1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

1.10

1.11

1.12

1.13

1.14

1.15

1.16

1.17

1.18

1.19

1.20

1.21

1.22

1.23

1.24

1.25

TableofContentsIntroduction

Dedication

Preface

AboutPython

Installation

FirstSteps

Basics

OperatorsandExpressions

Controlflow

Functions

Modules

DataStructures

ProblemSolving

ObjectOrientedProgramming

InputandOutput

Exceptions

StandardLibrary

More

WhatNext

Appendix:FLOSS

Appendix:About

Appendix:RevisionHistory

Appendix:Translations

Appendix:TranslationHow-to

Feedback

1

AByteofPython"AByteofPython"isafreebookonprogrammingusingthePythonlanguage.ItservesasatutorialorguidetothePythonlanguageforabeginneraudience.Ifallyouknowaboutcomputersishowtosavetextfiles,thenthisisthebookforyou.

ForPythonversion3

ThisbookwillteachyoutousePythonversion3.TherewillalsobeguidanceforyoutoadapttotheolderandmorecommonPythonversion2inthebook.

WhoreadsAByteofPython?

Herearewhatpeoplearesayingaboutthebook:

Somewherearound2004-05whenIwasconvincedIwasn’tsmartenoughtobeaprogrammer,IcamecameacrosstheoriginalAByteof#Python,andthatchangedmyentireperspectiveoncomputingandlife,Iowealottothatbook@swaroopchhadwritten.--RahulonJul30,2020

Thisisthebookthatgotmeintoprogrammingalmostadecadeago.Thankyou@swaroopch.Youchangedmylife.--StefanFroelichonAug2,2019

Iamwritingthisemailtothankyouforthegreathelpyourbookhasdoneforme!ItwasareallygoodbookthatIenjoyedthoroughly.Asa15yearoldwhohasneverdoneprogrammingbefore,tryingtolearnPythononlinewasdifficultandIcouldn'tunderstandanything.ButIfeltlikeyourbookgavewasmucheasiertounderstandandeasedmeintothewholenewworldofprogramming.Thankstoyou,Icannowwriteahighlevellanguagewithease.Ithoughtprogrammingwouldbehardandboring,butwithyourbook'shelp,Irealisedhowfunandinterestingyetsimpleitcanbe!Iwouldliketothankyouagainforyourhardworkonhelpingoutbeginnerslikeme.--ProttyashitaTahiyatonSep17,2019

Thisisthebestbeginner'stutorialI'veeverseen!Thankyouforyoureffort.--WaltMichalik

Thebestthingifoundwas"AByteofPython",whichissimplyabrilliantbookforabeginner.It'swellwritten,theconceptsarewellexplainedwithselfevidentexamples.--JoshuaRobin

Excellentgentleintroductiontoprogramming#Pythonforbeginners--ShanRajasekaran

starttolovepythonwitheverysinglepageread--HerbertFeutl

perfectbeginnersguideforpython,willgiveukeytounlockmagicalworldofpython--Dilip

Ishouldbedoingmyactual"work"butjustfound"AByteofPython".Agreatguidewithgreatexamples.--BiologistJohn

RecentlystartedreadingaByteofpython.Awesomework.Andthattooforfree.Highlyrecommendedforaspiringpythonistas.--Mangesh

AByteofPython,writtenbySwaroop.(thisisthebookI'mcurrentlyreading).Probablythebesttostartwith,andprobablythebestintheworldforeverynewbieorevenamoreexperienceduser.--Apostolos

EnjoyingReading#ByteOfPythonby@swaroopchbestbookever--YuvrajSharma

AByteofPythonby@swaroopchisstillthe"Bestnewbieguidetopython"--NicksonKaigi

Introduction

2

ThankyousomuchforwritingAByteOfPython.IjuststartedlearninghowtocodetwodaysagoandI'malreadybuildingsomesimplegames.YourguidehasbeenadreamandIjustwantedtoletyouknowhowvaluableithasbeen.--Franklin

I'mfromDayanandasagarCollegeofEngineering(7thsem,CSE).Firstlyiwanttosaythatyourbook"Thebyteofpython"istoogoodabookforabeginnerinpythonlikeme.Theconceptsaresowellexplainedwithsimpleexamplesthathelpedmetoeasilylearnpython.Thankyousomuch.--Madhura

Iama18yearoldITstudentstudyingatUniversityinIreland.Iwouldliketoexpressmygratitudetoyouforwritingyourbook"AByteofPython",Ialreadyhadknowledgeof3programminglangagues-C,JavaandJavascript,andPythonwasbyfartheeasiestlangagueIhaveeverlearned,andthatwasmainlybecauseyourbookwasfantasticandmadelearningpythonverysimpleandinteresting.ItisoneofthebestwrittenandeasytofollowprogrammingbooksIhaveeverread.Congratulationsandkeepupthegreatwork.--Matt

Hi,I'mfromDominicanRepublic.MynameisPavel,recentlyIreadyourbookAByteofPythonandIconsideritexcellent!!:).Ilearntmuchfromalltheexamples.Yourbookisofgreathelpfornewbieslikeme...--PavelSimo

IamastudentfromChina,Now,IhavereadyoubookAbyteofPython,Ohit'sbeautiful.Thebookisverysimplebutcanhelpallthefirstlearnners.YouknowIaminterestinginJavaandcloudcomputingmanytimes,ihavetocodingprogrammfortheserver,soithinkpythonisagoodchoice,finishyourbook,ithinkitsnotonlyagoodchoiceitsmustusethePython.MyEnglishisnotverywell,theemailtoyou,ijustwannathankyou!BestWishesforyouandyourfamily.--RoyLau

IrecentlyfinishedreadingByteofPython,andIthoughtIreallyoughttothankyou.IwasverysadtoreachthefinalpagesasInowhavetogobacktodull,tediousoreillyoretc.manualsforlearningaboutpython.Anyway,Ireallyappreciateyourbook.SamuelYoung

DearSwaroop,Iamtakingaclassfromaninstructorthathasnointerestinteaching.WeareusingLearningPython,secondedition,byO'Reilly.Itisnotatextforbeginnerwithoutanyprogrammingknowledge,andaninstructorthatshouldbeworkinginanotherfield.Thankyouverymuchforyourbook,withoutitIwouldbecluelessaboutPythonandprogramming.Thanksamillion,youareabletobreakthemessagedowntoalevelthatbeginnerscanunderstandandnoteveryonecan.--JosephDuarte

Iloveyourbook!ItisthegreatestPythontutorialever,andaveryusefulreference.Brilliant,atruemasterpiece!Keepupthegoodwork!--Chris-AndréSommerseth

Firstofall,Iwanttosaythankstoyouforthisgreatebook.Ithinkitisagoodbookforthosewhoarelookingforabeginner'stutorialforPython.Itisabouttwoorthereyearsago,Ithink,whenIfirstheardofthisbook.Atthattime,IamnotabletoreadsomebookinEnglishyet,soIgotachinesetranslation,whichtookmeintothegateofPythonprogramming.Recently,Irereadthisbook.Thistime,ofcourse,theenglishversion.Icouldn'tbelievethatIcanreadthewholebookwithoutmydictionaryathand.Ofcourse,itallduestoyourefforttomakethisbookaneasy-to-understandone.--myd7349

I'mjuste-mailingyoutothankyouforwritingByteofPythononline.IhadbeenattemptingPythonforafewmonthspriortostumblingacrossyourbook,andalthoughImadelimitedsuccesswithpyGame,Inevercompletedaprogram.Thankstoyoursimplificationofthecategories,Pythonactuallyseemsareachablegoal.ItseemslikeIhavefinallylearnedthefoundationsandIcancontinueintomyrealgoal,gamedevelopment....Onceagain,thanksVERYmuchforplacingsuchastructuredandhelpfulguidetobasicprogrammingontheweb.ItshovedmeintoandoutofOOPwithanunderstandingwheretwotextbookshadfailed.--MattGallivan

IwouldliketothankyouforyourbookAByteofPythonwhichimyselffindthebestwaytolearnpython.Iama15yearoldiliveinegyptmynameisAhmed.Pythonwasmysecondprogramminglanguageilearnvisualbasic6atschoolbutdidn'tenjoyit,howeverireallyenjoyedlearningpython.Imadetheaddressbookprogramandiwassucessful.iwilltrytostartmakemoreprogramsandreadpythonprograms(ifyoucouldtellmesourcethatwouldbehelpful).Iwillalsostartonlearningjavaandifyoucantellmewheretofindatutorialasgoodasyoursforjavathatwouldhelpmealot.Thanx.--AhmedMohammed

