexError: string index out of range

Negative indices may not seem useful at ﬁrst, but sometimes they’re

abetterchoicethanapositiveindex.

4.2. Concatenation, Indexing, and Slicing

For example, suppose a string input by a user is assigned to the vari-

ableuser_input. Ifyouneedtogetthelastcharacterofthestring,how

doyouknowwhatindextouse?

One way to get the last character of a string is to calculate the ﬁnal

indexusinglen():

final_index = len(user_input) - 1

last_character = user_input[final_index]

Getting the ﬁnal character with the index -1 takes less typing and

doesn’trequireanintermediatesteptocalculatetheﬁnalindex:

last_character = user_input[-1]

String Slicing

Supposeyouneed astringcontainingjustthe ﬁrstthreelettersofthe

string "fig pie". You could access each character by index and con-

catenatethemlikethis:

>>> first_three_letters = flavor[0] + flavor[1] + flavor[2]

>>> first_three_letters

'fig'

If you need more than just the ﬁrst few letters of a string, then get-

ting each character individually and concatenating them together is

clumsy and long-winded. Fortunately, Python provides a way to do

thiswithmuchlesstyping.

Youcanextractaportionofastring,calledasubstring,byinserting

a colon between two index numbers set inside square brackets like

this:

>>> flavor = "fig pie"

>>> flavor[0:3]

'fig'

4.2. Concatenation, Indexing, and Slicing

flavor[0:3] returns the ﬁrst three characters of the string assigned to

flavor,startingwiththecharacteratindex0 andgoinguptobutnotin-

cludingthecharacteratindex3. The[0:3] partof flavor[0:3] iscalled

aslice. Inthiscase,itreturnsasliceof "fig pie". Yum!

String slices can be confusing because the substring returned by

the slice includes the character whose index is the ﬁrst number but

doesn’tincludethecharacterwhoseindexisthesecondnumber.

To remember how slicing works, you can think of a string as a se-

quence of square slots. The left and right boundaries of each slot are

numbered sequentially from zero up to the length of the string, and

eachslotisﬁlledwithacharacterinthestring.

Here’swhatthislookslikeforthestring"fig pie":

| f | i | g | | p | i | e |

0 1 2 3 4 5 6 7

So, for "fig pie", the slice [0:3] returns the string "fig", and the slice

[3:7] returnsthestring" pie".

Ifyouomittheﬁrstindexinaslice,thenPythonassumesyouwantto

startatindex0:

>>> flavor[:3]

'fig'

Theslice[:3] isequivalenttotheslice[0:3],soflavor[:3] returnsthe

ﬁrstthreecharactersinthestring"fig pie".

Similarly, if you omit the second index in the slice, then Python as-

sumesyouwanttoreturnthesubstringthatbeginswiththecharacter

4.2. Concatenation, Indexing, and Slicing

whoseindexistheﬁrstnumberinthesliceandendswiththelastchar-

acterinthestring:

>>> flavor[3:]

' pie'

For "fig pie", the slice [3:] is equivalent to the slice [3:7]. Since the

character at index 3 is a space, flavor[3:9] returns the substring that

startswiththespaceandendswiththelastletter: " pie".

If you omit both the ﬁrst and second numbers in a slice, you get a

string that starts with the character at index 0 and ends with the last

character. In other words, omitting both numbers in a slice returns

theentirestring:

>>> flavor[:]

'fig pie'

It’simportant to note that, unlike with stringindexing, Python won’t

raise an IndexError when you try to slice between boundaries that fall

outsidethebeginningorendingboundariesofastring:

>>> flavor[:14]

'fig pie'

>>> flavor[13:15]

In this example, the ﬁrst line gets the slice from the beginning of the

string up to but not including the fourteenth character. The string

assigned to flavor has a length of seven, so you might expect Python

to throw an error. Instead, it ignores any nonexistent indices and re-

turnstheentirestring"fig pie".

Thethirdlineshowswhathappenswhenyoutrytogetasliceinwhich

the entire range is out of bounds. flavor[13:15] attempts to get the

thirteenth and fourteenth characters, which don’t exist. Instead of

raisinganerror,Pythonreturnstheempty string ("").

