A Byte of Python.pdf

1/4/2015 AByteofPython

http://www.swaroopch.com/notes/python/ 1/165

AByteofPython

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mailto:[email protected]



TableofContents

1.Welcome2.Dedication3.Preface4.Introduction5.Installation6.FirstSteps7.Basics8.OperatorsandExpressions9.ControlFlow10.Functions11.Modules12.DataStructures13.ProblemSolving14.ObjectOrientedProgramming15.InputandOutput16.Exceptions17.StandardLibrary18.More19.Appendix:FLOSS20.Appendix:Colophon21.Appendix:HistoryLesson22.Appendix:RevisionHistory23.Translations24.TranslationHowto



"AByteofPython"isafreebookonprogrammingusingthePythonlanguage.ItservesasatutorialorguidetothePythonlanguageforabeginneraudience.Ifallyouknowaboutcomputersishowtosavetextfiles,thenthisisthebookforyou.



1.Welcome

1.1.WhoreadsAByteofPython?Herearewhatpeoplearesayingaboutthebook:

ThisisthebestbeginnerstutorialIveeverseen!Thankyouforyoureffort.

WaltMichalik

Thebestthingifoundwas"AByteofPython",whichissimplyabrilliantbookfor

a beginner. Its well written, the concepts are well explained with self evident

examples.

JoshuaRobin

Excellentgentleintroductiontoprogramming#Pythonforbeginners

ShanRajasekaran (https://twitter.com/ShanRajasekaran/status/268910645842423809)

Bestnewbieguidetopython

NicksonKaigi (https://twitter.com/nickaigi/status/175508815729541120)

starttolovepythonwitheverysinglepageread

HerbertFeutl (https://twitter.com/HerbertFeutl/status/11901471389913088)

perfect beginners guide for python, will give u key to unlock magical world of

python

Dilip (https://twitter.com/Dili_mathilakam/status/220033783066411008)

I should be doing my actual "work" but just found "A Byte of Python". A great

guidewithgreatexamples.

BiologistJohn (https://twitter.com/BiologistJohn/statuses/194726001803132928)

Recently started readinga Byte of python.Awesome work. And that too for free.

Highlyrecommendedforaspiringpythonistas.

Mangesh (https://twitter.com/mangeshnanoti/status/225680668867321857)

https://twitter.com/HerbertFeutl/status/11901471389913088https://twitter.com/Dili_mathilakam/status/220033783066411008https://twitter.com/BiologistJohn/statuses/194726001803132928https://twitter.com/mangeshnanoti/status/225680668867321857mailto:[email protected]:[email protected]://twitter.com/ShanRajasekaran/status/268910645842423809https://twitter.com/nickaigi/status/175508815729541120



A Byte of Python, written by Swaroop. (this is the book Im currently reading).

Probably the best to start with, and probably the best in the world for every

newbieorevenamoreexperienceduser.

Apostolos (http://apas.gr/2010/04/27/learningpython/)

EnjoyingReading#ByteOfPythonby@swaroopchbestbookever

YuvrajSharma (https://twitter.com/YuvrajPoudyal/status/448050415356346368)

Thank youso much forwriting A ByteOf Python. I just started learninghow to

code two daysago and Imalready buildingsomesimple games.Your guidehas

beenadreamandIjustwantedtoletyouknowhowvaluableithasbeen.

Franklin

Im fromDayanandasagarCollegeof Engineering (7th sem,CSE).Firstly iwant

to say thatyourbook "Thebyteof python" is toogooda book fora beginner in

python like me.The concepts are so well explained with simple examples that

helpedmetoeasilylearnpython.Thankyousomuch.

Madhura

I am a 18 year old IT student studying at University in Ireland. I would like to

express mygratitude to you for writing yourbook "AByte of Python", I already

hadknowledgeof3programminglangaguesC,JavaandJavascript,andPython

wasbyfar theeasiest langagueIhaveever learned,and thatwasmainlybecause

yourbookwas fantasticandmade learningpythonverysimpleand interesting. It

isoneofthebestwrittenandeasyto followprogrammingbooks Ihaveeverread.

Congratulationsandkeepupthegreatwork.

Matt

Hi, ImfromDominicanRepublic.Myname isPavel, recentlyI readyourbookA

Byte of Pythonand I consider it excellent!! :). I learnt much from all the

examples.Yourbookisofgreathelpfornewbieslikeme

PavelSimo

https://twitter.com/YuvrajPoudyal/status/448050415356346368mailto:[email protected]://apas.gr/2010/04/27/learning-python/



Iam astudent fromChina, Now ,Ihave readyoubookAbyteofPython, Oh its

beautiful.Thebookisverysimplebutcanhelpall the first learnners.YouknowI

am interesting in Java and cloud computing many times, i have to coding

programm for the server, so i think python is a good choice, finish your book, i

think itsnot onlya goodchoice its mustuse thePython.MyEnglish isnot very

well, the email to you, i just wanna thank you! Best Wishes for you and your

family.

RoyLau

I recently finishedreading Byte of Python,and I thought I reallyought to thank

you. I was very sad to reach the final pages as I now have to go back to dull,

tedious oreilly or etc. manuals for learning about python. Anyway, I really

appreciateyourbook.

SamuelYoung

Dear Swaroop, I am taking a class from an instructor that has no interest in

teaching.We are using Learning Python, secondedition, by OReilly. It is not a

text for beginner without any programming knowledge, and an instructor that

shouldbeworking inanotherfield.Thankyouverymuch foryourbook,withoutit

I would be clueless about Python and programming. Thanks a million, you are

able tobreak themessagedowntoa level thatbeginnerscanunderstandandnot

everyonecan.

JosephDuarte

I love your book! It is the greatest Python tutorial ever, and a very useful

reference.Brilliant,atruemasterpiece!Keepupthegoodwork!

ChrisAndrSommerseth

mailto:[email protected]:[email protected]



Firstof all, Iwant to say thanks toyou for thisgreate book. I think it is agood

bookforthosewhoarelookingforabeginnerstutorialforPython.

It isabouttwoor thereyearsago, I think,when Ifirstheardof thisbook.At that

time, I am not able to read some book in English yet, so I got a chinese

translation,whichtookmeintothegateofPythonprogramming.

Recently, I reread this book. This time, of course, the english version. I couldnt

believe that Icanread thewholebookwithoutmydictionaryathand.Ofcourse,

itallduestoyourefforttomakethisbookaneasytounderstandone.

myd7349 (https://github.com/swaroopch/byte_of_python/pull/13)

Im justemailingyou to thankyou forwritingByteofPythononline. Ihadbeen

attempting Python for a few months prior to stumbling across your book, and

althoughImadelimitedsuccesswithpyGame,Inevercompletedaprogram.

Thankstoyoursimplificationofthecategories,Pythonactuallyseemsareachable

goal. It seems like Ihave finally learned the foundationsand Ican continue into

myrealgoal,gamedevelopment.

Onceagain, thanksVERYmuch forplacingsuchastructuredandhelpfulguideto

basic programming on the web. It shoved me into and out of OOP with an

understandingwheretwotextbookshadfailed.

MattGallivan

mailto:[email protected]://github.com/swaroopch/byte_of_python/pull/13



Iwould like to thank you foryourbookAByteofPythonwhich imyself find the

bestway to learn python. I ama 15yearold i live inegypt myname isAhmed.

Pythonwasmysecondprogramminglanguagei learnvisualbasic6at schoolbut

didntenjoy it,however i really enjoyed learningpython. Imade theaddressbook

program and i was sucessful. i will try to start make more programs and read

python programs (if you could tell me source that would be helpful). I will also

starton learning javaand if youcan tellme where to find a tutorialas goodas

yoursforjavathatwouldhelpmealot.Thanx.

AhmedMohammed

A wonderful resource for beginners wanting to learn more about Python is the

110pagePDF tutorialAByteofPythonbySwaroopCH.It iswellwritten,easy

tofollow,andmaybethebestintroductiontoPythonprogrammingavailable.

DrewAmes (http://www.linux.com/feature/126522)

Yesterday I got throughmost of Byte of Python on my Nokia N800 and its the

easiest and most concise introduction to Python I have yet encountered. Highly

recommendedasastartingpointforlearningPython.

JasonDelport (http://paxmodept.com/telesto/blogitem.htm?id=627)

Byte of Vim and Python by @swaroopch is by far the best works in technical

writingtome.Excellentreads#FeelGoodFactor

Surendran (http://twitter.com/suren/status/12840485454)

"Byteofpython"bestonebyfarman

(inresponse tothequestion"Cananyonesuggestagood, inexpensiveresourcefor

learningthebasicsofPython?")

JustinLoveTrue (http://www.facebook.com/pythonlang/posts/406873916788)

TheBookByteofpythonwasveryhelpful..Thanksbigtime:)

Chinmay (https://twitter.com/a_chinmay/status/258822633741762560)

http://www.linux.com/feature/126522http://paxmodept.com/telesto/blogitem.htm?id=627mailto:[email protected]://www.facebook.com/pythonlang/posts/406873916788https://twitter.com/a_chinmay/status/258822633741762560http://twitter.com/suren/status/12840485454



Always been a fan of A Byte of Python made for both new and experienced

programmers.

PatrickHarrington (http://stackoverflow.com/a/457785/4869)

I started learning python few days ago from your book..thanks for such a nice

book. it is so well written, you made my life easy..so you found a new fan of

yours..thatsme:)tonsofthanks.

GadadhariBheem (https://twitter.com/Pagal_e_azam/statuses/242865885256232960)

Before I started to learn Python, Ive acquired basic programming skills in

Assembly,C,C++,C#andJava.Theveryreason Iwanted to learnPythonis its

popular (peopleare talkingabout it) andpowerful (reality).Thisbookwrittenby

Mr. Swaroop is a very good guide for both brandnew programmers and new

pythonprogrammers.Took10halfdaystogothroughit.GreatHelp!

FangBiyi(PhDCandidateECE,MichiganStateUniversity)

Thankyoueversomuchforthisbook!!

Thisbook clearedup manyquestions I hadaboutcertain aspectsof Pythonsuch

asobjectorientedprogramming.

Idonot feel like anexpertatOObut Iknow thisbookhelped meona first step

ortwo.

Ihavenowwrittenseveralpythonprograms thatactuallydoreal thingsformeas

a system administrator. They are all procedural oriented but they are small by

mostpeoplesstandards.

Again,thanksforthisbook.Thankyouforhavingitontheweb.

Bob

mailto:[email protected]://twitter.com/Pagal_e_azam/statuses/242865885256232960http://stackoverflow.com/a/457785/4869



Ijustwanttothankyou forwritingthefirstbookonprogrammingIveeverreally

read.Python isnowmyfirst language,andIcanjust imagineall thepossibilities.

Sothankyouforgivingmethetools tocreatethings Ineverwouldhaveimagined

Icoulddobefore.

TheWalrus

Iwantedto thankyou forwritingAByteOfPython(2&3Versions). Ithasbeen

invaluabletomylearningexperienceinPython&Programmingingeneral.

Needlesstosay,Iamabeginnerintheprogrammingworld,acoupleofmonthsof

selfstudyuptothispoint.Ihadbeenusingyoutube tutorials&someotheronline

tutorials includingother freebooks. Idecided todig intoyour bookyesterday,&

Ive learned more on the first few pages than any other book or tutorial. A few

thingsIhadbeenconfusedabout,wereclearedrightupwithaGREATexample&

explanation.Cantwaittoread(andlearn)more!!

Thank you so much for not only writing the book, but for putting it under the

creative commons license (free). Thank goodness there are unselfish people like

yououttheretohelp&teachtherestofus.