Introduction

3

AwonderfulresourceforbeginnerswantingtolearnmoreaboutPythonisthe110-pagePDFtutorialAByteofPythonbySwaroopCH.Itiswell-written,easytofollow,andmaybethebestintroductiontoPythonprogrammingavailable.--DrewAmes

YesterdayIgotthroughmostofByteofPythononmyNokiaN800andit'stheeasiestandmostconciseintroductiontoPythonIhaveyetencountered.HighlyrecommendedasastartingpointforlearningPython.--JasonDelport

ByteofVimandPythonby@swaroopchisbyfarthebestworksintechnicalwritingtome.Excellentreads#FeelGoodFactor--Surendran

"Byteofpython"bestonebyfarman(inresponsetothequestion"Cananyonesuggestagood,inexpensiveresourceforlearningthebasicsofPython?")--JustinLoveTrue

TheBookByteofpythonwasveryhelpful..Thanksbigtime:)Chinmay

AlwaysbeenafanofAByteofPython-madeforbothnewandexperiencedprogrammers.--PatrickHarrington

Istartedlearningpythonfewdaysagofromyourbook..thanksforsuchanicebook.itissowellwritten,youmademylifeeasy..soyoufoundanewfanofyours..thatsme:)tonsofthanks.--GadadhariBheem

BeforeIstartedtolearnPython,I'veacquiredbasicprogrammingskillsinAssembly,C,C++,C#andJava.TheveryreasonIwantedtolearnPythonisit'spopular(peoplearetalkingaboutit)andpowerful(reality).ThisbookwrittenbyMr.Swaroopisaverygoodguideforbothbrand-newprogrammersandnewpythonprogrammers.Took10halfdaystogothroughit.GreatHelp!--FangBiyi(PhDCandidateECE,MichiganStateUniversity)

Thankyoueversomuchforthisbook!!ThisbookclearedupmanyquestionsIhadaboutcertainaspectsofPythonsuchasobjectorientedprogramming.IdonotfeellikeanexpertatOObutIknowthisbookhelpedmeonafirststeportwo.Ihavenowwrittenseveralpythonprogramsthatactuallydorealthingsformeasasystemadministrator.Theyareallproceduralorientedbuttheyaresmallbymostpeoplesstandards.Again,thanksforthisbook.Thankyouforhavingitontheweb.--Bob

IjustwanttothankyouforwritingthefirstbookonprogrammingI'veeverreallyread.Pythonisnowmyfirstlanguage,andIcanjustimagineallthepossibilities.SothankyouforgivingmethetoolstocreatethingsIneverwouldhaveimaginedIcoulddobefore.--"TheWalrus"

IwantedtothankyouforwritingAByteOfPython(2&3Versions).IthasbeeninvaluabletomylearningexperienceinPython&Programmingingeneral.Needlesstosay,Iamabeginnerintheprogrammingworld,acoupleofmonthsofselfstudyuptothispoint.Ihadbeenusingyoutubetutorials&someotheronlinetutorialsincludingotherfreebooks.Idecidedtodigintoyourbookyesterday,&I'velearnedmoreonthefirstfewpagesthananyotherbookortutorial.AfewthingsIhadbeenconfusedabout,wereclearedrightupwithaGREATexample&explanation.Can'twaittoread(andlearn)more!!Thankyousomuchfornotonlywritingthebook,butforputtingitunderthecreativecommonslicense(free).Thankgoodnessthereareunselfishpeoplelikeyououttheretohelp&teachtherestofus.--Chris

Iwroteyoubackin2011andIwasjustgettingintoPythonandwantedtothankyouforyourtutorial"AByteofPython".Withoutit,Iwouldhavefallenbythewayside.SincethenIhavegoneontoprogramanumberoffunctionsinmyorganizationwiththislanguagewithyetmoreonthehorizon.IwouldnotcallmyselfanadvancedprogrammerbyanystretchbutInoticetheoccasionalrequestforassistancenowfromotherssinceIstartedusingit.Idiscovered,whilereading"Byte"whyIhadceasedstudyingCandC++anditwasbecausethebookgiventomestartedoutwithanexamplecontaininganaugmentedassignment.Ofcourse,therewasnoexplanationforthisarrangementofoperatorsandIfellonmyheadtryingtomakesenseofwhatwasonthewrittenpage.AsIrecallitwasamostfrustratingexercisewhichIeventuallyabandoned.Doesn'tmeanCorC++isimpossibletolearn,oreventhatIamstupid,butitdoesmeanthatthedocumentationIworkedmywaythroughdidnotdefinethesymbolsandwordswhichisanessentialpartofanyinstruction.Justascomputerswillnotbeabletounderstandacomputerwordorcomputersymbolthatisoutsidethesyntaxforthelanguagebeingused,astudentnewtoanyfieldwillnotgrasphissubjectifheencounterswordsorsymbolsforwhichtherearenodefinitions.Yougeta"bluescreen"asitwereineithercase.Thesolutionissimple,though:findthewordorsymbolandgettheproperdefinitionorsymbolandloandbehold,thecomputerorstudentcanproceed.Yourbook

Introduction

4

wassowellputtogetherthatIfoundverylittleinitIcouldn'tgrasp.So,thankyou.Iencourageyoutocontinuetoincludefulldefinitionsofterms.ThedocumentationwithPythonisgood,onceyouknow,(theexamplesareitsstrengthfromwhatIsee)butinmanycasesitseemsthatyouhavetoknowinordertounderstandthedocumentationwhichtomymindisnotwhatshouldbe.Thirdpartytutorialsexpresstheneedforclarificationofthedocumentationandtheirsuccesslargelydependsonthewordsthatareusedtodescribetheterminology.Ihaverecommendedyourbooktomanyothers.SomeinAustralia,someintheCaribbeanandyetothersintheUS.Itfillsanichenoothersdo.Ihopeyouaredoingwellandwishyouallthesuccessinthefuture.--Nick

hey,thisisankush(19).Iwasfacingagreatdifficultytostartwithpython.Itriedalotofbooksbutallwerebulkierandnottargetoriented;andthenifoundthislovelyone,whichmademelovepythoninnotime.Thanksalotforthis"beautifulpieceofbook".--Ankush

IwouldliketothankyouforyourexcellentguideonPython.Iamamolecularbiologist(withlittleprogrammingbackground)andformyworkIneedtohandlebigdatasetsofDNAsequencesandtoanalysemicroscopeimages.Forboththings,programminginpythonhasbeenuseful,ifnotessentialtocompleteandpublisha6-yearsproject.Thatsuchaguideisfreelyavailableisaclearsignthattheforcesofevilarenotyetrulingtheworld!:)--Luca

Sincethisisgoingtobethefirstlanguageyoulearn,youshoulduseAByteofPython.ItreallygivesaproperintroductionintoprogramminginPythonanditispacedwellenoughfortheaveragebeginner.Themostimportantthingfromthenonwillbeactuallystartingtopracticemakingyourownlittleprograms.--"{Unregistered}"

Justtosayaloudandhappythankyouverymuchforpublishing"AByteofPython"and"AByteofVim".ThosebookswereveryusefultomefourorfiveyearsagowhenIstartinglearningprogramming.RightnowI'mdevelopingaprojectthatwasadreamforalong,longtimeandjustwanttosaythankyou.Keepwalking.Youareasourceofmotivation.Allthebest.--Jocimar

FinishedreadingAbyteofPythonin3days.Itisthoroughlyinteresting.Notasinglepagewasboring.IwanttounderstandtheOrcascreenreadercode.Yourbookhashopefullyequippedmeforit.--Dattatray

Hi,'Abyteofpython'isreallyagoodreadingforpythonbeginners.So,again,NICEWORK!i'ma4yearsexperiencedJava&CdeveloperfromChina.Recently,iwanttodosomeworkonzim-wikinoteprojectwhichusespygtktoimplement.ireadyourbookin6days,andicanreadandwritepythoncodeexamplesnow.thxforyourcontribution.plzkeepyourenthusiasmtomakethisworldbetter,thisisjustalittleencouragefromChina.--Lee

IamIsenfromTaiwan,whoisagraduatingPhDstudentinElectricalEngineeringDepartmentofNationalTaiwanUniversity.Iwouldliketothankyouforyourgreatbook.IthinkitisnotonlyjusteasytoreadbutalsocomprehensiveandcompleteforanewcomerofPython.ThereasonIreadyourbookisthatIamstartingtoworkontheGNURadioframework.YourbookletmecatchmostofimportantcoreideasandskillofPythonwithaminimumtime.Ialsosawthatyoudonotmindthatreaderssendyouathanknoteinyourbook.SoIreallylikeyourbookandappreciateit.Thanks.--IsenI-ChunChao

ThebookisevenusedbyNASA!ItisusedintheirJetPropulsionLaboratorywiththeirDeepSpaceNetworkproject.