4.2. Concatenation, Indexing, and Slicing

Note

Theemptystringiscalledemptybecauseitdoesn’tcontainany

characters. You can create it by writing two quotation marks

withnothingbetweenthem:

empty_string = ""

Astringwithanythinginit—evenaspace—isnotempty. Allthe

followingstringsarenon-empty:

non_empty_string1 = " "

non_empty_string2 = " "

non_empty_string3 = " "

Even though these strings don’t contain any visible characters,

theyarenon-emptybecausetheydocontainspaces.

Youcanusenegativenumbersinslices. Therulesforsliceswithnega-

tivenumbersareexactly the same as the rules for sliceswithpositive

numbers. It helps to visualize the string as slots with the boundaries

labeledwithnegativenumbers:

| f | i | g | | p | i | e |

-7 -6 -5 -4 -3 -2 -1

Justlikebefore, the slice [x:y] returns the substring starting at index

x and going up to but not including y. For instance, the slice [-7:-4]

returnstheﬁrstthreelettersofthestring"fig pie":

>>> flavor[-7:-4]

'fig'

Notice, however, that the rightmost boundary of the string does not

have a negative index. The logical choice for that boundary would

seemtobethenumber0,butthatdoesn’twork.

4.2. Concatenation, Indexing, and Slicing

Insteadofreturningtheentirestring,[-7:0] returnstheemptystring:

>>> flavor[-7:0]

This happens because the second number in a slice must correspond

toaboundarythatistotherightoftheboundarycorrespondingtothe

ﬁrst number, but both -7 and 0 correspond to the leftmost boundary

intheﬁgure.

Ifyouneedtoincludetheﬁnalcharacterofastringinyourslice,then

youcanomitthesecondnumber:

>>> flavor[-7:]

'fig pie'

Ofcourse,usingflavor[-7:] togettheentirestringisabitoddconsid-

ering that you can use the variable flavor without the slice to get the

sameresult!

Sliceswithnegativeindicesareuseful,though,forgettingthelastfew

charactersinastring. Forexample,flavor[-3:] is"pie".

Strings Are Immutable

To wrap this section up, let’s discuss an important property of string

objects. Strings are immutable, which means that you can’t change

themonceyou’vecreatedthem. Forinstance,seewhathappenswhen

youtrytoassignanewlettertooneparticularcharacterofastring:

>>> word = "goal"

>>> word[0] = "f"

Traceback (most recent call last):

File

"<pyshell#16>", line 1, in <module>

word[0] = "f"

TypeError: 'str' object does not support item assignment

4.2. Concatenation, Indexing, and Slicing

PythonthrowsaTypeError and tells you that str objectsdon’tsupport

itemassignment.

If you want to alter a string, then you must create an entirely new

string. To change the string "goal" to the string "foal", you can use a

string slice to concatenate the letter "f" with everything but the ﬁrst

letteroftheword"goal":

>>> word = "goal"

>>> word = "f" + word[1:]

>>> word

'foal'

First, you assign the string "goal" to the variable word. Then you con-

catenatethesliceword[1:],whichisthestring"oal",withtheletter"f"

to get the string "foal". If you’re getting a diﬀerent result here, then

makesureyou’reincludingthecoloncharacter(:) aspartofthestring

slice.

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Createastringandprintitslengthusinglen().

2. Create two strings, concatenate them, and print the resulting

string.

3. Createtwostrings,useconcatenationtoaddaspacebetweenthem,

andprinttheresult.

4. Printthestring"zing" byusingslicenotationtospecifythecorrect

rangeofcharactersinthestring"bazinga".

4.3. Manipulate Strings With Methods

4.3 Manipulate Strings With Methods

Stringscomebundledwithspecialfunctions called string methods

that you can use to work with and manipulate strings. There are nu-

merousstringmethods available,butwe’llfocuson someofthemost

commonlyusedones.

Inthissection,you’lllearnhowto:

• Convertastringtouppercaseorlowercase

• Removewhitespacefromastring

• Determineifastringbeginsorendswithcertaincharacters

Let’sgo!

Converting String Case

Toconvertastringtoalllowercaseletters,youusethestring’s.lower()

method. This is done by tacking .lower() onto the end of the string

itself:

>>> "Jean-Luc Picard".lower()

'jean-luc picard'

The dot (.) tells Python that what follows is the name of a method—

thelower() methodinthiscase.

Note

We’ll refer to string methods with a dot (.) at the beginning of

theirnames. Forexample,.lower() iswrittenwithaleadingdot

insteadofaslower().