Chris



Iwroteyouback in2011and Iwas justgetting into Pythonandwanted to thankyou for your tutorial "A Byte of Python". Without it, I would have fallen by thewayside. Since then I have gone on to program a number of functions in my

organization with this language with yet more on the horizon. I would not call

myselfanadvancedprogrammerbyanystretchbutInoticetheoccasionalrequest

forassistancenowfromotherssinceI startedusing it.Idiscovered,while reading

"Byte"why IhadceasedstudyingCandC[]+and itwasbecausethebookgiven

to me started out with an example containing an augmented assignment. Of

course, there wasno explanation for thisarrangement ofoperators and I fellon

myhead trying tomakesenseofwhatwasonthewrittenpage. As I recallitwas

amost frustratingexercisewhich Ieventuallyabandoned.Doesn'tmeanCorC+

is impossible to learn, or even that I am stupid, but it does mean that the

documentation I worked my way through did not define the symbols and words

whichisanessentialpartofanyinstruction.Justascomputerswillnotbeableto

understandacomputerwordorcomputersymbol thatisoutside thesyntax forthe

language being used, a student new to any field will not grasp his subject if he

encounters words or symbols for which there areno definitions. You get a "blue

screen"as itwereineithercase.Thesolution is simple, though:find thewordor

symbolandget theproperdefinitionor symbolandloandbehold,thecomputeror

studentcanproceed.Yourbookwassowellputtogether that Ifoundverylittle in

it I couldnt grasp. So, thank you. I encourage you to continue to include full

definitionsofterms.ThedocumentationwithPython isgood,onceyouknow, (the

examples are its strength from what I see) but in many cases it seems that you

have to know in order to understand thedocumentation which to mymind isnot

what should be. Third party tutorials express the need for clarification of the

documentation and their success largely depends on the words that are used to

describethe terminology.Ihaverecommendedyourbooktomanyothers.Somein

Australia, some in the Caribbean and yet others in the US. It fills a niche no

othersdo.Ihopeyouaredoingwellandwishyouallthesuccessinthefuture.

Nick



hey, this isankush(19). Iwas facingagreatdifficulty to startwithpython. I tried

a lot ofbooksbut allwerebulkier andnot targetoriented and then i found this

lovelyone,whichmademelovepythoninnotime.Thanksalotfor this"beautiful

pieceofbook".

Ankush

I would like to thank you for your excellent guide on Python. I am a molecular

biologist (with littleprogrammingbackground)and formywork Ineed tohandle

big datasets of DNA sequences and to analyse microscope images. For both

things, programming in python has been useful, if not essential to complete and

publisha6yearsproject.

That suchaguide is freelyavailable isaclear sign that the forcesofevilarenot

yetrulingtheworld!:)

Luca

Since this is going to be the first language you learn, you should use A Byte of

Python.It reallygivesaproper introductionintoprogramminginPythonand itis

pacedwell enough for the averagebeginner.The most important thing from then

onwillbeactuallystartingtopracticemakingyourownlittleprograms.

"{Unregistered}"(http://www.overclock.net/t/1177951/wanttolearnprogrammingwheredoistart#post_15837176)

Just to say a loud and happy thank you very muchfor publishing "A Byte of

Python" and "A Byte of Vim". Those books were very useful to me four or five

years ago when I starting learning programming. Right now Im developing a

project that was a dream for a long, long time and just want to say thank you.

Keepwalking.Youareasourceofmotivation.Allthebest.

Jocimar

Finished reading A byte of Python in 3 days. It is thoroughly interesting. Not a

singlepage wasboring. Iwant tounderstand theOrca screenreader code.Your

bookhashopefullyequippedmeforit.

http://www.overclock.net/t/1177951/want-to-learn-programming-where-do-i-start#post_15837176



Dattatray

Hi, 'A byte of python' is really a good reading for python beginners. So, again,

NICEWORK!

im a4 yearsexperienced Java&Cdeveloper fromChina. Recently, i want to do

someworkonzimwikinoteprojectwhichusespygtktoimplement.

i readyourbook in6days,and icanreadand writepythoncodeexamplesnow.

thx foryourcontribution.plzkeepyourenthusiasm tomake thisworldbetter, this

isjustalittleencouragefromChina.YourreaderLee

LEE

I am Isen from Taiwan, who is a graduating PhD student in Electrical

EngineeringDepartmentofNationalTaiwanUniversity. Iwould liketo thankyou

foryourgreatbook. Ithink itisnotonlyjusteasyto readbutalsocomprehensive

and complete fora new comer ofPython. The reason I read your book is that I

am starting to workon theGNU Radio framework. Yourbook letme catchmost

ofimportantcoreideasandskillofPythonwithaminimumtime.

Ialsosawthatyoudonotmind that readers sendyoua thanknoteinyourbook.

SoIreallylikeyourbookandappreciateit.Thanks.

IsenIChunChao

ThebookisevenusedbyNASA!ItisbeingusedintheirJetPropulsionLaboratory(http://dsnra.jpl.nasa.gov/software/Python/byteofpython/output/byteofpython_html/)withtheirDeepSpaceNetworkproject.

1.2.AcademicCoursesThisbookis/wasbeingusedasinstructionalmaterialinvariouseducationalinstitutions:

'PrinciplesofProgrammingLanguages'courseat'VrijeUniversiteit,Amsterdam'(http://www.few.vu.nl/~nsilvis/PPL/2007/index.html)

'BasicConceptsofComputing'courseat'UniversityofCalifornia,Davis'(http://www.cs.ucdavis.edu/courses/exp_course_desc/10.html)

http://dsnra.jpl.nasa.gov/software/Python/byte-of-python/output/byteofpython_html/http://www.few.vu.nl/~nsilvis/PPL/2007/index.htmlhttp://www.cs.ucdavis.edu/courses/exp_course_desc/10.htmlmailto:[email protected]:[email protected]



'ProgrammingWithPython'courseat'HarvardUniversity'(http://www.people.fas.harvard.edu/~preshman/python_winter.html)

'IntroductiontoProgramming'courseat'UniversityofLeeds' (http://www.comp.leeds.ac.uk/acom1900/)

'IntroductiontoApplicationProgramming'courseat'BostonUniversity'(http://www.cs.bu.edu/courses/cs108/materials.html)

'InformationTechnologySkillsforMeteorology'courseat'UniversityofOklahoma'(http://gentry.metr.ou.edu/byteofpython/)

'Geoprocessing'courseat'MichiganStateUniversity' (http://www.msu.edu/~ashton/classes/825/index.html)

'MultiAgentSemanticWebSystems'courseatthe'UniversityofEdinburgh'(http://homepages.inf.ed.ac.uk/ewan/masws/)

'IntroductiontoComputerScienceandProgramming'at'MITOpenCourseWare'(http://ocw.mit.edu/courses/electricalengineeringandcomputerscience/600scintroductiontocomputerscienceandprogrammingspring2011/references/)

'BasicprogrammingattheFacultyofSocialSciences,UniversityofLjubljana,Slovenia'Aleibernasays"I(andmypredecessor)havebeenusingyourbookasthemainliteratureforthiscourse"

1.3.LicenseThisbookislicensedunderaCreativeCommonsAttributionShareAlike4.0InternationalLicense(http://creativecommons.org/licenses/bysa/4.0/).

Thismeans:

YouarefreetoSharei.e.tocopy,distributeandtransmitthisbook

YouarefreetoRemixi.e.tomakechangestothisbook(especiallytranslations)

Youarefreetouseitforcommercialpurposes

Pleasenote:

Pleasedonotsellelectronicorprintedcopiesofthebookunlessyouhaveclearlyandprominentlymentionedinthedescriptionthatthesecopiesarenotfromtheoriginalauthorofthisbook.

Attributionmustbeshownintheintroductorydescriptionandfrontpageofthedocumentbylinkingbacktohttp://swaroopch.com/notes/pythonandclearlyindicatingthattheoriginaltextcanbefetchedfromthislocation.

http://www.msu.edu/~ashton/classes/825/index.htmlhttp://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/references/http://www.cs.bu.edu/courses/cs108/materials.htmlmailto:[email protected]://homepages.inf.ed.ac.uk/ewan/masws/http://www.people.fas.harvard.edu/~preshman/python_winter.htmlhttp://www.comp.leeds.ac.uk/acom1900/http://gentry.metr.ou.edu/byteofpython/http://creativecommons.org/licenses/by-sa/4.0/http://swaroopch.com/notes/python



Allthecode/scriptsprovidedinthisbookislicensedunderthe3clauseBSDLicense(http://www.opensource.org/licenses/bsdlicense.php)unlessotherwisenoted.

1.4.ReadNowYoucanreadthebookonlineathttp://swaroopch.com/notes/python

1.5.BuyTheBookAprintedhardcopyofthebookcanbepurchasedathttp://swaroopch.com/buybookforyourofflinereadingpleasure,andtosupportthecontinueddevelopmentandimprovementofthisbook.

1.6.DownloadPDF (http://files.swaroopch.com/python/byte_of_python.pdf)(fordesktopreading,etc.)

EPUB (http://files.swaroopch.com/python/byte_of_python.epub)(foriPhone/iPad,ebookreaders,etc.)

Mobi(forKindle) (http://files.swaroopch.com/python/byte_of_python.mobi)

GitHub (https://github.com/swaroopch/byte_of_python)(forrawtext,translating,etc.)

Ifyouwishtosupportthecontinueddevelopmentofthisbook,pleaseconsiderbuyingahardcopy(http://swaroopch.com/buybook).

1.7.ReadthebookinyournativelanguageIfyouareinterestedinreadingorcontributingtranslationsofthisbooktootherhumanlanguages,pleaseseeTranslations.

http://files.swaroopch.com/python/byte_of_python.pdfhttp://www.opensource.org/licenses/bsd-license.phphttp://swaroopch.com/buybookhttp://swaroopch.com/buybookhttp://files.swaroopch.com/python/byte_of_python.epubhttp://swaroopch.com/notes/pythonhttp://files.swaroopch.com/python/byte_of_python.mobihttps://github.com/swaroopch/byte_of_python



2.DedicationToKalyanVarma (http://www.kalyanvarma.net/)andmanyotherseniorsatPESIT (http://www.pes.edu/)whointroducedustoGNU/Linuxandtheworldofopensource.

TothememoryofAtulChitnis (http://www.nextbigwhat.com/atulchitnisobituary297/),afriendandguidewhoshallbemissedgreatly.

TothepioneerswhomadetheInternethappen (http://www.ibiblio.org/pioneers/index.html).Thisbookwasfirstwrittenin2003.Itstillremainspopular,thankstothenatureofsharingknowledgeontheInternetasenvisionedbythepioneers.

http://www.nextbigwhat.com/atul-chitnis-obituary-297/http://www.ibiblio.org/pioneers/index.htmlhttp://www.kalyanvarma.net/http://www.pes.edu/



3.PrefacePythonisprobablyoneofthefewprogramminglanguageswhichisbothsimpleandpowerful.Thisisgoodforbeginnersaswellasforexperts,andmoreimportantly,isfuntoprogramwith.Thisbookaimstohelpyoulearnthiswonderfullanguageandshowhowtogetthingsdonequicklyandpainlesslyineffect'TheAntivenomtoyourprogrammingproblems'.

3.1.WhoThisBookIsForThisbookservesasaguideortutorialtothePythonprogramminglanguage.Itismainlytargetedatnewbies.Itisusefulforexperiencedprogrammersaswell.

Theaimisthatifallyouknowaboutcomputersishowtosavetextfiles,thenyoucanlearnPythonfromthisbook.Ifyouhavepreviousprogrammingexperience,thenyoucanalsolearnPythonfromthisbook.