AcademicCourses

Thisbookis/wasbeingusedasinstructionalmaterialinvariouseducationalinstitutions:

'PrinciplesofProgrammingLanguages'courseatVrijeUniversiteit,Amsterdam'BasicConceptsofComputing'courseatUniversityofCalifornia,Davis'ProgrammingWithPython'courseatHarvardUniversity'IntroductiontoProgramming'courseatUniversityofLeeds'IntroductiontoApplicationProgramming'courseatBostonUniversity'InformationTechnologySkillsforMeteorology'courseatUniversityofOklahoma'Geoprocessing'courseatMichiganStateUniversity

Introduction

5

'MultiAgentSemanticWebSystems'courseattheUniversityofEdinburgh'IntroductiontoComputerScienceandProgramming'atMITOpenCourseWare'BasicprogrammingattheFacultyofSocialSciences,UniversityofLjubljana,Slovenia'--AlešŽibernasays"I(andmypredecessor)havebeenusingyourbookasthemainliteratureforthiscourse"'Introductiontoprogramming',DepartmentofInformationSciences,UniversityofZadar,Croatia--KrešimirZaudersays"IwouldliketoinformyouthatAByteofPythonisamandatoryreadatmycourse"

License

ThisbookislicensedunderaCreativeCommonsAttribution-ShareAlike4.0InternationalLicense.

Thismeans:

YouarefreetoSharei.e.tocopy,distributeandtransmitthisbookYouarefreetoRemixi.e.tomakechangestothisbook(especiallytranslations)Youarefreetouseitforcommercialpurposes

Pleasenote:

Pleasedonotsellelectronicorprintedcopiesofthebookunlessyouhaveclearlyandprominentlymentionedinthedescriptionthatthesecopiesarenotfromtheoriginalauthorofthisbook.Attributionmustbeshownintheintroductorydescriptionandfrontpageofthedocumentbylinkingbacktohttps://python.swaroopch.comandclearlyindicatingthattheoriginaltextcanbefetchedfromthislocation.Allthecode/scriptsprovidedinthisbookislicensedunderthe3-clauseBSDLicenseunlessotherwisenoted.

ReadNowYoucanreaditonlineathttps://python.swaroopch.com

BuyTheBookAprintedhardcopyofthebookcanbepurchasedathttps://swaroopch.com/buybookforyourofflinereadingpleasure,andtosupportthecontinueddevelopmentandimprovementofthisbook.

DownloadVisithttps://github.com/swaroopch/byte-of-python/releases/latesttodownloadaPDFfile(bestfordesktopreading)oranEPUBfile(bestfordevicessuchasmobile,tablet,ebookreaders).

Visithttps://github.com/swaroopch/byte-of-pythonfortherawcontent(forsuggestingcorrections,changes,translating,etc.)

Readthebookinyournativelanguage

Ifyouareinterestedinreadingorcontributingtranslationsofthisbooktootherhumanlanguages,pleaseseeTranslations.

Introduction

6

DedicationToKalyanVarmaandmanyotherseniorsatPESITwhointroducedustoGNU/Linuxandtheworldofopensource.

TothememoryofAtulChitnis,afriendandguidewhoshallbemissedgreatly.

TothepioneerswhomadetheInternethappen.Thisbookwasfirstwrittenin2003.Itstillremainspopular,thankstothenatureofsharingknowledgeontheInternetasenvisionedbythepioneers.

Dedication

7

PrefacePythonisprobablyoneofthefewprogramminglanguageswhichisbothsimpleandpowerful.Thisisgoodforbeginnersaswellasforexperts,andmoreimportantly,isfuntoprogramwith.Thisbookaimstohelpyoulearnthiswonderfullanguageandshowhowtogetthingsdonequicklyandpainlessly-ineffect'TheAnti-venomtoyourprogrammingproblems'.

WhoThisBookIsFor

ThisbookservesasaguideortutorialtothePythonprogramminglanguage.Itismainlytargetedatnewbies.Itisusefulforexperiencedprogrammersaswell.

Theaimisthatifallyouknowaboutcomputersishowtosavetextfiles,thenyoucanlearnPythonfromthisbook.Ifyouhavepreviousprogrammingexperience,thenyoucanalsolearnPythonfromthisbook.

Ifyoudohavepreviousprogrammingexperience,youwillbeinterestedinthedifferencesbetweenPythonandyourfavoriteprogramminglanguage-Ihavehighlightedmanysuchdifferences.Alittlewarningthough,Pythonissoongoingtobecomeyourfavoriteprogramminglanguage!

OfficialWebsiteTheofficialwebsiteofthebookishttps://python.swaroopch.comwhereyoucanreadthewholebookonline,downloadthelatestversionsofthebook,buyaprintedhardcopyandalsosendmefeedback.

SomethingToThinkAboutTherearetwowaysofconstructingasoftwaredesign:onewayistomakeitsosimplethatthereareobviouslynodeficiencies;theotheristomakeitsocomplicatedthattherearenoobviousdeficiencies.--C.A.R.Hoare

Successinlifeisamatternotsomuchoftalentandopportunityasofconcentrationandperseverance.--C.W.Wendte

Preface

8

AboutPythonPythonisoneofthoserarelanguageswhichcanclaimtobebothsimpleandpowerful.Youwillfindyourselfpleasantlysurprisedtoseehoweasyitistoconcentrateonthesolutiontotheproblemratherthanthesyntaxandstructureofthelanguageyouareprogrammingin.

TheofficialintroductiontoPythonis:

Pythonisaneasytolearn,powerfulprogramminglanguage.Ithasefficienthigh-leveldatastructuresandasimplebuteffectiveapproachtoobject-orientedprogramming.Python'selegantsyntaxanddynamictyping,togetherwithitsinterpretednature,makeitanideallanguageforscriptingandrapidapplicationdevelopmentinmanyareasonmostplatforms.

Iwilldiscussmostofthesefeaturesinmoredetailinthenextsection.

Storybehindthename

GuidovanRossum,thecreatorofthePythonlanguage,namedthelanguageaftertheBBCshow"MontyPython'sFlyingCircus".Hedoesn'tparticularlylikesnakesthatkillanimalsforfoodbywindingtheirlongbodiesaroundthemandcrushingthem.

FeaturesofPython

Simple

Pythonisasimpleandminimalisticlanguage.ReadingagoodPythonprogramfeelsalmostlikereadingEnglish,althoughverystrictEnglish!Thispseudo-codenatureofPythonisoneofitsgreateststrengths.Itallowsyoutoconcentrateonthesolutiontotheproblemratherthanthelanguageitself.

EasytoLearn

Asyouwillsee,Pythonisextremelyeasytogetstartedwith.Pythonhasanextraordinarilysimplesyntax,asalreadymentioned.

FreeandOpenSource

PythonisanexampleofaFLOSS(Free/LibreandOpenSourceSoftware).Insimpleterms,youcanfreelydistributecopiesofthissoftware,readitssourcecode,makechangestoit,andusepiecesofitinnewfreeprograms.FLOSSisbasedontheconceptofacommunitywhichsharesknowledge.ThisisoneofthereasonswhyPythonissogood-ithasbeencreatedandisconstantlyimprovedbyacommunitywhojustwanttoseeabetterPython.

High-levelLanguage

WhenyouwriteprogramsinPython,youneverneedtobotheraboutthelow-leveldetailssuchasmanagingthememoryusedbyyourprogram,etc.