This makes it easier to diﬀerentiate functions that are string

methodsfrombuilt-infunctionslikeprint() andtype().

4.3. Manipulate Strings With Methods

String methods don’t just work on string literals. You can also use

.lower() onastringassignedtoavariable:

>>> name = "Jean-Luc Picard"

>>> name.lower()

'jean-luc picard'

Theoppositeof.lower() is.upper(),whichconvertseverycharacterin

astringtouppercase:

>>> name.upper()

'JEAN-LUC PICARD'

Compare the .upper() and .lower() string methods to the len() func-

tion you saw in the last section. Aside from the diﬀerent results of

thesefunctions,theimportantdistinctionhereishowthey’reused.

len() isastand-alonefunction. Ifyouwanttodeterminethelengthof

thename string,thenyoucallthelen() functiondirectly:

>>> len(name)

Onthe otherhand,.upper() and.lower() mustbeusedinconjunction

withastring. Theydonotexistindependently.

Removing Whitespace From a String

Whitespace is any character that is printed as blank space. This in-

cludesthingslikespacesandline feeds,whicharespecialcharacters

thatmoveoutputtoanewline.

Sometimesyouneedtoremovewhitespacefromthebeginningorend

of a string. This is especially useful when working with strings that

comefromuserinput,whichmayincludeextrawhitespacecharacters

byaccident.

4.3. Manipulate Strings With Methods

Therearethreestringmethodsthatyoucanusetoremovewhitespace

fromastring:

1. .rstrip()

2. .lstrip()

3. .strip()

.rstrip() removeswhitespacefromtherightsideofastring:

>>> name = "Jean-Luc Picard "

>>> name

'Jean-Luc Picard '

>>> name.rstrip()

'Jean-Luc Picard'

Inthisexample,thestring"Jean-Luc Picard " hasﬁvetrailingspaces.

You use .rstrip() to remove trailing spaces from the right-hand side

of the string. This returns the new string "Jean-Luc Picard", which no

longerhasthespacesattheend.

.lstrip() works just like .rstrip(), except that it removes whitespace

fromtheleft-handsideofthestring:

>>> name = " Jean-Luc Picard"

>>> name

' Jean-Luc Picard'

>>> name.lstrip()

'Jean-Luc Picard'

To remove whitespace from both the left and the right sides of the

stringatthesametime,use.strip():

>>> name = " Jean-Luc Picard "

>>> name

' Jean-Luc Picard '

>>> name.strip()

'Jean-Luc Picard'

4.3. Manipulate Strings With Methods

It’s important to note that none of .rstrip(), .lstrip(), or .strip() re-

moveswhitespacefrom the middle of the string. In each ofthe previ-

ousexamples,the spacebetween"Jean-Luc" and"Picard" ispreserved.

Determine If a String Starts or Ends With a

Particular String

Whenyouworkwithtext,sometimesyouneedtodetermineifagiven

string starts with or ends with certain characters. You can use two

stringmethodstosolvethisproblem: .startswith() and.endswith().

Let’slookatanexample. Considerthestring"Enterprise". Here’show

you use .startswith() to determine if the string starts with the letters

e andn:

>>> starship = "Enterprise"

>>> starship.startswith("en")

False

You tell .startswith() which characters to search for by providing a

string containing those characters. So, to determine if "Enterprise"

startswith the letters e and n, you call .startswith("en"). This returns

False. Whydoyouthinkthatis?

If you guessed that .startswith("en") returns False because "En-

terprise" starts with a capital E, then you’re absolutely right! The

.startswith() method is case sensitive. To get .startswith() to

returnTrue,youneedtoprovideitwiththestring"En":

>>> starship.startswith("En")

True

Youcanuse.endswith() todetermineifastringendswithcertainchar-

acters:

>>> starship.endswith("rise")

True

4.3. Manipulate Strings With Methods

Justlike.startswith(),the.endswith() methodiscasesensitive:

>>> starship.endswith("risE")

False

Note

TheTrue andFalse valuesarenotstrings. Theyareaspecialkind

of data type called a Boolean value. You’ll learn more about

Booleanvaluesinchapter8.

String Methods and Immutability

Recall from the previous section that strings are immutable—they

can’t be changed once they’ve been created. Most string methods

thataltera string, like .upper() and .lower(),actually return copies of

theoriginalstringwiththeappropriatemodiﬁcations.