Ifyoudohavepreviousprogrammingexperience,youwillbeinterestedinthedifferencesbetweenPythonandyourfavoriteprogramminglanguageIhavehighlightedmanysuchdifferences.Alittlewarningthough,Pythonissoongoingtobecomeyourfavoriteprogramminglanguage!

3.2.OfficialWebsiteTheofficialwebsiteofthebookishttp://swaroopch.com/notes/pythonwhereyoucanreadthewholebookonline,downloadthelatestversionsofthebook,buyaprintedhardcopy(http://swaroopch.com/buybook)andalsosendmefeedback.

3.3.SomethingToThinkAbout

There are two waysof constructing a software design: one way is to make it so

simple that there are obviously no deficiencies the other is to make it so

complicatedthattherearenoobviousdeficiencies.

C.A.R.Hoare

Success in life is a matter not so much of talent and opportunity as of

concentrationandperseverance.

C.W.Wendte

http://swaroopch.com/buybookhttp://swaroopch.com/notes/python



4.IntroductionPythonisoneofthoserarelanguageswhichcanclaimtobebothsimpleandpowerful.Youwillfindyourselfpleasantlysurprisedtoseehoweasyitistoconcentrateonthesolutiontotheproblemratherthanthesyntaxandstructureofthelanguageyouareprogrammingin.

TheofficialintroductiontoPythonis:

Python isaneasy tolearn,powerfulprogramminglanguage. Ithasefficienthigh

level data structures and a simple but effective approach to objectoriented

programming. Pythons elegant syntax and dynamic typing, together with its

interpretednature, make it an ideal language for scriptingand rapidapplication

developmentinmanyareasonmostplatforms.

Iwilldiscussmostofthesefeaturesinmoredetailinthenextsection.

StorybehindthenameGuidovanRossum,thecreatorofthePythonlanguage,namedthelanguageaftertheBBCshow"MontyPythonsFlyingCircus".Hedoesntparticularlylikesnakesthatkillanimalsforfoodbywindingtheirlongbodiesaroundthemandcrushingthem.

4.1.FeaturesofPythonSimple

Pythonisasimpleandminimalisticlanguage.ReadingagoodPythonprogramfeelsalmostlikereadingEnglish,althoughverystrictEnglish!ThispseudocodenatureofPythonisoneofitsgreateststrengths.Itallowsyoutoconcentrateonthesolutiontotheproblemratherthanthelanguageitself.

EasytoLearn

Asyouwillsee,Pythonisextremelyeasytogetstartedwith.Pythonhasanextraordinarilysimplesyntax,asalreadymentioned.

FreeandOpenSource

PythonisanexampleofaFLOSS(Free/LibrandOpenSourceSoftware).Insimpleterms,youcanfreelydistributecopiesofthissoftware,readitssourcecode,makechangestoit,andusepiecesofitinnewfreeprograms.FLOSSisbasedontheconceptofacommunitywhichsharesknowledge.Thisis



oneofthereasonswhyPythonissogoodithasbeencreatedandisconstantlyimprovedbyacommunitywhojustwanttoseeabetterPython.

HighlevelLanguage

WhenyouwriteprogramsinPython,youneverneedtobotheraboutthelowleveldetailssuchasmanagingthememoryusedbyyourprogram,etc.

Portable

Duetoitsopensourcenature,Pythonhasbeenportedto(i.e.changedtomakeitworkon)manyplatforms.AllyourPythonprogramscanworkonanyoftheseplatformswithoutrequiringanychangesatallifyouarecarefulenoughtoavoidanysystemdependentfeatures.

YoucanusePythononGNU/Linux,Windows,FreeBSD,Macintosh,Solaris,OS/2,Amiga,AROS,AS/400,BeOS,OS/390,z/OS,PalmOS,QNX,VMS,Psion,AcornRISCOS,VxWorks,PlayStation,SharpZaurus,WindowsCEandPocketPC!

YoucanevenuseaplatformlikeKivy (http://kivy.org)tocreategamesforyourcomputerandforiPhone,iPad,andAndroid.

Interpreted

Thisrequiresabitofexplanation.

AprogramwritteninacompiledlanguagelikeCorC[]+isconvertedfromthesourcelanguagei.e.CorC+intoalanguagethatisspokenbyyourcomputer(binarycodei.e.0sand1s)usingacompilerwithvariousflagsandoptions.Whenyouruntheprogram,thelinker/loadersoftwarecopiestheprogramfromharddisktomemoryandstartsrunningit.

Python,ontheotherhand,doesnotneedcompilationtobinary.Youjustruntheprogramdirectlyfromthesourcecode.Internally,Pythonconvertsthesourcecodeintoanintermediateformcalledbytecodesandthentranslatesthisintothenativelanguageofyourcomputerandthenrunsit.Allthis,actually,makesusingPythonmucheasiersinceyoudonthavetoworryaboutcompilingtheprogram,makingsurethattheproperlibrariesarelinkedandloaded,etc.ThisalsomakesyourPythonprogramsmuchmoreportable,sinceyoucanjustcopyyourPythonprogramontoanothercomputeranditjustworks!

ObjectOriented

Pythonsupportsprocedureorientedprogrammingaswellasobjectorientedprogramming.Inprocedureorientedlanguages,theprogramisbuiltaroundproceduresorfunctionswhicharenothingbutreusablepiecesofprograms.Inobjectorientedlanguages,theprogramisbuiltaroundobjects

http://kivy.org/



whichcombinedataandfunctionality.PythonhasaverypowerfulbutsimplisticwayofdoingOOP,especiallywhencomparedtobiglanguageslikeC++orJava.

Extensible

Ifyouneedacriticalpieceofcodetorunveryfastorwanttohavesomepieceofalgorithmnottobeopen,youcancodethatpartofyourprograminCorC\++andthenuseitfromyourPythonprogram.

Embeddable

YoucanembedPythonwithinyourC/C\++programstogivescriptingcapabilitiesforyourprogramsusers.

ExtensiveLibraries

ThePythonStandardLibraryishugeindeed.Itcanhelpyoudovariousthingsinvolvingregularexpressions,documentationgeneration,unittesting,threading,databases,webbrowsers,CGI,FTP,email,XML,XMLRPC,HTML,WAVfiles,cryptography,GUI(graphicaluserinterfaces),andothersystemdependentstuff.Remember,allthisisalwaysavailablewhereverPythonisinstalled.ThisiscalledtheBatteriesIncludedphilosophyofPython.

Besidesthestandardlibrary,therearevariousotherhighqualitylibrarieswhichyoucanfindatthePythonPackageIndex (http://pypi.python.org/pypi).

Summary

Pythonisindeedanexcitingandpowerfullanguage.IthastherightcombinationofperformanceandfeaturesthatmakewritingprogramsinPythonbothfunandeasy.

4.2.Python2versus3Youcanignorethissectionifyourenotinterestedinthedifferencebetween"Pythonversion2"and"Pythonversion3".Butpleasedobeawareofwhichversionyouareusing.ThisbookiswrittenforPython2.

Rememberthatonceyouhaveproperlyunderstoodandlearntouseoneversion,youcaneasilylearnthedifferencesandusetheotherone.ThehardpartislearningprogrammingandunderstandingthebasicsofPythonlanguageitself.Thatisourgoalinthisbook,andonceyouhaveachievedthatgoal,youcaneasilyusePython2orPython3dependingonyoursituation.

FordetailsondifferencesbetweenPython2andPython3,see:

ThefutureofPython2 (http://lwn.net/Articles/547191/)

http://lwn.net/Articles/547191/http://pypi.python.org/pypi



Python/3pageontheUbuntuwiki (https://wiki.ubuntu.com/Python/3)

4.3.WhatProgrammersSayYoumayfinditinterestingtoreadwhatgreathackerslikeESRhavetosayaboutPython:

1. EricS.Raymondistheauthorof"TheCathedralandtheBazaar"andisalsothepersonwhocoinedthetermOpenSource.HesaysthatPythonhasbecomehisfavoriteprogramminglanguage(http://www.python.org/about/success/esr/).ThisarticlewastherealinspirationformyfirstbrushwithPython.

2. BruceEckelistheauthorofthefamous'ThinkinginJava'and'ThinkinginC++'books.HesaysthatnolanguagehasmadehimmoreproductivethanPython.HesaysthatPythonisperhapstheonlylanguagethatfocusesonmakingthingseasierfortheprogrammer.Readthecompleteinterview(http://www.artima.com/intv/aboutme.html)formoredetails.

3. PeterNorvigisawellknownLispauthorandDirectorofSearchQualityatGoogle(thankstoGuidovanRossumforpointingthatout).HesaysthatwritingPythonislikewritinginpseudocode(https://news.ycombinator.com/item?id=1803815).HesaysthatPythonhasalwaysbeenanintegralpartofGoogle.YoucanactuallyverifythisstatementbylookingattheGoogleJobs(http://www.google.com/jobs/index.html)pagewhichlistsPythonknowledgeasarequirementforsoftwareengineers.

https://news.ycombinator.com/item?id=1803815http://www.artima.com/intv/aboutme.htmlhttp://www.python.org/about/success/esr/https://wiki.ubuntu.com/Python/3http://www.google.com/jobs/index.html



5.InstallationWhenwereferto"Python2"inthisbook,wewillbereferringtoanyversionofPythonequaltoorgreaterthanversion2.7 (https://www.python.org/downloads/).

5.1.InstallationonWindowsVisithttps://www.python.org/downloads/anddownloadthelatestversion.TheinstallationisjustlikeanyotherWindowsbasedsoftware.

CAUTIONWhenyouaregiventheoptionofuncheckingany"optional"components,dontuncheckany.

5.1.1.DOSPrompt

IfyouwanttobeabletousePythonfromtheWindowscommandlinei.e.theDOSprompt,thenyouneedtosetthePATHvariableappropriately.

ForWindows2000,XP,2003,clickonControlPanelSystemAdvancedEnvironmentVariables.ClickonthevariablenamedPATHintheSystemVariablessection,thenselectEditandadd;C:\Python27 (pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatedirectoryname.

ForolderversionsofWindows,openthefile C:\AUTOEXEC.BAT andaddthelinePATH=%PATH%;C:\Python33 andrestartthesystem.ForWindowsNT,usetheAUTOEXEC.NTfile.

ForWindowsVista:

1. ClickStartandchooseControlPanel

2. ClickSystem,ontherightyoullsee"Viewbasicinformationaboutyourcomputer"

3. Ontheleftisalistoftasks,thelastofwhichisAdvancedsystemsettings.Clickthat.

4. TheAdvancedtaboftheSystemPropertiesdialogboxisshown.ClicktheEnvironmentVariablesbuttononthebottomright.

5. InthelowerboxtitledSystemVariablesscrolldowntoPathandclicktheEditbutton.

6. Changeyourpathasneedbe.

7. Restartyoursystem.VistadidntpickupthesystempathenvironmentvariablechangeuntilIrestarted.

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



ForWindows7and8:

1. RightclickonComputerfromyourdesktopandselectPropertiesorclickStartandchooseControlPanelSystemandSecuritySystem.ClickonAdvancedsystemsettingsontheleftandthenclickontheAdvancedtab.AtthebottomclickonEnvironmentVariablesandunderSystemvariables,lookforthePATHvariable,selectandthenpressEdit.

2. GototheendofthelineunderVariablevalueandappend ;C:\Python27 (pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatefoldername.