Portable

Duetoitsopen-sourcenature,Pythonhasbeenportedto(i.e.changedtomakeitworkon)manyplatforms.AllyourPythonprogramscanworkonanyoftheseplatformswithoutrequiringanychangesatallifyouarecarefulenoughtoavoidanysystem-dependentfeatures.

AboutPython

9

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!

YoucanevenuseaplatformlikeKivytocreategamesforyourcomputerandforiPhone,iPad,andAndroid.

Interpreted

Thisrequiresabitofexplanation.

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

Python,ontheotherhand,doesnotneedcompilationtobinary.Youjustruntheprogramdirectlyfromthesourcecode.Internally,Pythonconvertsthesourcecodeintoanintermediateformcalledbytecodesandthentranslatesthisintothenativelanguageofyourcomputerandthenrunsit.Allthis,actually,makesusingPythonmucheasiersinceyoudon'thavetoworryaboutcompilingtheprogram,makingsurethattheproperlibrariesarelinkedandloaded,etc.ThisalsomakesyourPythonprogramsmuchmoreportable,sinceyoucanjustcopyyourPythonprogramontoanothercomputeranditjustworks!

ObjectOriented

Pythonsupportsprocedure-orientedprogrammingaswellasobject-orientedprogramming(OOP).Inprocedure-orientedlanguages,theprogramisbuiltaroundproceduresorfunctionswhicharenothingbutreusablepiecesofprograms.Inobject-orientedlanguages,theprogramisbuiltaroundobjectswhichcombinedataandfunctionality.PythonhasaverypowerfulbutsimplisticwayofdoingOOP,especiallywhencomparedtobiglanguageslikeC++orJava.

Extensible

Ifyouneedacriticalpieceofcodetorunveryfastorwanttohavesomepieceofalgorithmnottobeopen,youcancodethatpartofyourprograminCorC++andthenuseitfromyourPythonprogram.

Embeddable

YoucanembedPythonwithinyourC/C++programstogivescriptingcapabilitiesforyourprogram'susers.

ExtensiveLibraries

ThePythonStandardLibraryishugeindeed.Itcanhelpyoudovariousthingsinvolvingregularexpressions,documentationgeneration,unittesting,threading,databases,webbrowsers,CGI,FTP,email,XML,XML-RPC,HTML,WAVfiles,cryptography,GUI(graphicaluserinterfaces),andothersystem-dependentstuff.Remember,allthisisalwaysavailablewhereverPythonisinstalled.ThisiscalledtheBatteriesIncludedphilosophyofPython.

Besidesthestandardlibrary,therearevariousotherhigh-qualitylibrarieswhichyoucanfindatthePythonPackageIndex.

Summary

Pythonisindeedanexcitingandpowerfullanguage.IthastherightcombinationofperformanceandfeaturesthatmakewritingprogramsinPythonbothfunandeasy.

Python3versus2Youcanignorethissectionifyou'renotinterestedinthedifferencebetween"Pythonversion2"and"Pythonversion3".Butpleasedobeawareofwhichversionyouareusing.ThisbookiswrittenforPythonversion3.

AboutPython

10

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

FordetailsondifferencesbetweenPython2andPython3,see:

ThefutureofPython2PortingPython2CodetoPython3WritingcodethatrunsunderbothPython2and3SupportingPython3:Anin-depthguide

WhatProgrammersSayYoumayfinditinterestingtoreadwhatgreathackerslikeEricS.Raymond(ESR)havetosayaboutPython:

EricS.Raymondistheauthorof"TheCathedralandtheBazaar"andisalsothepersonwhocoinedthetermOpenSource.HesaysthatPythonhasbecomehisfavoriteprogramminglanguage.ThisarticlewastherealinspirationformyfirstbrushwithPython.BruceEckelistheauthorofthefamous'ThinkinginJava'and'ThinkinginC++'books.HesaysthatnolanguagehasmadehimmoreproductivethanPython.HesaysthatPythonisperhapstheonlylanguagethatfocusesonmakingthingseasierfortheprogrammer.Readthecompleteinterviewformoredetails.PeterNorvigisawell-knownLispauthorandDirectorofSearchQualityatGoogle(thankstoGuidovanRossumforpointingthatout).HesaysthatwritingPythonislikewritinginpseudocode.HesaysthatPythonhasalwaysbeenanintegralpartofGoogle.YoucanactuallyverifythisstatementbylookingattheGoogleJobspagewhichlistsPythonknowledgeasarequirementforsoftwareengineers.

AboutPython

11

InstallationWhenwereferto"Python3"inthisbook,wewillbereferringtoanyversionofPythonequaltoorgreaterthanversionPython3.6.0.

InstallationonWindows

Visithttps://www.python.org/downloads/anddownloadthelatestversion.Atthetimeofthiswriting,itwasPython3.5.1TheinstallationisjustlikeanyotherWindows-basedsoftware.

NotethatifyourWindowsversionispre-Vista,youshoulddownloadPython3.4onlyaslaterversionsrequirenewerversionsofWindows.

CAUTION:Makesureyoucheckoption AddPython3.5toPATH.

Tochangeinstalllocation,clickon Customizeinstallation,then Nextandenter C:\python35(oranotherappropriatelocation)astheinstalllocation.

Ifyoudidn�tcheckthe AddPython3.5PATHoptionearlier,check AddPythontoenvironmentvariables.Thisdoesthesamethingas AddPython3.5toPATHonthefirstinstallscreen.

YoucanchoosetoinstallLauncherforallusersornot,itdoesnotmattermuch.LauncherisusedtoswitchbetweendifferentversionsofPythoninstalled.

Ifyourpathwasnotsetcorrectly(bycheckingthe AddPython3.5Pathor AddPythontoenvironmentvariablesoptions),thenfollowthestepsinthenextsection( DOSPrompt)tofixit.Otherwise,gotothe RunningPythonpromptonWindowssectioninthisdocument.

NOTE:Forpeoplewhoalreadyknowprogramming,ifyouarefamiliarwithDocker,checkoutPythoninDockerandDockeronWindows.

DOSPrompt

IfyouwanttobeabletousePythonfromtheWindowscommandlinei.e.theDOSprompt,thenyouneedtosetthePATHvariableappropriately.

ForWindows2000,XP,2003,clickon ControlPanel-> System-> Advanced-> EnvironmentVariables.Clickonthevariablenamed PATHintheSystemVariablessection,thenselect Editandadd ;C:\Python35(pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatedirectoryname.

ForolderversionsofWindows,openthefile C:\AUTOEXEC.BATandaddtheline PATH=%PATH%;C:\Python35andrestartthesystem.ForWindowsNT,usethe AUTOEXEC.NTfile.

ForWindowsVista:

ClickStartandchoose ControlPanelClickSystem,ontherightyou'llsee"Viewbasicinformationaboutyourcomputer"Ontheleftisalistoftasks,thelastofwhichis Advancedsystemsettings.Clickthat.The Advancedtabofthe SystemPropertiesdialogboxisshown.Clickthe EnvironmentVariablesbuttononthebottomright.Inthelowerboxtitled SystemVariablesscrolldowntoPathandclickthe Editbutton.Changeyourpathasneedbe.Restartyoursystem.Vistadidn'tpickupthesystempathenvironmentvariablechangeuntilIrestarted.

Installation

12

ForWindows7and8:

RightclickonComputerfromyourdesktopandselect Propertiesorclick Startandchoose ControlPanel->SystemandSecurity-> System.Clickon Advancedsystemsettingsontheleftandthenclickonthe Advancedtab.Atthebottomclickon EnvironmentVariablesandunder Systemvariables,lookforthe PATHvariable,selectandthenpress Edit.GototheendofthelineunderVariablevalueandappend ;C:\Python35(pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatefoldername.Ifthevaluewas %SystemRoot%\system32;Itwillnowbecome %SystemRoot%\system32;C:\Python36Click OKandyouaredone.Norestartisrequired,howeveryoumayhavetocloseandreopenthecommandline.

ForWindows10:

WindowsStartMenu> Settings> About> SystemInfo(thisisallthewayovertotheright)> AdvancedSystemSettings> EnvironmentVariables(thisistowardsthebottom)>(thenhighlight Pathvariableandclick Edit)> New>(typeinwhateveryourpythonlocationis.Forexample, C:\Python35\)

RunningPythonpromptonWindows

ForWindowsusers,youcanruntheinterpreterinthecommandlineifyouhavesetthe PATHvariableappropriately.