Ifyouaren’tcareful,thiscanintroducesubtlebugsintoyourprogram.

TrythisoutinIDLE’sinteractivewindow:

>>> name = "Picard"

>>> name.upper()

'PICARD'

>>> name

'Picard'

When you call name.upper(), nothing about name actually changes. If

youneedtokeeptheresult,thenyouneedtoassignittoavariable:

>>> name = "Picard"

>>> name = name.upper()

>>> name

'PICARD'

name.upper() returns a new string "PICARD", which is reassigned to the

name variable. Thisoverrides theoriginalstring"Picard" thatyouﬁrst

assignedtoname.

4.3. Manipulate Strings With Methods

Use IDLE to Discover Additional String Methods

Strings have lots of methods associated with them, and the methods

introduced in this section barely scratch the surface. IDLE can help

you ﬁnd new string methods. To see how, ﬁrst assign a string literal

toavariableintheinteractivewindow:

>>> starship = "Enterprise"

Next, type starship followed by a period, but do not hit

Enter

. You

shouldseethefollowingintheinteractivewindow:

>>> starship.

Now wait for a couple of seconds. IDLE displays a list of every string

method,whichyoucanscrollthroughusingthearrowkeys.

A related shortcut in IDLE is the ability to use

Tab

to automatically

ﬁll in text without having to type long names. For instance, if you

typeonlystarship.u andhit

Tab

,thenIDLEautomaticallyﬁllsinstar-

ship.upper because only one method that begins with a u belongs to

starship.

Thisevenworkswithvariable names. Try typing just the ﬁrstfewlet-

ters of starship and pressing

Tab

. If you haven’t deﬁned any other

names that share those ﬁrst letters, then IDLE completes the name

starship foryou.

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Write a program that converts the following strings to lowercase:

"Animals", "Badger", "Honey Bee", "Honey Badger". Print each lower-

casestringonaseparateline.

2. Repeat exercise 1, but convert each string to uppercase instead of

lowercase.

4.4. Interact With User Input

3. Write a program that removes whitespace from the following

strings,thenprintoutthestringswiththewhitespaceremoved:

string1 = " Filet Mignon"

string2 = "Brisket "

string3 = " Cheeseburger "

4. Write a program that prints out the result of .startswith("be") on

eachofthefollowingstrings:

string1 = "Becomes"

string2 = "becomes"

string3 = "BEAR"

string4 = " bEautiful"

5. Usingthe same four strings from exercise4,write a program that

uses string methods to alter each string so that .startswith("be")

returnsTrue forallofthem.

4.4 Interact With User Input

Now that you’ve seen how to work with string methods, let’s make

thingsinteractive!

In this section, you’ll learn how to get some input from a user with

input(). You’llwriteaprogramthatasksausertoinputsometextand

thendisplaysthattextbacktotheminuppercase.

EnterthefollowingintoIDLE’sinteractivewindow:

>>> input()

When you press

Enter

, it looks like nothing happens. The cursor

moves to a new line, but a new >>> doesn’t appear. Python is waiting

foryoutoentersomething!

4.4. Interact With User Input

Goaheadandtypesometextandpress

Enter

>>> input()

Hello there!

'Hello there!'

>>>

The text you entered is repeated on a new line with single quotes.

That’sbecauseinput() returnsasastringanytextenteredbytheuser.

Tomakeinput() abitmoreuser-friendly,youcangiveitaprompt to

display to the user. The prompt is just a string that you put between

the parentheses of input(). It can be anything you want: a word, a

symbol,aphrase—anythingthatisavalidPythonstring.

input() displays the prompt and waits for the user to type something.

When the user hits

Enter

, input() returns their input as a string that

can be assigned to a variable and used to do something in your pro-

gram.

To see how input() works, type the following code into IDLE’s editor

window:

prompt = "Hey, what's up? "

user_input = input(prompt)

print("You said: " + user_input)

Press

toruntheprogram. ThetextHey, what's up? displaysinthe

interactivewindowwithablinkingcursor.

Thesinglespaceattheendofthestring"Hey, what's up? " makessure

thatwhentheuserstartstotype,thetextisseparatedfromtheprompt

withaspace. Whentheusertypesaresponseandpresses

Enter

,their

responseisassignedtotheuser_input variable.