3. Ifthevaluewas %SystemRoot%\system32; Itwillnowbecome%SystemRoot%\system32;C:\Python27

4. ClickOKandyouaredone.Norestartisrequired,howeveryoumayhavetocloseandreopenthecommandline.

5.1.2.RunningPythonpromptonWindows

ForWindowsusers,youcanruntheinterpreterinthecommandlineifyouhavesetthe PATH variableappropriately.

ToopentheterminalinWindows,clickthestartbuttonandclickRun.Inthedialogbox,typecmdandpress enter key.

Then,typepythonandensuretherearenoerrors.

5.2.InstallationonMacOSXForMacOSXusers,Pythonmustbeinstalledalready.

Toverify,opentheterminalbypressing Command + Space keys(toopenSpotlightsearch),typeTerminalandpress enter key.Now,runpythonandensuretherearenoerrors.

5.3.InstallationonGNU/LinuxForGNU/Linuxusers,Pythonmustbeinstalledalready.

Toverify,opentheterminalbyopeningtheTerminalapplicationorbypressing Alt + F2 andenteringgnometerminal.Ifthatdoesntwork,pleasereferthedocumentationofyourparticularGNU/Linuxdistribution.Now,runpythonandensuretherearenoerrors.

YoucanseetheversionofPythononthescreenbyrunning:



NOTE$ isthepromptoftheshell.Itwillbedifferentforyoudependingonthesettingsoftheoperatingsystemonyourcomputer,henceIwillindicatethepromptbyjustthe $symbol.

CAUTIONOutputmaybedifferentonyourcomputer,dependingontheversionofPythonsoftwareinstalledonyourcomputer.

5.4.SummaryFromnowon,wewillassumethatyouhavePythoninstalledonyoursystem.

Next,wewillwriteourfirstPythonprogram.

$pythonVPython2.7.6



6.FirstStepsWewillnowseehowtorunatraditional'HelloWorld'programinPython.Thiswillteachyouhowtowrite,saveandrunPythonprograms.

TherearetwowaysofusingPythontorunyourprogramusingtheinteractiveinterpreterpromptorusingasourcefile.Wewillnowseehowtousebothofthesemethods.

6.1.UsingTheInterpreterPromptOpentheterminalinyouroperatingsystem(asdiscussedpreviouslyintheInstallationchapter)andthenopenthePythonpromptbytypingpythonandpressing enter key.

OnceyouhavestartedPython,youshouldsee >>> whereyoucanstarttypingstuff.ThisiscalledthePythoninterpreterprompt.

AtthePythoninterpreterprompt,type:

followedbythe enter key.YoushouldseethewordsHelloWorldprintedtothescreen.

Hereisanexampleofwhatyoushouldbeseeing,whenusingaMacOSXcomputer.ThedetailsaboutthePythonsoftwarewilldifferbasedonyourcomputer,butthepartfromtheprompt(i.e.from >>>onwards)shouldbethesameregardlessoftheoperatingsystem.

NoticethatPythongivesyoutheoutputofthelineimmediately!WhatyoujustenteredisasinglePythonstatement.Weuseprintto(unsurprisingly)printanyvaluethatyousupplytoit.Here,wearesupplyingthetexthelloworldandthisispromptlyprintedtothescreen.

HowtoQuittheInterpreterPrompt

print"HelloWorld"PYTHON

$pythonPython2.7.6(default,Feb232014,16:08:15)[GCC4.2.1CompatibleAppleLLVM5.0(clang500.2.79)]ondarwinType"help","copyright","credits"or"license"formoreinformation.>>>print"helloworld"helloworld>>>



NOTEIfyouareusingaGNU/LinuxorOSXshell,youcanexittheinterpreterpromptbypressing ctrl + d orenteringexit()(note:remembertoincludetheparentheses,())followedbythe enter key.

IfyouareusingtheWindowscommandprompt,press ctrl + z followedbytheenter key.

6.2.ChoosingAnEditorWecannottypeoutourprogramattheinterpreterprompteverytimewewanttorunsomething,sowehavetosavetheminfilesandcanrunourprogramsanynumberoftimes.

TocreateourPythonsourcefiles,weneedaneditorsoftwarewhereyoucantypeandsave.Agoodprogrammerseditorwillmakeyourlifeeasierinwritingthesourcefiles.Hence,thechoiceofaneditoriscrucialindeed.Youhavetochooseaneditorasyouwouldchooseacaryouwouldbuy.AgoodeditorwillhelpyouwritePythonprogramseasily,makingyourjourneymorecomfortableandhelpsyoureachyourdestination(achieveyourgoal)inamuchfasterandsaferway.

OneoftheverybasicrequirementsissyntaxhighlightingwhereallthedifferentpartsofyourPythonprogramarecolorizedsothatyoucanseeyourprogramandvisualizeitsrunning.

Ifyouhavenoideawheretostart,IwouldrecommendusingPyCharmEducationalEdition(https://www.jetbrains.com/pycharmeducational/)softwarewhichisavailableonWindows,MacOSXandGNU/Linux.Detailsinthenextsection.

IfyouareusingWindows,donotuseNotepaditisabadchoicebecauseitdoesnotdosyntaxhighlightingandalsoimportantlyitdoesnotsupportindentationofthetextwhichisveryimportantinourcaseaswewillseelater.Goodeditorswillautomaticallydothis.

Ifyouareanexperiencedprogrammer,thenyoumustbealreadyusingVim (http://www.vim.org)orEmacs(http://www.gnu.org/software/emacs/).Needlesstosay,thesearetwoofthemostpowerfuleditorsandyouwillbenefitfromusingthemtowriteyourPythonprograms.Ipersonallyusebothformostofmyprograms,andhaveevenwrittenanentirebookonVim (http://swaroopch.com/notes/vim).

IncaseyouarewillingtotakethetimetolearnVimorEmacs,thenIhighlyrecommendthatyoudolearntouseeitherofthemasitwillbeveryusefulforyouinthelongrun.However,asImentionedbefore,beginnerscanstartwithPyCharmandfocusthelearningonPythonratherthantheeditoratthismoment.

Toreiterate,pleasechooseapropereditoritcanmakewritingPythonprogramsmorefunandeasy.

http://swaroopch.com/notes/vimhttp://www.vim.org/http://www.gnu.org/software/emacs/https://www.jetbrains.com/pycharm-educational/



6.3.PyCharmPyCharmEducationalEdition (https://www.jetbrains.com/pycharmeducational/)isafreeeditorwhichyoucanuseforwritingPythonprograms.

WhenyouopenPyCharm,youllseethis,clickonCreateNewProject:

SelectPurePython:

https://www.jetbrains.com/pycharm-educational/



Changeuntitledtohelloworldasthelocationoftheproject,youshouldseedetailssimilartothis:

ClicktheCreatebutton.

RightclickonthehelloworldinthesidebarandselectNewPythonFile:



PyCharm>New>PythonFile

Youwillbeaskedtotypethename,typehello:



PyCharmNewFiledialogbox

Youcannowseeafileopenedforyou:



PyCharmhello.pyfile

Deletethelinesthatarealreadypresent,andnowtypethefollowing:

Nowrightclickonwhatyoutyped(withoutselectingthetext),andclickonRun'hello'.

print"helloworld"PYTHON



PyCharmRun'hello'

Youshouldnowseetheoutput(whatitprints)ofyourprogram:



PyCharmoutput

Phew!Thatwasquiteafewstepstogetstarted,buthenceforth,everytimeweaskyoutocreateanewfile,remembertojustrightclickonhelloworldontheleftNewPythonFileandcontinuethesamestepstotypeandrunasshownabove.

YoucanfindmoreinformationaboutPyCharminthePyCharmQuickstart(https://www.jetbrains.com/pycharmeducational/quickstart/)page.

6.4.Vim1. InstallVim (http://www.vim.org)

a. MacOSXusersshouldinstallmacvimpackageviaHomeBrew (http://brew.sh/)

http://www.vim.org/http://brew.sh/https://www.jetbrains.com/pycharm-educational/quickstart/



b. Windowsusersshoulddownloadthe"selfinstallingexecutable"fromhttp://www.vim.org/download.php

c. GNU/LinuxusersshouldgetVimfromtheirdistributionssoftwarerepositories,e.g.DebianandUbuntuuserscaninstallthevim (http://packages.ubuntu.com/saucy/vim)package.

2. ReadtheVimasPythonIDE (http://blog.sontek.net/blog/detail/turningvimintoamodernpythonide)articlebyJohnMAnderson.

3. Installjedivim (https://github.com/davidhalter/jedivim)pluginforautocompletion.

6.5.Emacs1. InstallEmacs24 (http://www.gnu.org/software/emacs/).

a. MacOSXusersshouldgetEmacsfromhttp://emacsformacosx.com

b. WindowsusersshouldgetEmacsfromhttp://ftp.gnu.org/gnu/emacs/windows/

c. GNU/LinuxusersshouldgetEmacsfromtheirdistributionssoftwarerepositories,e.g.DebianandUbuntuuserscaninstalltheemacs24 (http://packages.ubuntu.com/saucy/emacs24)package.

2. InstallELPY (https://github.com/jorgenschaefer/elpy/wiki).

6.6.UsingASourceFileNowletsgetbacktoprogramming.Thereisatraditionthatwheneveryoulearnanewprogramminglanguage,thefirstprogramthatyouwriteandrunisthe'HelloWorld'programallitdoesisjustsay

'HelloWorld'whenyourunit.AsSimonCozens[1]says,itisthe"traditionalincantationtotheprogramminggodstohelpyoulearnthelanguagebetter."

Startyourchoiceofeditor,enterthefollowingprogramandsaveitashello.py.

IfyouareusingPyCharm,wehavealreadydiscussedhowtorunfromasourcefile.

Forothereditors,openanewfilehello.pyandtypethis:

Whereshouldyousavethefile?Toanyfolderforwhichyouknowthelocationofthefolder.Ifyoudontunderstandwhatthatmeans,createanewfolderandusethatlocationtosaveandrunallyourPythonprograms:

/tmp/py onMacOSX

print"helloworld"PYTHON

http://blog.sontek.net/blog/detail/turning-vim-into-a-modern-python-idehttps://github.com/jorgenschaefer/elpy/wikihttp://www.vim.org/download.phphttp://packages.ubuntu.com/saucy/emacs24http://ftp.gnu.org/gnu/emacs/windows/http://packages.ubuntu.com/saucy/vimhttp://emacsformacosx.com/https://github.com/davidhalter/jedi-vimhttp://www.gnu.org/software/emacs/



/tmp/py onGNU/Linux

C:\\py onWindows

Tocreatetheabovefolder(fortheoperatingsystemyouareusing),usethemkdircommandintheterminal,forexample,mkdir/tmp/py.

IMPORTANT Alwaysensurethatyougiveitthefileextensionof.py,forexample,foo.py.

TorunyourPythonprogram:

1. Openaterminalwindow(seethepreviousInstallationchapteronhowtodothat)

2. Changedirectorytowhereyousavedthefile,forexample,cd/tmp/py

3. Runtheprogrambyenteringthecommandpythonhello.py.Theoutputisasshownbelow.

$pythonhello.pyhelloworld



Screenshotofrunningprograminterminal

Ifyougottheoutputasshownabove,congratulations!youhavesuccessfullyrunyourfirstPythonprogram.Youhavesuccessfullycrossedthehardestpartoflearningprogramming,whichis,gettingstartedwithyourfirstprogram!

Incaseyougotanerror,pleasetypetheaboveprogramexactlyasshownaboveandruntheprogramagain.NotethatPythoniscasesensitivei.e.printisnotthesameasPrintnotethelowercasepintheformerandtheuppercasePinthelatter.Also,ensuretherearenospacesortabsbeforethefirstcharacterineachlinewewillseewhythisisimportantlater.