ToopentheterminalinWindows,clickthestartbuttonandclick Run.Inthedialogbox,type cmdandpress [enter]key.

Then,type pythonandensuretherearenoerrors.

InstallationonMacOSX

ForMacOSXusers,useHomebrew: brewinstallpython3.

Toverify,opentheterminalbypressing [Command+Space]keys(toopenSpotlightsearch),type Terminalandpress[enter]key.Now,run python3andensuretherearenoerrors.

InstallationonGNU/Linux

ForGNU/Linuxusers,useyourdistribution'spackagemanagertoinstallPython3,e.g.onDebian&Ubuntu: sudoapt-getupdate&&sudoapt-getinstallpython3.

Toverify,opentheterminalbyopeningthe Terminalapplicationorbypressing Alt+F2andentering gnome-terminal.Ifthatdoesn'twork,pleasereferthedocumentationofyourparticularGNU/Linuxdistribution.Now,run python3andensuretherearenoerrors.

YoucanseetheversionofPythononthescreenbyrunning:

$python3-V

Python3.6.0

NOTE: $isthepromptoftheshell.Itwillbedifferentforyoudependingonthesettingsoftheoperatingsystemonyourcomputer,henceIwillindicatethepromptbyjustthe $symbol.

CAUTION:Outputmaybedifferentonyourcomputer,dependingontheversionofPythonsoftwareinstalledonyourcomputer.

Summary

Installation

13

Fromnowon,wewillassumethatyouhavePythoninstalledonyoursystem.

Next,wewillwriteourfirstPythonprogram.

Installation

14

FirstStepsWewillnowseehowtorunatraditional'HelloWorld'programinPython.Thiswillteachyouhowtowrite,saveandrunPythonprograms.

TherearetwowaysofusingPythontorunyourprogram-usingtheinteractiveinterpreterpromptorusingasourcefile.Wewillnowseehowtousebothofthesemethods.

UsingTheInterpreterPrompt

Opentheterminalinyouroperatingsystem(asdiscussedpreviouslyintheInstallationchapter)andthenopenthePythonpromptbytyping python3andpressing [enter]key.

OnceyouhavestartedPython,youshouldsee >>>whereyoucanstarttypingstuff.ThisiscalledthePythoninterpreterprompt.

AtthePythoninterpreterprompt,type:

print("HelloWorld")

followedbythe [enter]key.Youshouldseethewords HelloWorldprintedtothescreen.

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

$python3

Python3.6.0(default,Jan122017,11:26:36)

[GCC4.2.1CompatibleAppleLLVM8.0.0(clang-800.0.38)]ondarwin

Type"help","copyright","credits"or"license"formoreinformation.

>>>print("HelloWorld")

HelloWorld

NoticethatPythongivesyoutheoutputofthelineimmediately!WhatyoujustenteredisasinglePythonstatement.Weuseprintto(unsurprisingly)printanyvaluethatyousupplytoit.Here,wearesupplyingthetext HelloWorldandthisispromptlyprintedtothescreen.

HowtoQuittheInterpreterPrompt

IfyouareusingaGNU/LinuxorOSXshell,youcanexittheinterpreterpromptbypressing [ctrl+d]orentering exit()(note:remembertoincludetheparentheses, ())followedbythe [enter]key.

IfyouareusingtheWindowscommandprompt,press [ctrl+z]followedbythe [enter]key.

ChoosingAnEditor

Wecannottypeoutourprogramattheinterpreterprompteverytimewewanttorunsomething,sowehavetosavetheminfilesandcanrunourprogramsanynumberoftimes.

TocreateourPythonsourcefiles,weneedaneditorsoftwarewhereyoucantypeandsave.Agoodprogrammer'seditorwillmakeyourlifeeasierinwritingthesourcefiles.Hence,thechoiceofaneditoriscrucialindeed.Youhavetochooseaneditorasyouwouldchooseacaryouwouldbuy.AgoodeditorwillhelpyouwritePythonprogramseasily,makingyourjourneymorecomfortableandhelpsyoureachyourdestination(achieveyourgoal)inamuchfasterandsaferway.

FirstSteps

15

OneoftheverybasicrequirementsissyntaxhighlightingwhereallthedifferentpartsofyourPythonprogramarecolorizedsothatyoucanseeyourprogramandvisualizeitsrunning.

Ifyouhavenoideawheretostart,IwouldrecommendusingPyCharmEducationalEditionsoftwarewhichisavailableonWindows,MacOSXandGNU/Linux.Detailsinthenextsection.

IfyouareusingWindows,donotuseNotepad-itisabadchoicebecauseitdoesnotdosyntaxhighlightingandalsoimportantlyitdoesnotsupportindentationofthetextwhichisveryimportantinourcaseaswewillseelater.Goodeditorswillautomaticallydothis.

Ifyouareanexperiencedprogrammer,thenyoumustbealreadyusingVimorEmacs.Needlesstosay,thesearetwoofthemostpowerfuleditorsandyouwillbenefitfromusingthemtowriteyourPythonprograms.Ipersonallyusebothformostofmyprograms,andhaveevenwrittenanentirebookonVim.

IncaseyouarewillingtotakethetimetolearnVimorEmacs,thenIhighlyrecommendthatyoudolearntouseeitherofthemasitwillbeveryusefulforyouinthelongrun.However,asImentionedbefore,beginnerscanstartwithPyCharmandfocusthelearningonPythonratherthantheeditoratthismoment.

Toreiterate,pleasechooseapropereditor-itcanmakewritingPythonprogramsmorefunandeasy.

PyCharm

PyCharmEducationalEditionisafreeeditorwhichyoucanuseforwritingPythonprograms.

WhenyouopenPyCharm,you'llseethis,clickon CreateNewProject:

FirstSteps

16

Select PurePython:

FirstSteps

17

Change untitledto helloworldasthelocationoftheproject,youshouldseedetailssimilartothis:

FirstSteps

18

Clickthe Createbutton.

Right-clickonthe helloworldinthesidebarandselect New-> PythonFile:

FirstSteps

19

Youwillbeaskedtotypethename,type hello:

Youcannowseeafileopenedforyou:

FirstSteps

20

Deletethelinesthatarealreadypresent,andnowtypethefollowing:

print("helloworld")

Nowright-clickonwhatyoutyped(withoutselectingthetext),andclickon Run'hello'.

Youshouldnowseetheoutput(whatitprints)ofyourprogram:

FirstSteps

21

Phew!Thatwasquiteafewstepstogetstarted,buthenceforth,everytimeweaskyoutocreateanewfile,remembertojustright-clickon helloworldontheleft-> New-> PythonFileandcontinuethesamestepstotypeandrunasshownabove.

YoucanfindmoreinformationaboutPyCharminthePyCharmQuickstartpage.

Vim1. InstallVim

MacOSXusersshouldinstall macvimpackageviaHomeBrewWindowsusersshoulddownloadthe"self-installingexecutable"fromVimwebsiteGNU/LinuxusersshouldgetVimfromtheirdistribution'ssoftwarerepositories,e.g.DebianandUbuntuuserscaninstallthe vimpackage.

2. Installjedi-vimpluginforautocompletion.3. Installcorresponding jedipythonpackage: pipinstall-Ujedi

Emacs1. InstallEmacs24+.

MacOSXusersshouldgetEmacsfromhttp://emacsformacosx.comWindowsusersshouldgetEmacsfromhttp://ftp.gnu.org/gnu/emacs/windows/GNU/LinuxusersshouldgetEmacsfromtheirdistribution'ssoftwarerepositories,e.g.DebianandUbuntuuserscaninstallthe emacs24package.

2. InstallELPY

UsingASourceFile

FirstSteps

22

Nowlet'sgetbacktoprogramming.Thereisatraditionthatwheneveryoulearnanewprogramminglanguage,thefirstprogramthatyouwriteandrunisthe'HelloWorld'program-allitdoesisjustsay'HelloWorld'whenyourunit.AsSimonCozens says,itisthe"traditionalincantationtotheprogramminggodstohelpyoulearnthelanguagebetter."

Startyourchoiceofeditor,enterthefollowingprogramandsaveitas hello.py.