4.4. Interact With User Input

Here’sasamplerunoftheprogram:

Hey, what's up? Mind your own business.

You said: Mind your own business.

Once you have input from a user, you can do something with it. For

example,thefollowingprogramtakesuserinput,convertsittoupper-

casewith.upper(),andprintstheresult:

response = input("What should I shout? ")

shouted_response = response.upper()

print(

"Well, if you insist..." + shouted_response)

Try typing this program into IDLE’s editor window and running it.

Whatelsecanyouthinkoftodowiththeinput?

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Write a program that takes input from the user and displays that

inputback.

2. Write a program that takes input from the user and displays the

inputinlowercase.

3. Write a program that takes input from the user and displays the

numberofcharactersintheinput.

4.5. Challenge: Pick Apart Your User’s Input

4.5 Challenge: Pick Apart Your User’s

Input

Write a program named first_letter.py that prompts the user for in-

putwiththestring"Tell me your password:". Theprogramshouldthen

determine the ﬁrst letter of the user’s input, convert that letter to up-

percase,anddisplayitback.

Forexample,iftheuserinputis"no",thentheprogramshoulddisplay

thefollowingoutput:

The first letter you entered was: N

Fornow,it’sokayifyourprogramcrasheswhen theuserentersnoth-

ingasinput—thatis, when theyjusthit

Enter

insteadoftypingsome-

thing. You’ll learn a couple of ways to deal with this situation in an

upcomingchapter.

You can nd the solutions to this code challenge and many other bonus

resources online at

realpython.com/python-basics/resources

4.6 Working With Strings and Numbers

When you get user input using input(), the result is always a string.

Therearemanyothersituations in which input is given to a program

asa string. Sometimesthosestringscontainnumbersthatneedtobe

fedintocalculations.

Inthissection,you’lllearnhowtodealwithstringsofnumbers. You’ll

seehowarithmeticoperationsworkonstringsandhowtheyoftenlead

tosurprisingresults. You’llalsolearnhowtoconvertbetweenstrings

andnumbertypes.

Using Strings With Arithmetic Operators

You’ve seen that string objects can hold many types of characters, in-

cluding numbers. However, don’t confuse numerals in a string with

4.6. Working With Strings and Numbers

actual numbers. For instance, try this bit of code out in IDLE’s inter-

activewindow:

>>> num = "2"

>>> num + num

'22'

The + operator concatenates two strings together, which is why the

resultof "2" + "2" is "22" andnot"4".

You can multiply strings by a number as long as that number is an

integerorwholenumber. Typethefollowingintothe interactive win-

dow:

>>> num = "12"

>>> num * 3

'121212'

num * 3 concatenatesthreeinstancesofthestring"12" andreturnsthe

string"121212".

Compare this operation to arithmetic with numbers. When you mul-

tiplythenumber12bythe number3,theresultisthesameasadding

three12stogether. The same is true for a string. Thatis,"12" * 3 can

be interpreted as "12" + "12" + "12". In general, multiplying a string

byanintegern concatenatesn copiesofthatstring.

Youcanmovethenumberontheright-handsideoftheexpressionnum

* 3 totheleft,andtheresultisunchanged:

>>> 3 * num

'121212'

What do you think happens if you use the * operator between two

strings?

4.6. Working With Strings and Numbers

Type"12" * "3" intheinteractivewindowandpress

Enter

>>> "12" * "3"

Traceback (most recent call last):

File

"<stdin>", line 1, in <module>

TypeError: can't multiply sequence by non-int of type 'str'

Python raises a TypeError and tells you that you can’t multiply a se-

quencebyanon-integer.

Note

A sequence is any Python object that supports accessing ele-

mentsbyindex. Stringsaresequences. You’lllearnaboutother

sequencetypesinchapter9.

When you use the * operator with a string, Python always expects an

integerontheothersideoftheoperator.

What do you think happens when you try to add a string and a num-

ber?

>>> "3" + 3

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: can only concatenate str (not "int") to str

PythonthrowsaTypeError becauseitexpectstheobjectsonbothsides

ofthe+ operatortobeofthesametype.

Ifanobjectoneithersideof + isastring,thenPythontriestoperform

stringconcatenation. Itwillonly perform additionifbothobjectsare

numbers. So,toadd"3" + 3 andget6,youmustﬁrstconvertthestring

"3" toanumber.