HowItWorks

APythonprogramiscomposedofstatements.Inourfirstprogram,wehaveonlyonestatement.Inthisstatement,wecalltheprintstatementtowhichwesupplythetext"helloworld".

6.7.GettingHelp



IfyouneedquickinformationaboutanyfunctionorstatementinPython,thenyoucanusethebuiltinhelpfunctionality.Thisisveryusefulespeciallywhenusingtheinterpreterprompt.Forexample,runhelp('len') thisdisplaysthehelpforthelenfunctionwhichisusedtocountnumberofitems.

TIP Pressqtoexitthehelp.

Similarly,youcanobtaininformationaboutalmostanythinginPython.Usehelp()tolearnmoreaboutusinghelpitself!

Incaseyouneedtogethelpforoperatorslikereturn,thenyouneedtoputthoseinsidequotessuchashelp('return') sothatPythondoesntgetconfusedonwhatweretryingtodo.

6.8.SummaryYoushouldnowbeabletowrite,saveandrunPythonprogramsatease.

NowthatyouareaPythonuser,letslearnsomemorePythonconcepts.



7.BasicsJustprintinghelloworldisnotenough,isit?Youwanttodomorethanthatyouwanttotakesomeinput,manipulateitandgetsomethingoutofit.WecanachievethisinPythonusingconstantsandvariables,andwelllearnsomeotherconceptsaswellinthischapter.

7.1.CommentsCommentsareanytexttotherightofthe # symbolandismainlyusefulasnotesforthereaderoftheprogram.

Forexample:

or:

Useasmanyusefulcommentsasyoucaninyourprogramto:

explainassumptions

explainimportantdecisions

explainimportantdetails

explainproblemsyouretryingtosolve

explainproblemsyouretryingtoovercomeinyourprogram,etc.

Codetellsyouhow,commentsshouldtellyouwhy(http://www.codinghorror.com/blog/2006/12/codetellsyouhowcommentstellyouwhy.html).

Thisisusefulforreadersofyourprogramsothattheycaneasilyunderstandwhattheprogramisdoing.Remember,thatpersoncanbeyourselfaftersixmonths!

7.2.LiteralConstantsAnexampleofaliteralconstantisanumberlike 5 , 1.23 ,orastringlike 'Thisisastring' or"Itsastring!" .

print'helloworld'#NotethatprintisastatementPYTHON

#Notethatprintisastatementprint'helloworld'

PYTHON

http://www.codinghorror.com/blog/2006/12/code-tells-you-how-comments-tell-you-why.html



Itiscalledaliteralbecauseitisliteralyouuseitsvalueliterally.Thenumber 2 alwaysrepresentsitselfandnothingelseitisaconstantbecauseitsvaluecannotbechanged.Hence,allthesearereferredtoasliteralconstants.

7.3.NumbersNumbersaremainlyoftwotypesintegersandfloats.

Anexamplesofanintegeris 2 whichisjustawholenumber.

Examplesoffloatingpointnumbers(orfloatsforshort)are 3.23 and 52.3E4 .TheEnotationindicatespowersof10.Inthiscase, 52.3E4 means52.3*10^4^.

NOTENoteforExperiencedProgrammers

Thereisnoseparatelongtype.Theinttypecanbeanintegerofanysize.

7.4.StringsAstringisasequenceofcharacters.Stringsarebasicallyjustabunchofwords.

YouwillbeusingstringsinalmosteveryPythonprogramthatyouwrite,sopayattentiontothefollowingpart.

7.4.1.SingleQuote

Youcanspecifystringsusingsinglequotessuchas 'Quotemeonthis' .

Allwhitespacei.e.spacesandtabs,withinthequotes,arepreservedasis.

7.4.2.DoubleQuotes

Stringsindoublequotesworkexactlythesamewayasstringsinsinglequotes.Anexampleis "Whatsyourname?" .

7.4.3.TripleQuotes

Youcanspecifymultilinestringsusingtriplequotes( """ or ''' ).Youcanusesinglequotesanddoublequotesfreelywithinthetriplequotes.Anexampleis:



7.4.4.StringsAreImmutable

Thismeansthatonceyouhavecreatedastring,youcannotchangeit.Althoughthismightseemlikeabadthing,itreallyisnt.Wewillseewhythisisnotalimitationinthevariousprogramsthatweseelateron.

NOTENoteforC/C++Programmers

ThereisnoseparatechardatatypeinPython.ThereisnorealneedforitandIamsureyouwontmissit.

NOTENoteforPerl/PHPProgrammers

Rememberthatsinglequotedstringsanddoublequotedstringsarethesametheydonotdifferinanyway.

7.4.5.Theformatmethod

Sometimeswemaywanttoconstructstringsfromotherinformation.Thisiswheretheformat()methodisuseful.

Savethefollowinglinesasafilestr_format.py:

Output:

HowItWorks

'''Thisisamultilinestring.Thisisthefirstline.Thisisthesecondline."What'syourname?,"Iasked.Hesaid"Bond,JamesBond."'''

PYTHON

age=20name='Swaroop'

print'{0}was{1}yearsoldwhenhewrotethisbook'.format(name,age)print'Whyis{0}playingwiththatpython?'.format(name)

PYTHON

$pythonstr_format.pySwaroopwas20yearsoldwhenhewrotethisbookWhyisSwaroopplayingwiththatpython?



Astringcanusecertainspecificationsandsubsequently,theformatmethodcanbecalledtosubstitutethosespecificationswithcorrespondingargumentstotheformatmethod.

Observethefirstusagewhereweuse{0}andthiscorrespondstothevariablenamewhichisthefirstargumenttotheformatmethod.Similarly,thesecondspecificationis{1}correspondingtoagewhichisthesecondargumenttotheformatmethod.NotethatPythonstartscountingfrom0whichmeansthatfirstpositionisatindex0,secondpositionisatindex1,andsoon.

Noticethatwecouldhaveachievedthesameusingstringconcatenation:

butthatismuchuglieranderrorprone.Second,theconversiontostringwouldbedoneautomaticallybytheformatmethodinsteadoftheexplicitconversiontostringsneededinthiscase.Third,whenusingtheformatmethod,wecanchangethemessagewithouthavingtodealwiththevariablesusedandviceversa.

Alsonotethatthenumbersareoptional,soyoucouldhavealsowrittenas:

whichwillgivethesameexactoutputasthepreviousprogram.

WhatPythondoesintheformatmethodisthatitsubstituteseachargumentvalueintotheplaceofthespecification.Therecanbemoredetailedspecificationssuchas:

Output:

name+'is'+str(age)+'yearsold'PYTHON

age=20name='Swaroop'

print'{}was{}yearsoldwhenhewrotethisbook'.format(name,age)print'Whyis{}playingwiththatpython?'.format(name)

PYTHON

#decimal(.)precisionof3forfloat'0.333'print'{0:.3f}'.format(1.0/3)#fillwithunderscores(_)withthetextcentered#(^)to11width'___hello___'print'{0:_^11}'.format('hello')#keywordbased'SwaroopwroteAByteofPython'print'{name}wrote{book}'.format(name='Swaroop',book='AByteofPython')

PYTHON



Sincewearediscussingformatting,notethatprintalwaysendswithaninvisible"newline"character(\n)sothatrepeatedcallstoprintwillallprintonaseparatelineeach.Topreventthisnewlinecharacterfrombeingprinted,youcanendthestatementwithacomma:

Outputis:

7.4.6.EscapeSequences

Suppose,youwanttohaveastringwhichcontainsasinglequote( ' ),howwillyouspecifythisstring?Forexample,thestringis "Whatsyourname?" .Youcannotspecify 'Whatsyourname?'becausePythonwillbeconfusedastowherethestringstartsandends.So,youwillhavetospecifythatthissinglequotedoesnotindicatetheendofthestring.Thiscanbedonewiththehelpofwhatiscalledanescapesequence.Youspecifythesinglequoteas \' :noticethebackslash.Now,youcanspecifythestringas 'What'syourname?' .

Anotherwayofspecifyingthisspecificstringwouldbe "Whatsyourname?" i.e.usingdoublequotes.Similarly,youhavetouseanescapesequenceforusingadoublequoteitselfinadoublequotedstring.Also,youhavetoindicatethebackslashitselfusingtheescapesequence \\ .

Whatifyouwantedtospecifyatwolinestring?Onewayistouseatriplequotedstringasshownpreviouslyoryoucanuseanescapesequenceforthenewlinecharacter\ntoindicatethestartofanewline.Anexampleis:

Anotherusefulescapesequencetoknowisthetab:\t.TherearemanymoreescapesequencesbutIhavementionedonlythemostusefuloneshere.

Onethingtonoteisthatinastring,asinglebackslashattheendofthelineindicatesthatthestringiscontinuedinthenextline,butnonewlineisadded.Forexample:

0.333___hello___SwaroopwroteAByteofPython

print"a",print"b",

PYTHON

ab

'Thisisthefirstline\nThisisthesecondline'PYTHON



isequivalentto

7.4.7.RawString

Ifyouneedtospecifysomestringswherenospecialprocessingsuchasescapesequencesarehandled,thenwhatyouneedistospecifyarawstringbyprefixingrorRtothestring.Anexampleis:

NOTE

NoteforRegularExpressionUsers

Alwaysuserawstringswhendealingwithregularexpressions.Otherwise,alotofbackwhackingmayberequired.Forexample,backreferencescanbereferredtoas'\\1' or r'\1' .

7.5.VariableUsingjustliteralconstantscansoonbecomeboringweneedsomewayofstoringanyinformationandmanipulatethemaswell.Thisiswherevariablescomeintothepicture.Variablesareexactlywhatthenameimpliestheirvaluecanvary,i.e.,youcanstoreanythingusingavariable.Variablesarejustpartsofyourcomputersmemorywhereyoustoresomeinformation.Unlikeliteralconstants,youneedsomemethodofaccessingthesevariablesandhenceyougivethemnames.

7.6.IdentifierNamingVariablesareexamplesofidentifiers.Identifiersarenamesgiventoidentifysomething.Therearesomerulesyouhavetofollowfornamingidentifiers:

Thefirstcharacteroftheidentifiermustbealetterofthealphabet(uppercaseASCIIorlowercaseASCIIorUnicodecharacter)oranunderscore( _ ).

Therestoftheidentifiernamecanconsistofletters(uppercaseASCIIorlowercaseASCIIorUnicodecharacter),underscores( _ )ordigits(09).

Identifiernamesarecasesensitive.Forexample, myname and myName arenotthesame.Notethelowercase n intheformerandtheuppercase N inthelatter.

"Thisisthefirstsentence.\Thisisthesecondsentence."

PYTHON

"Thisisthefirstsentence.Thisisthesecondsentence."PYTHON

r"Newlinesareindicatedby\n"PYTHON



Examplesofvalididentifiernamesare i , name_2_3 .Examplesofinvalididentifiernamesare2things , thisisspacedout , myname and >a1b2_c3 .

7.7.DataTypesVariablescanholdvaluesofdifferenttypescalleddatatypes.Thebasictypesarenumbersandstrings,whichwehavealreadydiscussed.Inlaterchapters,wewillseehowtocreateourowntypesusingclasses.

7.8.ObjectRemember,Pythonreferstoanythingusedinaprogramasanobject.Thisismeantinthegenericsense.Insteadofsaying"thesomething"',wesay"theobject".

NOTENoteforObjectOrientedProgrammingusers

Pythonisstronglyobjectorientedinthesensethateverythingisanobjectincludingnumbers,stringsandfunctions.

Wewillnowseehowtousevariablesalongwithliteralconstants.Savethefollowingexampleandruntheprogram.