IfyouareusingPyCharm,wehavealreadydiscussedhowtorunfromasourcefile.

Forothereditors,openanewfile hello.pyandtypethis:

print("helloworld")

Whereshouldyousavethefile?Toanyfolderforwhichyouknowthelocationofthefolder.Ifyoudon'tunderstandwhatthatmeans,createanewfolderandusethatlocationtosaveandrunallyourPythonprograms:

/tmp/pyonMacOSX/tmp/pyonGNU/LinuxC:\pyonWindows

Tocreatetheabovefolder(fortheoperatingsystemyouareusing),usethe mkdircommandintheterminal,forexample, mkdir/tmp/py.

IMPORTANT:Alwaysensurethatyougiveitthefileextensionof .py,forexample, foo.py.

TorunyourPythonprogram:

1. Openaterminalwindow(seethepreviousInstallationchapteronhowtodothat)2. Changedirectorytowhereyousavedthefile,forexample, cd/tmp/py3. Runtheprogrambyenteringthecommand pythonhello.py.Theoutputisasshownbelow.

$pythonhello.py

helloworld

1

FirstSteps

23

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

Incaseyougotanerror,pleasetypetheaboveprogramexactlyasshownaboveandruntheprogramagain.NotethatPythoniscase-sensitivei.e. printisnotthesameas Print-notethelowercase pintheformerandtheuppercase Pinthelatter.Also,ensuretherearenospacesortabsbeforethefirstcharacterineachline-wewillseewhythisisimportantlater.

HowItWorks

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

GettingHelp

IfyouneedquickinformationaboutanyfunctionorstatementinPython,thenyoucanusethebuilt-in helpfunctionality.Thisisveryusefulespeciallywhenusingtheinterpreterprompt.Forexample,run help('len')-thisdisplaysthehelpforthe lenfunctionwhichisusedtocountnumberofitems.

TIP:Press qtoexitthehelp.

Similarly,youcanobtaininformationaboutalmostanythinginPython.Use help()tolearnmoreaboutusing helpitself!

Incaseyouneedtogethelpforoperatorslike return,thenyouneedtoputthoseinsidequotessuchas help('return')sothatPythondoesn'tgetconfusedonwhatwe'retryingtodo.

Summary

Youshouldnowbeabletowrite,saveandrunPythonprogramsatease.

NowthatyouareaPythonuser,let'slearnsomemorePythonconcepts.

.theauthoroftheamazing'BeginningPerl'book↩1

FirstSteps

24

BasicsJustprinting helloworldisnotenough,isit?Youwanttodomorethanthat-youwanttotakesomeinput,manipulateitandgetsomethingoutofit.WecanachievethisinPythonusingconstantsandvariables,andwe'lllearnsomeotherconceptsaswellinthischapter.

Comments

Commentsareanytexttotherightofthe #symbolandismainlyusefulasnotesforthereaderoftheprogram.

Forexample:

print('helloworld')#Notethatprintisafunction

or:

#Notethatprintisafunction

print('helloworld')

Useasmanyusefulcommentsasyoucaninyourprogramto:

explainassumptionsexplainimportantdecisionsexplainimportantdetailsexplainproblemsyou'retryingtosolveexplainproblemsyou'retryingtoovercomeinyourprogram,etc.

Codetellsyouhow,commentsshouldtellyouwhy.

Thisisusefulforreadersofyourprogramsothattheycaneasilyunderstandwhattheprogramisdoing.Remember,thatpersoncanbeyourselfaftersixmonths!

LiteralConstants

Anexampleofaliteralconstantisanumberlike 5, 1.23,orastringlike 'Thisisastring'or "It'sastring!".

Itiscalledaliteralbecauseitisliteral-youuseitsvalueliterally.Thenumber 2alwaysrepresentsitselfandnothingelse-itisaconstantbecauseitsvaluecannotbechanged.Hence,allthesearereferredtoasliteralconstants.

Numbers

Numbersaremainlyoftwotypes-integersandfloats.

Anexampleofanintegeris 2whichisjustawholenumber.

Examplesoffloatingpointnumbers(orfloatsforshort)are 3.23and 52.3E-4.The Enotationindicatespowersof10.Inthiscase, 52.3E-4means 52.3*10^-4^.

NoteforExperiencedProgrammers

Thereisnoseparate longtype.The inttypecanbeanintegerofanysize.

Basics

25

Strings

Astringisasequenceofcharacters.Stringsarebasicallyjustabunchofwords.

YouwillbeusingstringsinalmosteveryPythonprogramthatyouwrite,sopayattentiontothefollowingpart.

SingleQuote

Youcanspecifystringsusingsinglequotessuchas 'Quotemeonthis'.

Allwhitespacei.e.spacesandtabs,withinthequotes,arepreservedas-is.

DoubleQuotes

Stringsindoublequotesworkexactlythesamewayasstringsinsinglequotes.Anexampleis "What'syourname?".

TripleQuotes

Youcanspecifymulti-linestringsusingtriplequotes-( """or ''').Youcanusesinglequotesanddoublequotesfreelywithinthetriplequotes.Anexampleis:

'''Thisisamulti-linestring.Thisisthefirstline.

Thisisthesecondline.

"What'syourname?,"Iasked.

Hesaid"Bond,JamesBond."

'''

StringsAreImmutable

Thismeansthatonceyouhavecreatedastring,youcannotchangeit.Althoughthismightseemlikeabadthing,itreallyisn't.Wewillseewhythisisnotalimitationinthevariousprogramsthatweseelateron.

NoteforC/C++Programmers

Thereisnoseparate chardatatypeinPython.ThereisnorealneedforitandIamsureyouwon'tmissit.

NoteforPerl/PHPProgrammers

Rememberthatsingle-quotedstringsanddouble-quotedstringsarethesame-theydonotdifferinanyway.

Theformatmethod

Sometimeswemaywanttoconstructstringsfromotherinformation.Thisiswherethe format()methodisuseful.

Savethefollowinglinesasafile str_format.py:

age=20

name='Swaroop'

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

print('Whyis{0}playingwiththatpython?'.format(name))

Output:

$pythonstr_format.py

Swaroopwas20yearsoldwhenhewrotethisbook

WhyisSwaroopplayingwiththatpython?

Basics

26

HowItWorks

Astringcanusecertainspecificationsandsubsequently,the formatmethodcanbecalledtosubstitutethosespecificationswithcorrespondingargumentstothe formatmethod.

Observethefirstusagewhereweuse {0}andthiscorrespondstothevariable namewhichisthefirstargumenttotheformatmethod.Similarly,thesecondspecificationis {1}correspondingto agewhichisthesecondargumenttotheformatmethod.NotethatPythonstartscountingfrom0whichmeansthatfirstpositionisatindex0,secondpositionisatindex1,andsoon.

Noticethatwecouldhaveachievedthesameusingstringconcatenation:

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

butthatismuchuglierandmoreerror-prone.Second,theconversiontostringwouldbedoneautomaticallybythe formatmethodinsteadoftheexplicitconversiontostringsneededinthiscase.Third,whenusingthe formatmethod,wecanchangethemessagewithouthavingtodealwiththevariablesusedandvice-versa.

Alsonotethatthenumbersareoptional,soyoucouldhavealsowrittenas:

age=20

name='Swaroop'

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

print('Whyis{}playingwiththatpython?'.format(name))

whichwillgivethesameexactoutputasthepreviousprogram.

Wecanalsonametheparameters:

age=20

name='Swaroop'

print('{name}was{age}yearsoldwhenhewrotethisbook'.format(name=name,age=age))

print('Whyis{name}playingwiththatpython?'.format(name=name))

whichwillgivethesameexactoutputasthepreviousprogram.

Python3.6introducedashorterwaytodonamedparameters,called"f-strings":

age=20

name='Swaroop'

print(f'{name}was{age}yearsoldwhenhewrotethisbook')#noticethe'f'beforethestring

print(f'Whyis{name}playingwiththatpython?')#noticethe'f'beforethestring

whichwillgivethesameexactoutputasthepreviousprogram.