4.6. Working With Strings and Numbers

Converting Strings to Numbers

The TypeError examples in the previous section highlight a common

problem when applying user input to an operation that requires a

numberandnotastring: typemismatches.

Let’slookatanexample. Saveandrunthefollowingprogram:

num = input("Enter a number to be doubled: ")

doubled_num

= num * 2

print(doubled_num)

Ifyouenteredthenumber2 attheprompt,thenyouwouldexpectthe

outputto be 4. But in this case, youwould get 22. Remember, input()

alwaysreturnsastring,soifyouinput2,thennum isassignedthestring

"2", not the integer 2. Therefore, the expression num * 2 returns the

string"2" concatenatedwithitself,whichis"22".

To perform arithmetic on numbers contained in a string, you must

ﬁrstconvertthem fromastringtype toanumbertype. Therearetwo

functionsthatyoucanusetodothis: int() andfloat().

int() stands for integer and converts objects into whole numbers,

whereasfloat() standsforoating-point number andconvertsob-

jects into numbers with decimal points. Here’s what using each one

lookslikeintheinteractivewindow:

>>> int("12")

>>> float("12")

12.0

Notice how float() adds a decimal point to the number. Floating-

pointnumbersalwayshaveatleastonedecimalplaceofprecision. For

this reason, you can’t change a string that looks like a floating-point

number into an integer because you would lose everything after the

decimalpoint.

4.6. Working With Strings and Numbers

Tryconvertingthestring"12.0" toaninteger:

>>> int("12.0")

Traceback (most recent call last):

File

"<stdin>", line 1, in <module>

ValueError: invalid literal for int() with base 10: '12.0'

Eventhoughtheextra0 afterthe decimalplacedoesn’taddanyvalue

to the number, Python won’t change 12.0 into 12 because it would re-

sultinalossofprecision.

Let’s revisit the program from the beginning of this section and see

howtoﬁxit. Here’sthecodeagain:

num = input("Enter a number to be doubled: ")

doubled_num = num * 2

print(doubled_num)

The issue is on the line doubled_num = num * 2 because num is a string

and2 isaninteger.

Youcanﬁxtheproblembypassingnum toeitherint() orfloat(). Since

the prompts asks the user to input a number, and not speciﬁcally an

integer,let’sconvertnum toafloating-pointnumber:

num = input("Enter a number to be doubled: ")

doubled_num = float(num) * 2

print(doubled_num)

Nowwhen you run this program andinput 2, you get 4.0 as expected.

Tryitout!

Converting Numbers to Strings

Sometimes you need to convert a number to a string. You might do

this, for example, if you need to build a string from some preexisting

variablesthatareassignedtonumericvalues.

4.6. Working With Strings and Numbers

As you’ve already seen, concatenating a number with a string pro-

ducesaTypeError:

>>> num_pancakes = 10

>>> "I am going to eat " + num_pancakes + " pancakes."

Traceback (most recent call last):

File

"<stdin>", line 1, in <module>

TypeError: can only concatenate str (not "int") to str

Since num_pancakes is a number, Python can’t concatenate it with the

string "I'm going to eat". To build the string, you need to convert

num_pancakes toastringusingstr():

>>> num_pancakes = 10

>>> "I am going to eat " + str(num_pancakes) + " pancakes."

'I am going to eat 10 pancakes.'

Youcanalsocallstr() onanumberliteral:

>>> "I am going to eat " + str(10) + " pancakes."

'I am going to eat 10 pancakes.'

str() canevenhandlearithmeticexpressions:

>>> total_pancakes = 10

>>> pancakes_eaten = 5

>>> "Only " + str(total_pancakes - pancakes_eaten) + " pancakes left."

'Only 5 pancakes left.'

Inthenextsection,you’lllearnhowtoformatstringsneatlytodisplay

valuesinanice,readablemanner. Beforeyoumoveon,though,check

yourunderstandingwiththefollowingreviewexercises.

4.7. Streamline Your Print Statements

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Createastringcontaininganinteger,thenconvertthatstringinto

an actual integer object using int(). Test that your new object is

anumberbymultiplyingitby anothernumberanddisplayingthe

result.

2. Repeatthepreviousexercise,butuseafloating-pointnumberand

float().

3. Createastringobjectandanintegerobject,thendisplaythemside

bysidewithasingleprintstatementusingstr().