7.9.HowtowritePythonprogramsHenceforth,thestandardproceduretosaveandrunaPythonprogramisasfollows:

7.10.ForPyCharm1. OpenPyCharm.

2. Createnewfilewiththefilenamementioned.

3. Typetheprogramcodegivenintheexample.

4. Rightclickandrunthecurrentfile.

Wheneveryouhavetoprovidecommandlinearguments,clickonRunEditConfigurationsandtypetheargumentsintheScriptparameters:sectionandclicktheOKbutton:



NOTE

PyCharmcommandlinearguments

7.11.Forothereditors1. Openyoureditorofchoice.

2. Typetheprogramcodegivenintheexample.

3. Saveitasafilewiththefilenamementioned.

4. Runtheinterpreterwiththecommandpythonprogram.pytoruntheprogram.

7.12.Example:UsingVariablesAndLiteralConstantsTypeandrunthefollowingprogram:



Output:

HowItWorks

Hereshowthisprogramworks.First,weassigntheliteralconstantvalue5tothevariableiusingtheassignmentoperator( = ).Thislineiscalledastatementbecauseitstatesthatsomethingshouldbedoneandinthiscase,weconnectthevariablenameitothevalue5.Next,weprintthevalueofiusingtheprintstatementwhich,unsurprisingly,justprintsthevalueofthevariabletothescreen.

Thenweadd1tothevaluestorediniandstoreitback.Wethenprintitandexpectedly,wegetthevalue6.

Similarly,weassigntheliteralstringtothevariablesandthenprintit.

NOTENoteforstaticlanguageprogrammers

Variablesareusedbyjustassigningthemavalue.Nodeclarationordatatypedefinitionisneeded/used.

7.13.LogicalAndPhysicalLineAphysicallineiswhatyouseewhenyouwritetheprogram.AlogicallineiswhatPythonseesasasinglestatement.Pythonimplicitlyassumesthateachphysicallinecorrespondstoalogicalline.

Anexampleofalogicallineisastatementlike print'helloworld' ifthiswasonalinebyitself(asyouseeitinaneditor),thenthisalsocorrespondstoaphysicalline.

Implicitly,Pythonencouragestheuseofasinglestatementperlinewhichmakescodemorereadable.

#Filename:var.pyi=5printii=i+1printi

s='''Thisisamultilinestring.Thisisthesecondline.'''prints

PYTHON

56Thisisamultilinestring.Thisisthesecondline.



Ifyouwanttospecifymorethanonelogicallineonasinglephysicalline,thenyouhavetoexplicitlyspecifythisusingasemicolon( ; )whichindicatestheendofalogicalline/statement.Forexample:

iseffectivelysameas

whichisalsosameas

andsameas

However,Istronglyrecommendthatyousticktowritingamaximumofasinglelogicallineoneachsinglephysicalline.Theideaisthatyoushouldneverusethesemicolon.Infact,IhaveneverusedorevenseenasemicoloninaPythonprogram.

Thereisonekindofsituationwherethisconceptisreallyuseful:ifyouhavealonglineofcode,youcanbreakitintomultiplephysicallinesbyusingthebackslash.Thisisreferredtoasexplicitlinejoining:

Output:

Similarly,

i=5printi

PYTHON

i=5;printi;

PYTHON

i=5;printi;PYTHON

i=5;printiPYTHON

s='Thisisastring.\Thiscontinuesthestring.'prints

PYTHON

Thisisastring.Thiscontinuesthestring.



isthesameas

Sometimes,thereisanimplicitassumptionwhereyoudontneedtouseabackslash.Thisisthecasewherethelogicallinehasastartingparentheses,startingsquarebracketsorastartingcurlybracesbutnotanendingone.Thisiscalledimplicitlinejoining.Youcanseethisinactionwhenwewriteprogramsusinglistsinlaterchapters.

7.14.IndentationWhitespaceisimportantinPython.Actually,whitespaceatthebeginningofthelineisimportant.Thisiscalledindentation.Leadingwhitespace(spacesandtabs)atthebeginningofthelogicallineisusedtodeterminetheindentationlevelofthelogicalline,whichinturnisusedtodeterminethegroupingofstatements.

Thismeansthatstatementswhichgotogethermusthavethesameindentation.Eachsuchsetofstatementsiscalledablock.Wewillseeexamplesofhowblocksareimportantinlaterchapters.

Onethingyoushouldrememberisthatwrongindentationcangiverisetoerrors.Forexample:

Whenyourunthis,yougetthefollowingerror:

Noticethatthereisasinglespaceatthebeginningofthesecondline.TheerrorindicatedbyPythontellsusthatthesyntaxoftheprogramisinvalidi.e.theprogramwasnotproperlywritten.Whatthismeanstoyouisthatyoucannotarbitrarilystartnewblocksofstatements(exceptforthedefaultmainblockwhich

print\i

PYTHON

printiPYTHON

i=5#Errorbelow!Noticeasinglespaceatthestartofthelineprint'Valueis',iprint'Irepeat,thevalueis',i

PYTHON

File"whitespace.py",line5print'Valueis',i^IndentationError:unexpectedindent



youhavebeenusingallalong,ofcourse).CaseswhereyoucanusenewblockswillbedetailedinlaterchapterssuchastheControlFlow.

Howtoindent

Usefourspacesforindentation.ThisistheofficialPythonlanguagerecommendation.Goodeditorswillautomaticallydothisforyou.Makesureyouuseaconsistentnumberofspacesforindentation,otherwiseyourprogramwillshowerrors.

NOTENotetostaticlanguageprogrammers

Pythonwillalwaysuseindentationforblocksandwillneverusebraces.Run fromfutureimportbraces tolearnmore.

7.15.SummaryNowthatwehavegonethroughmanynittygrittydetails,wecanmoveontomoreinterestingstuffsuchascontrolflowstatements.Besuretobecomecomfortablewithwhatyouhavereadinthischapter.



8.OperatorsandExpressionsMoststatements(logicallines)thatyouwritewillcontainexpressions.Asimpleexampleofanexpressionis 2+3 .Anexpressioncanbebrokendownintooperatorsandoperands.

Operatorsarefunctionalitythatdosomethingandcanberepresentedbysymbolssuchas + orbyspecialkeywords.Operatorsrequiresomedatatooperateonandsuchdataiscalledoperands.Inthiscase, 2and 3 aretheoperands.

8.1.OperatorsWewillbrieflytakealookattheoperatorsandtheirusage.

Notethatyoucanevaluatetheexpressionsgivenintheexamplesusingtheinterpreterinteractively.Forexample,totesttheexpression 2+3 ,usetheinteractivePythoninterpreterprompt:

Hereisaquickoverviewoftheavailableoperators:

+ (plus)

Addstwoobjects

3+5 gives 8 . 'a'+'b' gives 'ab' .

(minus)

Givesthesubtractionofonenumberfromtheotherifthefirstoperandisabsentitisassumedtobezero.

5.2 givesanegativenumberand 5024 gives 26 .

*(multiply)

Givesthemultiplicationofthetwonumbersorreturnsthestringrepeatedthatmanytimes.

2*3 gives 6 . 'la'*3 gives 'lalala' .

**(power)

>>>2+35>>>3*515>>>

PYTHON



Returnsxtothepowerofy

3**4 gives 81 (i.e. 3*3*3*3 )

/ (divide)

Dividexbyy

13/3 gives 4 . 13.0/3 gives 4.333333333333333

% (modulo)

Returnstheremainderofthedivision

13%3 gives 1 . 25.5%2.25 gives 1.5 .

>1 gives 5 .

11 isrepresentedinbitsby 1011 whichwhenrightshiftedby1bitgives 101`whichisthedecimal 5 .

& (bitwiseAND)

BitwiseANDofthenumbers

5&3 gives 1 .

| (bitwiseOR)

BitwiseORofthenumbers

5|3 gives 7



^ (bitwiseXOR)

BitwiseXORofthenumbers

5 3 gives 6

~ (bitwiseinvert)

Thebitwiseinversionofxis(x+1)

~5 gives 6 .Moredetailsathttp://stackoverflow.com/a/11810203

< (lessthan)

Returnswhetherxislessthany.Allcomparisonoperatorsreturn True or False .Notethecapitalizationofthesenames.

5=3 returns True .

== (equalto)

Comparesiftheobjectsareequal

x=2;y=2;x==y returns True .

x='str';y='stR';x==y returns False .

http://stackoverflow.com/a/11810203



x='str';y='str';x==y returns True .

!= (notequalto)

Comparesiftheobjectsarenotequal

x=2;y=3;x!=y returns True .

not (booleanNOT)

Ifxis True ,itreturns False .Ifxis False ,itreturns True .

x=True;notx returns False .

and (booleanAND)

xandy returns False ifxis False ,elseitreturnsevaluationofy

x=False;y=True;xandy returns False sincexisFalse.Inthiscase,Pythonwillnotevaluateysinceitknowsthatthelefthandsideofthe'and'expressionis False whichimpliesthatthewholeexpressionwillbe False irrespectiveoftheothervalues.Thisiscalledshortcircuitevaluation.

or (booleanOR)

Ifxis True ,itreturnsTrue,elseitreturnsevaluationofy

x=True;y=False;xory returns True .Shortcircuitevaluationapplieshereaswell.

8.2.ShortcutformathoperationandassignmentItiscommontorunamathoperationonavariableandthenassigntheresultoftheoperationbacktothevariable,hencethereisashortcutforsuchexpressions:

canbewrittenas:

Noticethat var=varoperationexpression becomes varoperation=expression .

a=2a=a*3

PYTHON

a=2a*=3

PYTHON



8.3.EvaluationOrderIfyouhadanexpressionsuchas 2+3*4 ,istheadditiondonefirstorthemultiplication?Ourhighschoolmathstellsusthatthemultiplicationshouldbedonefirst.Thismeansthatthemultiplicationoperatorhashigherprecedencethantheadditionoperator.

ThefollowingtablegivestheprecedencetableforPython,fromthelowestprecedence(leastbinding)tothehighestprecedence(mostbinding).Thismeansthatinagivenexpression,Pythonwillfirstevaluatetheoperatorsandexpressionslowerinthetablebeforetheoneslistedhigherinthetable.

Thefollowingtable,takenfromthePythonreferencemanual(http://docs.python.org/3/reference/expressions.html#operatorprecedence),isprovidedforthesakeofcompleteness.Itisfarbettertouseparenthesestogroupoperatorsandoperandsappropriatelyinordertoexplicitlyspecifytheprecedence.Thismakestheprogrammorereadable.SeeChangingtheOrderofEvaluationbelowfordetails.

lambda

LambdaExpression

ifelse

Conditionalexpression

or

BooleanOR

and

BooleanAND

notx

BooleanNOT

in,notin,is,isnot,,>=,!=,==

Comparisons,includingmembershiptestsandidentitytests

|

BitwiseOR

^

BitwiseXOR

http://docs.python.org/3/reference/expressions.html#operator-precedence



&

BitwiseAND

Shifts

+,

Additionandsubtraction

*,/,//,%

Multiplication,Division,FloorDivisionandRemainder

+x,x,~x

Positive,Negative,bitwiseNOT

**

Exponentiation

x[index],x[index:index],x(arguments),x.attribute

Subscription,slicing,call,attributereference

(expressions),[expressions],{key:value},{expressions}

Bindingortupledisplay,listdisplay,dictionarydisplay,setdisplay

Theoperatorswhichwehavenotalreadycomeacrosswillbeexplainedinlaterchapters.

Operatorswiththesameprecedencearelistedinthesamerowintheabovetable.Forexample, + and havethesameprecedence.