WhatPythondoesinthe formatmethodisthatitsubstituteseachargumentvalueintotheplaceofthespecification.Therecanbemoredetailedspecificationssuchas:

#decimal(.)precisionof3forfloat'0.333'

print('{0:.3f}'.format(1.0/3))

#fillwithunderscores(_)withthetextcentered

#(^)to11width'___hello___'

print('{0:_^11}'.format('hello'))

#keyword-based'SwaroopwroteAByteofPython'

print('{name}wrote{book}'.format(name='Swaroop',book='AByteofPython'))

Basics

27

Output:

0.333

___hello___

SwaroopwroteAByteofPython

Sincewearediscussingformatting,notethat printalwaysendswithaninvisible"newline"character( \n)sothatrepeatedcallsto printwillallprintonaseparatelineeach.Topreventthisnewlinecharacterfrombeingprinted,youcanspecifythatitshould endwithablank:

print('a',end='')

print('b',end='')

Outputis:

ab

Oryoucan endwithaspace:

print('a',end='')

print('b',end='')

print('c')

Outputis:

abc

EscapeSequences

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

Anotherwayofspecifyingthisspecificstringwouldbe "What'syourname?"i.e.usingdoublequotes.Similarly,youhavetouseanescapesequenceforusingadoublequoteitselfinadoublequotedstring.Also,youhavetoindicatethebackslashitselfusingtheescapesequence \\.

Whatifyouwantedtospecifyatwo-linestring?Onewayistouseatriple-quotedstringasshownpreviouslyoryoucanuseanescapesequenceforthenewlinecharacter- \ntoindicatethestartofanewline.Anexampleis:

'Thisisthefirstline\nThisisthesecondline'

Anotherusefulescapesequencetoknowisthetab: \t.TherearemanymoreescapesequencesbutIhavementionedonlythemostusefuloneshere.

Onethingtonoteisthatinastring,asinglebackslashattheendofthelineindicatesthatthestringiscontinuedinthenextline,butnonewlineisadded.Forexample:

"Thisisthefirstsentence.\

Thisisthesecondsentence."

Basics

28

isequivalentto

"Thisisthefirstsentence.Thisisthesecondsentence."

RawString

Ifyouneedtospecifysomestringswherenospecialprocessingsuchasescapesequencesarehandled,thenwhatyouneedistospecifyarawstringbyprefixing ror Rtothestring.Anexampleis:

r"Newlinesareindicatedby\n"

NoteforRegularExpressionUsers

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

Variable

Usingjustliteralconstantscansoonbecomeboring-weneedsomewayofstoringanyinformationandmanipulatethemaswell.Thisiswherevariablescomeintothepicture.Variablesareexactlywhatthenameimplies-theirvaluecanvary,i.e.,youcanstoreanythingusingavariable.Variablesarejustpartsofyourcomputer'smemorywhereyoustoresomeinformation.Unlikeliteralconstants,youneedsomemethodofaccessingthesevariablesandhenceyougivethemnames.

IdentifierNaming

Variablesareexamplesofidentifiers.Identifiersarenamesgiventoidentifysomething.Therearesomerulesyouhavetofollowfornamingidentifiers:

Thefirstcharacteroftheidentifiermustbealetterofthealphabet(uppercaseASCIIorlowercaseASCIIorUnicodecharacter)oranunderscore( _).Therestoftheidentifiernamecanconsistofletters(uppercaseASCIIorlowercaseASCIIorUnicodecharacter),underscores( _)ordigits(0-9).Identifiernamesarecase-sensitive.Forexample, mynameand myNamearenotthesame.Notethelowercase nintheformerandtheuppercase Ninthelatter.Examplesofvalididentifiernamesare i, name_2_3.Examplesofinvalididentifiernamesare 2things, thisisspacedout, my-nameand >a1b2_c3.

DataTypesVariablescanholdvaluesofdifferenttypescalleddatatypes.Thebasictypesarenumbersandstrings,whichwehavealreadydiscussed.Inlaterchapters,wewillseehowtocreateourowntypesusingclasses.

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

NoteforObjectOrientedProgrammingusers:

Pythonisstronglyobject-orientedinthesensethateverythingisanobjectincludingnumbers,stringsandfunctions.

Basics

29

Wewillnowseehowtousevariablesalongwithliteralconstants.Savethefollowingexampleandruntheprogram.

HowtowritePythonprograms

Henceforth,thestandardproceduretosaveandrunaPythonprogramisasfollows:

ForPyCharm

1. OpenPyCharm.2. Createnewfilewiththefilenamementioned.3. Typetheprogramcodegivenintheexample.4. Right-clickandrunthecurrentfile.

NOTE:Wheneveryouhavetoprovidecommandlinearguments,clickon Run-> EditConfigurationsandtypetheargumentsinthe Scriptparameters:sectionandclickthe OKbutton:

Forothereditors

1. Openyoureditorofchoice.2. Typetheprogramcodegivenintheexample.3. Saveitasafilewiththefilenamementioned.4. Runtheinterpreterwiththecommand pythonprogram.pytoruntheprogram.

Example:UsingVariablesAndLiteralConstants

Typeandrunthefollowingprogram:

#Filename:var.py

i=5

print(i)

i=i+1

Basics

30

print(i)

s='''Thisisamulti-linestring.

Thisisthesecondline.'''

print(s)

Output:

5

6

Thisisamulti-linestring.

Thisisthesecondline.

HowItWorks

Here'showthisprogramworks.First,weassigntheliteralconstantvalue 5tothevariable iusingtheassignmentoperator( =).Thislineiscalledastatementbecauseitstatesthatsomethingshouldbedoneandinthiscase,weconnectthevariablenameitothevalue 5.Next,weprintthevalueof iusingthe printstatementwhich,unsurprisingly,justprintsthevalueofthevariabletothescreen.

Thenweadd 1tothevaluestoredin iandstoreitback.Wethenprintitandexpectedly,wegetthevalue 6.

Similarly,weassigntheliteralstringtothevariable sandthenprintit.

Noteforstaticlanguageprogrammers

Variablesareusedbyjustassigningthemavalue.Nodeclarationordatatypedefinitionisneeded/used.

LogicalAndPhysicalLine

Aphysicallineiswhatyouseewhenyouwritetheprogram.AlogicallineiswhatPythonseesasasinglestatement.Pythonimplicitlyassumesthateachphysicallinecorrespondstoalogicalline.

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

Implicitly,Pythonencouragestheuseofasinglestatementperlinewhichmakescodemorereadable.

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

i=5

print(i)

iseffectivelysameas

i=5;

print(i);

whichisalsosameas

i=5;print(i);

andsameas

i=5;print(i)

Basics

31

However,Istronglyrecommendthatyousticktowritingamaximumofasinglelogicallineoneachsinglephysicalline.Theideaisthatyoushouldneverusethesemicolon.Infact,IhaveneverusedorevenseenasemicoloninaPythonprogram.

Thereisonekindofsituationwherethisconceptisreallyuseful:ifyouhavealonglineofcode,youcanbreakitintomultiplephysicallinesbyusingthebackslash.Thisisreferredtoasexplicitlinejoining:

s='Thisisastring.\

Thiscontinuesthestring.'

print(s)

Output:

Thisisastring.Thiscontinuesthestring.

Similarly,

i=\

5

isthesameas

i=5

Sometimes,thereisanimplicitassumptionwhereyoudon'tneedtouseabackslash.Thisisthecasewherethelogicallinehasastartingparentheses,startingsquarebracketsorastartingcurlybracesbutnotanendingone.Thisiscalledimplicitlinejoining.Youcanseethisinactionwhenwewriteprogramsusinglistinlaterchapters.

Indentation

WhitespaceisimportantinPython.Actually,whitespaceatthebeginningofthelineisimportant.Thisiscalledindentation.Leadingwhitespace(spacesandtabs)atthebeginningofthelogicallineisusedtodeterminetheindentationlevelofthelogicalline,whichinturnisusedtodeterminethegroupingofstatements.

Thismeansthatstatementswhichgotogethermusthavethesameindentation.Eachsuchsetofstatementsiscalledablock.Wewillseeexamplesofhowblocksareimportantinlaterchapters.