4. Write a program that uses input() twice to get two numbers from

theuser,multipliesthenumbers together, and displaystheresult.

If the user enters 2 and 4, then your program should print the

followingtext:

The product of 2 and 4 is 8.0.

4.7 Streamline Your Print Statements

Suppose you have a string, name = "Zaphod", and two integers, heads

= 2 and arms = 3. You want to display them in the string "Zaphod has

2 heads and 3 arms". This is called string interpolation, which is

justa fancy way of saying that you want to insert some variables into

speciﬁclocationsinastring.

Onewaytodothisiswithstringconcatenation:

>>> name + " has " + str(heads) + " heads and " + str(arms) + " arms"

'Zaphod has 2 heads and 3 arms'

This code isn’t the prettiest, and keeping track of what goes inside or

outside the quotes can be tough. Fortunately, there’s another way of

interpolating strings: formatted string literals, more commonly

4.7. Streamline Your Print Statements

knownasf-strings.

Theeasiestwaytounderstandf-stringsistoseetheminaction. Here’s

whattheabovestringlookslikewhenwrittenasanf-string:

>>> f"{name} has {heads} heads and {arms} arms"

'Zaphod has 2 heads and 3 arms'

Therearetwoimportantthingstonoticeabouttheaboveexample:

1. The string literal starts with the letter f before the opening quota-

tionmark.

2. Variable names surrounded by curly braces ({}) are replaced by

theircorrespondingvalueswithoutusingstr().

YoucanalsoinsertPythonexpressionsbetweenthecurlybraces. The

expressionsarereplacedwiththeirresultinthestring:

>>> n = 3

>>> m = 4

>>> f"{n} times {m} is {n*m}"

'3 times 4 is 12'

It’s a good idea to keep any expressions used in an f-string as simple

aspossible. Packing abunchofcomplicatedexpressionsintoastring

literal can result in code that is diﬃcult to read and diﬃcult to main-

tain.

f-strings are available only in Python version 3.6 and above. In ear-

lier versions of Python, you can use .format() to get the same results.

ReturningtotheZaphodexample,youcanuse.format() toformatthe

stringlikethis:

>>> "{} has {} heads and {} arms".format(name, heads, arms)

'Zaphod has 2 heads and 3 arms'

f-strings are shorter and sometimes more readable than using .for-

mat(). You’llseef-stringsusedthroughoutthisbook.

4.8. Find a String in a String

Foranin-depthguidetof-stringsandcomparisonstootherstringfor-

mattingtechniques,checkoutReal Python’s

“Python3’sf-Strings: An

ImprovedStringFormattingSyntax(Guide).”

Review Exercises

You can nd the solutions to these exercises and many other bonus

resources online at

realpython.com/python-basics/resources

1. Create a float object named weight with the value 0.2, and create

astringobject named animal withthevalue"newt". Then usethese

objects to print the following string using only string concatena-

tion:

0.2 kg is the weight of the newt.

2. Displaythesamestringbyusing.format() andempty{} placehold-

ers.

3. Displaythesamestringusinganf-string.

4.8 Find a String in a String

Oneofthemostusefulstringmethodsis .find(). Asitsnameimplies,

this method allows you to ﬁnd the location of one string in another

string—commonlyreferredtoasasubstring.

To use .find(), tack it to the end of a variable or a string literal with

thestringyouwanttoﬁndtypedbetweentheparentheses:

>>> phrase = "the surprise is in here somewhere"

>>> phrase.find("surprise")

Thevaluethat.find() returnsistheindexoftheﬁrstoccurrenceofthe

stringyoupasstoit. Inthiscase,"surprise" startsattheﬁfthcharacter

of the string "the surprise is in here somewhere", which has index 4

becausecountingstartsatzero.

This is a sample from “Python Basics: A Practical

Introduction to Python 3”

WiththefullversionofthebookyougetacompletePythoncurriculum

togoallthewayfrombeginnertointermediate-level. Everystepalong

thewayisexplainedandillustratedwithshort&clearcodesamples.

Codingexerciseswithineachchapterandourinteractivequizzeshelp

fast-trackyourprogressandensureyoualwaysknowwhattofocuson

next.

Become a fluent Pythonista and gain programming knowledge you

canapplyinthereal-world,today:

If you enjoyed the sample chapters you can purchase a full

version of the book at

realpython.com/pybasics-book