8.4.ChangingtheOrderOfEvaluationTomaketheexpressionsmorereadable,wecanuseparentheses.Forexample, 2+(3*4) isdefinitelyeasiertounderstandthan 2+3*4 whichrequiresknowledgeoftheoperatorprecedences.Aswitheverythingelse,theparenthesesshouldbeusedreasonably(donotoverdoit)andshouldnotberedundant,asin (2+(3*4)) .

Thereisanadditionaladvantagetousingparenthesesithelpsustochangetheorderofevaluation.Forexample,ifyouwantadditiontobeevaluatedbeforemultiplicationinanexpression,thenyoucanwritesomethinglike (2+3)*4 .



8.5.AssociativityOperatorsareusuallyassociatedfromlefttoright.Thismeansthatoperatorswiththesameprecedenceareevaluatedinalefttorightmanner.Forexample, 2+3+4 isevaluatedas (23)+4 .Someoperatorslikeassignmentoperatorshaverighttoleftassociativityi.e. a=b=c istreatedas a=(b=c) .

8.6.ExpressionsExample(saveasexpression.py):

Output:

HowItWorks

Thelengthandbreadthoftherectanglearestoredinvariablesbythesamename.Weusethesetocalculatetheareaandperimeteroftherectanglewiththehelpofexpressions.Westoretheresultoftheexpression length*breadth inthevariableareaandthenprintitusingtheprintfunction.Inthesecondcase,wedirectlyusethevalueoftheexpression 2*(length+breadth) intheprintstatement.

Also,noticehowPythonprettyprintstheoutput.Eventhoughwehavenotspecifiedaspacebetween'Areais' andthevariable area ,Pythonputsitforussothatwegetacleanniceoutputandtheprogramismuchmorereadablethisway(sincewedontneedtoworryaboutspacinginthestringsweuseforoutput).ThisisanexampleofhowPythonmakeslifeeasyfortheprogrammer.

8.7.SummaryWehaveseenhowtouseoperators,operandsandexpressionsthesearethebasicbuildingblocksofanyprogram.Next,wewillseehowtomakeuseoftheseinourprogramsusingstatements.

length=5breadth=2

area=length*breadthprint'Areais',areaprint'Perimeteris',2*(length+breadth)

PYTHON

$pythonexpression.pyAreais10Perimeteris14



9.ControlFlowIntheprogramswehaveseentillnow,therehasalwaysbeenaseriesofstatementsfaithfullyexecutedbyPythoninexacttopdownorder.Whatifyouwantedtochangetheflowofhowitworks?Forexample,youwanttheprogramtotakesomedecisionsanddodifferentthingsdependingondifferentsituations,suchasprinting'GoodMorning'or'GoodEvening'dependingonthetimeoftheday?

Asyoumighthaveguessed,thisisachievedusingcontrolflowstatements.TherearethreecontrolflowstatementsinPythonif,forandwhile.

9.1.TheifstatementTheifstatementisusedtocheckacondition:iftheconditionistrue,werunablockofstatements(calledtheifblock),elseweprocessanotherblockofstatements(calledtheelseblock).Theelseclauseisoptional.

Example(saveasif.py):

Output:

number=23guess=int(raw_input('Enteraninteger:'))

ifguess==number:#Newblockstartshereprint'Congratulations,youguessedit.'print'(butyoudonotwinanyprizes!)'#Newblockendshereelifguessnumbertoreachhere

print'Done'#Thislaststatementisalwaysexecuted,#aftertheifstatementisexecuted.

PYTHON



HowItWorks

Inthisprogram,wetakeguessesfromtheuserandcheckifitisthenumberthatwehave.Wesetthevariablenumbertoanyintegerwewant,say 23 .Then,wetaketheusersguessusingtheraw_input() function.Functionsarejustreusablepiecesofprograms.Wellreadmoreabouttheminthenextchapter.

Wesupplyastringtothebuiltin raw_input functionwhichprintsittothescreenandwaitsforinputfromtheuser.Onceweentersomethingandpress enter key,the raw_input() functionreturnswhatweentered,asastring.Wethenconvertthisstringtoanintegerusing int andthenstoreitinthevariable guess .Actually,the int isaclassbutallyouneedtoknowrightnowisthatyoucanuseittoconvertastringtoaninteger(assumingthestringcontainsavalidintegerinthetext).

Next,wecomparetheguessoftheuserwiththenumberwehavechosen.Iftheyareequal,weprintasuccessmessage.NoticethatweuseindentationlevelstotellPythonwhichstatementsbelongtowhichblock.ThisiswhyindentationissoimportantinPython.Ihopeyouarestickingtothe"consistentindentation"rule.Areyou?

Noticehowthe if statementcontainsacolonattheendweareindicatingtoPythonthatablockofstatementsfollows.

Then,wecheckiftheguessislessthanthenumber,andifso,weinformtheuserthattheymustguessalittlehigherthanthat.Whatwehaveusedhereisthe elif clausewhichactuallycombinestworelatedifelseifelse statementsintoonecombined ifelifelse statement.Thismakestheprogrameasierandreducestheamountofindentationrequired.

$pythonif.pyEnteraninteger:50No,itisalittlelowerthanthatDone

$pythonif.pyEnteraninteger:22No,itisalittlehigherthanthatDone

$pythonif.pyEnteraninteger:23Congratulations,youguessedit.(butyoudonotwinanyprizes!)Done



The elif and else statementsmustalsohaveacolonattheendofthelogicallinefollowedbytheircorrespondingblockofstatements(withproperindentation,ofcourse)

Youcanhaveanother if statementinsidetheifblockofan if statementandsoonthisiscalledanested if statement.

Rememberthatthe elif and else partsareoptional.Aminimalvalid if statementis:

AfterPythonhasfinishedexecutingthecomplete if statementalongwiththeassociated elif andelse clauses,itmovesontothenextstatementintheblockcontainingthe if statement.Inthiscase,itisthemainblock(whereexecutionoftheprogramstarts),andthenextstatementisthe print'Done'statement.Afterthis,Pythonseestheendsoftheprogramandsimplyfinishesup.

Eventhoughthisisaverysimpleprogram,Ihavebeenpointingoutalotofthingsthatyoushouldnotice.Alltheseareprettystraightforward(andsurprisinglysimpleforthoseofyoufromC/C++backgrounds).Youwillneedtobecomeawareofallthesethingsinitially,butaftersomepracticeyouwillbecomecomfortablewiththem,anditwillallfeel'natural'toyou.


Thereisno switch statementinPython.Youcanusean if..elif..elsestatementtodothesamething(andinsomecases,useadictionarytodoitquickly)

9.2.ThewhileStatementThe while statementallowsyoutorepeatedlyexecuteablockofstatementsaslongasaconditionistrue.A while statementisanexampleofwhatiscalledaloopingstatement.A while statementcanhaveanoptional else clause.

Example(saveas while.py ):

ifTrue:print'Yes,itistrue'

PYTHON



Output:

HowItWorks

Inthisprogram,wearestillplayingtheguessinggame,buttheadvantageisthattheuserisallowedtokeepguessinguntilheguessescorrectlythereisnoneedtorepeatedlyruntheprogramforeachguess,aswehavedoneintheprevioussection.Thisaptlydemonstratestheuseofthe while statement.

Wemovethe raw_input and if statementstoinsidethe while loopandsetthevariable runningto True beforethewhileloop.First,wecheckifthevariable running is True andthenproceedtoexecutethecorrespondingwhileblock.Afterthisblockisexecuted,theconditionisagaincheckedwhichinthiscaseisthe running variable.Ifitistrue,weexecutethewhileblockagain,elsewecontinuetoexecutetheoptionalelseblockandthencontinuetothenextstatement.

number=23running=True

whilerunning:guess=int(raw_input('Enteraninteger:'))

ifguess==number:print'Congratulations,youguessedit.'#thiscausesthewhilelooptostoprunning=Falseelifguess



The else blockisexecutedwhenthe while loopconditionbecomes False thismayevenbethefirsttimethattheconditionischecked.Ifthereisan else clausefora while loop,itisalwaysexecutedunlessyoubreakoutoftheloopwitha break statement.

The True and False arecalledBooleantypesandyoucanconsiderthemtobeequivalenttothevalue1 and 0 respectively.


Rememberthatyoucanhavean else clauseforthe while loop.

9.3.TheforloopThe for..in statementisanotherloopingstatementwhichiteratesoverasequenceofobjectsi.e.gothrougheachiteminasequence.Wewillseemoreaboutsequencesindetailinlaterchapters.Whatyouneedtoknowrightnowisthatasequenceisjustanorderedcollectionofitems.

Example(saveas for.py ):

Output:

HowItWorks

Inthisprogram,weareprintingasequenceofnumbers.Wegeneratethissequenceofnumbersusingthebuiltin range function.

Whatwedohereissupplyittwonumbersand range returnsasequenceofnumbersstartingfromthefirstnumberanduptothesecondnumber.Forexample, range(1,5) givesthesequence [1,2,3,4] .Bydefault, range takesastepcountof1.Ifwesupplyathirdnumberto range ,thenthat

foriinrange(1,5):printielse:print'Theforloopisover'

PYTHON

$pythonfor.py1234Theforloopisover



becomesthestepcount.Forexample, range(1,5,2) gives [1,3] .Rememberthattherangeextendsuptothesecondnumberi.e.itdoesnotincludethesecondnumber.

Notethat range() generatesasequenceofnumbers,butitwillgenerateonlyonenumberatatime,whentheforlooprequestsforthenextitem.Ifyouwanttoseethefullsequenceofnumbersimmediately,use list(range()) .Listsareexplainedinthedatastructureschapter.

The for looptheniteratesoverthisrange foriinrange(1,5) isequivalentto foriin[1,2,3,4] whichislikeassigningeachnumber(orobject)inthesequencetoi,oneatatime,andthenexecutingtheblockofstatementsforeachvalueof i .Inthiscase,wejustprintthevalueintheblockofstatements.

Rememberthatthe else partisoptional.Whenincluded,itisalwaysexecutedonceafterthe for loopisoverunlessabreakstatementisencountered.

Rememberthatthe for..in loopworksforanysequence.Here,wehavealistofnumbersgeneratedbythebuiltin range function,butingeneralwecanuseanykindofsequenceofanykindofobjects!Wewillexplorethisideaindetailinlaterchapters.

NOTE

NoteforC/C++/Java/C#Programmers

ThePython for loopisradicallydifferentfromtheC/C++ for loop.C#programmerswillnotethatthe for loopinPythonissimilartothe foreach loopinC#.Javaprogrammerswillnotethatthesameissimilarto for(inti:IntArray) inJava1.5.

InC/C,ifyouwanttowrite`for(inti=0i



Output:

HowItWorks

Inthisprogram,werepeatedlytaketheusersinputandprintthelengthofeachinputeachtime.Weareprovidingaspecialconditiontostoptheprogrambycheckingiftheuserinputis 'quit' .Westoptheprogrambybreakingoutoftheloopandreachtheendoftheprogram.

Thelengthoftheinputstringcanbefoundoutusingthebuiltin len function.

Rememberthatthe break statementcanbeusedwiththe for loopaswell.

SwaroopsPoeticPythonTheinputIhaveusedhereisaminipoemIhavewritten:

ProgrammingisfunWhentheworkisdoneifyouwannamakeyourworkalsofun:usePython!

9.5.ThecontinueStatement

whileTrue:s=raw_input('Entersomething:')ifs=='quit':breakprint'Lengthofthestringis',len(s)print'Done'

PYTHON

$pythonbreak.pyEntersomething:ProgrammingisfunLengthofthestringis18Entersomething:WhentheworkisdoneLengthofthestringis21Entersomething:ifyouwannamakeyourworkalsofun:Lengthofthestringis37Entersomething:usePython!Lengthofthestringis11Entersomething:quitDone



The continue statementisusedtotellPythontoskiptherestofthestatementsinthecurrentloopblockandtocontinuetothenextiterationoftheloop.

Example(saveas continue.py ):

Output:

HowItWorks

Inthisprogram,weacceptinputfromtheuser,butweprocesstheinputstringonlyifitisatleast3characterslong.So,weusethebuiltin len functiontogetthelengthandifthelengthislessthan3,weskiptherestofthestatementsintheblockbyusingthe continue statement.Otherwise,therestofthestatementsintheloopareexecuted,doinganykindofprocessingwewanttodohere.

Notethatthe continue statementworkswiththe for loopaswell.

9.6.SummaryWehaveseenhowtousethethreecontrolflowstatements if , while and for alongwiththeirassociated break and continue statements.ThesearesomeofthemostcommonlyusedpartsofPythonandhence,becomingcomfortablewiththemisessential.

Next,wewillseehowtocreateandusefunctions.

whileTrue:s=raw_input('Entersomething:')ifs=='quit':breakiflen(s)



10.FunctionsFunctionsarereusablepiecesofprograms.Theyallowyoutogiveanametoablockofstatements,allowingyoutorunthatblockusingthespecifiednameanywhereinyourprogramandanynumberoftimes.Thisisknownascallingthefunction.Wehavealreadyusedmanybuiltinfunctionssuchas lenand range .

Thefunctionconceptisprobablythemostimportantbuildingblockofanynontrivialsoftware(inanyprogramminglanguage),sowewillexplorevariousaspectsoffunctionsinthischapter.

Functionsaredefinedusingthe def keyword.Afterthiskeywordcomesanidentifiernameforthefunction,followedbyapairofparentheseswhichmayenclosesomenamesofvariables,andbythefinalcolonthatendstheline.Nextfollowstheblockofstatementsthatarepartofthisfunction.Anexamplewillshowthatthisisactuallyverysimple:

Example(saveas function1.py ):

Output:

HowItWorks

Wedefineafunctioncalled say_hello usingthesyntaxasexplainedabove.Thisfunctiontakesnoparametersandhencetherearenovariablesdeclaredintheparentheses.Parameterstofunctionsarejustinputtothefunctionsothatwecanpassindifferentvaluestoitandgetbackcorrespondingresults.

Noticethatwecancallthesamefunctiontwicewhichmeanswedonothavetowritethesamecodeagain.

10.1.FunctionParameters

defsay_hello():#blockbelongingtothefunctionprint'helloworld'#Endoffunction

say_hello()#callthefunctionsay_hello()#callthefunctionagain

PYTHON

$pythonfunction1.pyhelloworldhelloworld



Afunctioncantakeparameters,whicharevaluesyousupplytothefunctionsothatthefunctioncandosomethingutilisingthosevalues.Theseparametersarejustlikevariablesexceptthatthevaluesofthesevariablesaredefinedwhenwecallthefunctionandarealreadyassignedvalueswhenthefunctionruns.

Parametersarespecifiedwithinthepairofparenthesesinthefunctiondefinition,separatedbycommas.Whenwecallthefunction,wesupplythevaluesinthesameway.Notetheterminologyusedthenamesgiveninthefunctiondefinitionarecalledparameterswhereasthevaluesyousupplyinthefunctioncallarecalledarguments.

Example(saveas function_param.py ):

Output:

HowItWorks

Here,wedefineafunctioncalled print_max thatusestwoparameterscalled a and b .Wefindoutthegreaternumberusingasimple if..else statementandthenprintthebiggernumber.

Thefirsttimewecallthefunction print_max ,wedirectlysupplythenumbersasarguments.Inthesecondcase,wecallthefunctionwithvariablesasarguments. print_max(x,y) causesthevalueofargument x tobeassignedtoparameter a andthevalueofargument y tobeassignedtoparameter b .TheprintMaxfunctionworksthesamewayinbothcases.

defprint_max(a,b):ifa>b:printa,'ismaximum'elifa==b:printa,'isequalto',belse:printb,'ismaximum'

#directlypassliteralvaluesprint_max(3,4)

x=5y=7

#passvariablesasargumentsprint_max(x,y)

PYTHON

$pythonfunction_param.py4ismaximum7ismaximum



10.2.LocalVariablesWhenyoudeclarevariablesinsideafunctiondefinition,theyarenotrelatedinanywaytoothervariableswiththesamenamesusedoutsidethefunctioni.e.variablenamesarelocaltothefunction.Thisiscalledthescopeofthevariable.Allvariableshavethescopeoftheblocktheyaredeclaredinstartingfromthepointofdefinitionofthename.

Example(saveas function_local.py ):

Output:

HowItWorks

Thefirsttimethatweprintthevalueofthenamexwiththefirstlineinthefunctionsbody,Pythonusesthevalueoftheparameterdeclaredinthemainblock,abovethefunctiondefinition.

Next,weassignthevalue 2 to x .Thename x islocaltoourfunction.So,whenwechangethevalueof x inthefunction,the x definedinthemainblockremainsunaffected.

Withthelast print statement,wedisplaythevalueof x asdefinedinthemainblock,therebyconfirmingthatitisactuallyunaffectedbythelocalassignmentwithinthepreviouslycalledfunction.

10.3.TheglobalstatementIfyouwanttoassignavaluetoanamedefinedatthetopleveloftheprogram(i.e.notinsideanykindofscopesuchasfunctionsorclasses),thenyouhavetotellPythonthatthenameisnotlocal,butitisglobal.Wedothisusingthe global statement.Itisimpossibletoassignavaluetoavariabledefinedoutsideafunctionwithoutthe global statement.

x=50

deffunc(x):print'xis',xx=2print'Changedlocalxto',x

func(x)print'xisstill',x

PYTHON

$pythonfunction_local.pyxis50Changedlocalxto2xisstill50



Youcanusethevaluesofsuchvariablesdefinedoutsidethefunction(assumingthereisnovariablewiththesamenamewithinthefunction).However,thisisnotencouragedandshouldbeavoidedsinceitbecomesuncleartothereaderoftheprogramastowherethatvariablesdefinitionis.Usingthe globalstatementmakesitamplyclearthatthevariableisdefinedinanoutermostblock.

Example(saveas function_global.py ):

Output:

HowItWorks

The global statementisusedtodeclarethat x isaglobalvariablehence,whenweassignavaluetox insidethefunction,thatchangeisreflectedwhenweusethevalueof x inthemainblock.

Youcanspecifymorethanoneglobalvariableusingthesame global statemente.g. globalx,y,z .

10.4.DefaultArgumentValuesForsomefunctions,youmaywanttomakesomeparametersoptionalandusedefaultvaluesincasetheuserdoesnotwanttoprovidevaluesforthem.Thisisdonewiththehelpofdefaultargumentvalues.Youcanspecifydefaultargumentvaluesforparametersbyappendingtotheparameternameinthefunctiondefinitiontheassignmentoperator( = )followedbythedefaultvalue.

Notethatthedefaultargumentvalueshouldbeaconstant.Moreprecisely,thedefaultargumentvalueshouldbeimmutablethisisexplainedindetailinlaterchapters.Fornow,justrememberthis.

x=50

deffunc():globalx

print'xis',xx=2print'Changedglobalxto',x

func()print'Valueofxis',x

PYTHON

$pythonfunction_global.pyxis50Changedglobalxto2Valueofxis2



Example(saveas function_default.py ):

Output:

HowItWorks

Thefunctionnamed say isusedtoprintastringasmanytimesasspecified.Ifwedontsupplyavalue,thenbydefault,thestringisprintedjustonce.Weachievethisbyspecifyingadefaultargumentvalueof1 totheparameter times .

Inthefirstusageof say ,wesupplyonlythestringanditprintsthestringonce.Inthesecondusageofsay ,wesupplyboththestringandanargument 5 statingthatwewanttosaythestringmessage5times.

CAUTION

Onlythoseparameterswhichareattheendoftheparameterlistcanbegivendefaultargumentvaluesi.e.youcannothaveaparameterwithadefaultargumentvalueprecedingaparameterwithoutadefaultargumentvalueinthefunctionsparameterlist.

Thisisbecausethevaluesareassignedtotheparametersbyposition.Forexample, deffunc(a,b=5) isvalid,but deffunc(a=5,b) isnotvalid.

10.5.KeywordArgumentsIfyouhavesomefunctionswithmanyparametersandyouwanttospecifyonlysomeofthem,thenyoucangivevaluesforsuchparametersbynamingthemthisiscalledkeywordargumentsweusethename(keyword)insteadoftheposition(whichwehavebeenusingallalong)tospecifytheargumentstothefunction.

defsay(message,times=1):printmessage*times

say('Hello')say('World',5)

PYTHON

$pythonfunction_default.pyHelloWorldWorldWorldWorldWorld



Therearetwoadvantagesone,usingthefunctioniseasiersincewedonotneedtoworryabouttheorderofthearguments.Two,wecangivevaluestoonlythoseparameterstowhichwewantto,providedthattheotherparametershavedefaultargumentvalues.

Example(saveas function_keyword.py ):

Output:

HowItWorks

Thefunctionnamed func hasoneparameterwithoutadefaultargumentvalue,followedbytwoparameterswithdefaultargumentvalues.

Inthefirstusage, func(3,7) ,theparameter a getsthevalue 3 ,theparameter b getsthevalue 7and c getsthedefaultvalueof 10 .

Inthesecondusage func(25,c=24) ,thevariable a getsthevalueof25duetothepositionoftheargument.Then,theparameter c getsthevalueof 24 duetonamingi.e.keywordarguments.Thevariable b getsthedefaultvalueof 5 .

Inthethirdusage func(c=50,a=100) ,weusekeywordargumentsforallspecifiedvalues.Noticethatwearespecifyingthevalueforparameter c beforethatfor a eventhough a isdefinedbefore c inthefunctiondefinition.

10.6.VarArgsparametersSometimesyoumightwanttodefineafunctionthatcantakeanynumberofparameters,i.e.variablenumberofarguments,thiscanbeachievedbyusingthestars(saveas function_varargs.py ):

deffunc(a,b=5,c=10):print'ais',a,'andbis',b,'andcis',c

func(3,7)func(25,c=24)func(c=50,a=100)

PYTHON

$pythonfunction_keyword.pyais3andbis7andcis10ais25andbis5andcis24ais100andbis5andcis50



Output:

HowItWorks

Whenwedeclareastarredparametersuchas *param ,thenallthepositionalargumentsfromthatpointtilltheendarecollectedasatuplecalled'param'.

Similarly,whenwedeclareadoublestarredparametersuchas **param ,thenallthekeywordargumentsfromthatpointtilltheendarecollectedasadictionarycalled'param'.

Wewillexploretuplesanddictionariesinalaterchapter.

10.7.ThereturnstatementThe return statementisusedtoreturnfromafunctioni.e.breakoutofthefunction.Wecanoptionallyreturnavaluefromthefunctionaswell.

Example(saveas function_return.py ):

Output:

deftotal(initial=5,*numbers,**keywords):count=initialfornumberinnumbers:count+=numberforkeyinkeywords:count+=keywords[key]returncount

printtotal(10,1,2,3,vegetables=50,fruits=100)

PYTHON

$pythonfunction_varargs.py166

defmaximum(x,y):ifx>y:returnxelifx==y:return'Thenumbersareequal'else:returny

printmaximum(2,3)

PYTHON



HowItWorks

The maximum functionreturnsthemaximumoftheparameters,inthiscasethenumbe

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