Onethingyoushouldrememberisthatwrongindentationcangiverisetoerrors.Forexample:

i=5

#Errorbelow!Noticeasinglespaceatthestartoftheline

print('Valueis',i)

print('Irepeat,thevalueis',i)

Whenyourunthis,yougetthefollowingerror:

File"whitespace.py",line3

print('Valueis',i)

^

IndentationError:unexpectedindent

Noticethatthereisasinglespaceatthebeginningofthesecondline.TheerrorindicatedbyPythontellsusthatthesyntaxoftheprogramisinvalidi.e.theprogramwasnotproperlywritten.Whatthismeanstoyouisthatyoucannotarbitrarilystartnewblocksofstatements(exceptforthedefaultmainblockwhichyouhavebeenusingallalong,ofcourse).Caseswhereyoucanuse

Basics

32

newblockswillbedetailedinlaterchapterssuchasthecontrolflow.

Howtoindent

Usefourspacesforindentation.ThisistheofficialPythonlanguagerecommendation.Goodeditorswillautomaticallydothisforyou.Makesureyouuseaconsistentnumberofspacesforindentation,otherwiseyourprogramwillnotrunorwillhaveunexpectedbehavior.

Notetostaticlanguageprogrammers

Pythonwillalwaysuseindentationforblocksandwillneverusebraces.Run from__future__importbracestolearnmore.

SummaryNowthatwehavegonethroughmanynitty-grittydetails,wecanmoveontomoreinterestingstuffsuchascontrolflowstatements.Besuretobecomecomfortablewithwhatyouhavereadinthischapter.

Basics

33

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

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

Operators

Wewillbrieflytakealookattheoperatorsandtheirusage.

Notethatyoucanevaluatetheexpressionsgivenintheexamplesusingtheinterpreterinteractively.Forexample,totesttheexpression 2+3,usetheinteractivePythoninterpreterprompt:

>>>2+3

5

>>>3*5

15

>>>

Hereisaquickoverviewoftheavailableoperators:

+(plus)

Addstwoobjects3+5gives 8. 'a'+'b'gives 'ab'.

-(minus)

Givesthesubtractionofonenumberfromtheother;ifthefirstoperandisabsentitisassumedtobezero.-5.2givesanegativenumberand 50-24gives 26.

*(multiply)

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

**(power)

Returnsxtothepowerofy3**4gives 81(i.e. 3*3*3*3)

/(divide)

Dividexbyy13/3gives 4.333333333333333

//(divideandfloor)

Dividexbyyandroundtheanswerdowntothenearestintegervalue.Notethatifoneofthevaluesisafloat,you'llgetbackafloat.13//3gives 4-13//3gives -59//1.81gives 4.0

%(modulo)

Returnstheremainderofthedivision13%3gives 1. -25.5%2.25gives 1.5.

OperatorsandExpressions

34

<<(leftshift)

Shiftsthebitsofthenumbertotheleftbythenumberofbitsspecified.(Eachnumberisrepresentedinmemorybybitsorbinarydigitsi.e.0and1)2<<2gives 8. 2isrepresentedby 10inbits.Leftshiftingby2bitsgives 1000whichrepresentsthedecimal 8.

>>(rightshift)

Shiftsthebitsofthenumbertotherightbythenumberofbitsspecified.11>>1gives 5.11isrepresentedinbitsby 1011whichwhenrightshiftedby1bitgives 101whichisthedecimal 5.

&(bit-wiseAND)

Bit-wiseANDofthenumbers:ifbothbitsare 1,theresultis 1.Otherwise,it's 0.5&3gives 1( 0101&0011gives 0001)

|(bit-wiseOR)

BitwiseORofthenumbers:ifbothbitsare 0,theresultis 0.Otherwise,it's 1.5|3gives 7( 0101|0011gives 0111)

(bit-wiseXOR)

BitwiseXORofthenumbers:ifbothbits( 1or0)arethesame,theresultis 0.Otherwise,it's 1.5^3gives 6( O101^0011gives 0110)

~(bit-wiseinvert)

Thebit-wiseinversionofxis-(x+1)~5gives -6.Moredetailsathttp://stackoverflow.com/a/11810203

<(lessthan)

Returnswhetherxislessthany.Allcomparisonoperatorsreturn Trueor False.Notethecapitalizationofthesenames.5<3gives Falseand 3<5gives True.Comparisonscanbechainedarbitrarily: 3<5<7gives True.

>(greaterthan)

Returnswhetherxisgreaterthany5>3returns True.Ifbothoperandsarenumbers,theyarefirstconvertedtoacommontype.Otherwise,italwaysreturns False.

<=(lessthanorequalto)

Returnswhetherxislessthanorequaltoyx=3;y=6;x<=yreturns True

>=(greaterthanorequalto)

Returnswhetherxisgreaterthanorequaltoyx=4;y=3;x>=3returns True

==(equalto)

Comparesiftheobjectsareequalx=2;y=2;x==yreturns Truex='str';y='stR';x==yreturns Falsex='str';y='str';x==yreturns True

!=(notequalto)

Comparesiftheobjectsarenotequalx=2;y=3;x!=yreturns True

not(booleanNOT)

OperatorsandExpressions

35

Ifxis True,itreturns False.Ifxis False,itreturns True.x=True;notxreturns False.

and(booleanAND)

xandyreturns Falseifxis False,elseitreturnsevaluationofyx=False;y=True;xandyreturns FalsesincexisFalse.Inthiscase,Pythonwillnotevaluateysinceitknowsthatthelefthandsideofthe'and'expressionis Falsewhichimpliesthatthewholeexpressionwillbe Falseirrespectiveoftheothervalues.Thisiscalledshort-circuitevaluation.

or(booleanOR)

Ifxis True,itreturnsTrue,elseitreturnsevaluationofyx=True;y=False;xoryreturns True.Short-circuitevaluationapplieshereaswell.

ShortcutformathoperationandassignmentItiscommontorunamathoperationonavariableandthenassigntheresultoftheoperationbacktothevariable,hencethereisashortcutforsuchexpressions:

a=2

a=a*3

canbewrittenas:

a=2

a*=3

Noticethat var=varoperationexpressionbecomes varoperation=expression.

EvaluationOrder

Ifyouhadanexpressionsuchas 2+3*4,istheadditiondonefirstorthemultiplication?Ourhighschoolmathstellsusthatthemultiplicationshouldbedonefirst.Thismeansthatthemultiplicationoperatorhashigherprecedencethantheadditionoperator.

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

Thefollowingtable,takenfromthePythonreferencemanual,isprovidedforthesakeofcompleteness.Itisfarbettertouseparenthesestogroupoperatorsandoperandsappropriatelyinordertoexplicitlyspecifytheprecedence.Thismakestheprogrammorereadable.SeeChangingtheOrderofEvaluationbelowfordetails.

lambda:LambdaExpressionif-else:Conditionalexpressionor:BooleanORand:BooleanANDnotx:BooleanNOTin,notin,is,isnot,<,<=,>,>=,!=,==:Comparisons,includingmembershiptestsandidentitytests|:BitwiseOR :BitwiseXOR&:BitwiseAND<<,>>:Shifts

OperatorsandExpressions

36

+,-:Additionandsubtraction*,/,//,%:Multiplication,Division,FloorDivisionandRemainder+x,-x,~x:Positive,Negative,bitwiseNOT**:Exponentiationx[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.

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

Thereisanadditionaladvantagetousingparentheses-ithelpsustochangetheorderofevaluation.Forexample,ifyouwantadditiontobeevaluatedbeforemultiplicationinanexpression,thenyoucanwritesomethinglike (2+3)*4.

Associativity

Operatorsareusuallyassociatedfromlefttoright.Thismeansthatoperatorswiththesameprecedenceareevaluatedinalefttorightmanner.Forexample, 2+3+4isevaluatedas (2+3)+4.

Expressions

Example(saveas expression.py):

length=5

breadth=2

area=length*breadth

print('Areais',area)

print('Perimeteris',2*(length+breadth))

Output:

$pythonexpression.py

Areais10

Perimeteris14

HowItWorks

Thelengthandbreadthoftherectanglearestoredinvariablesbythesamename.Weusethesetocalculatetheareaandperimeteroftherectanglewiththehelpofexpressions.Westoretheresultoftheexpression length*breadthinthevariable areaandthenprintitusingthe printfunction.Inthesecondcase,wedirectlyusethevalueoftheexpression 2*(length+breadth)intheprintfunction.

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37

Also,noticehowPythonpretty-printstheoutput.Eventhoughwehavenotspecifiedaspacebetween 'Areais'andthevariable area,Pythonputsitforussothatwegetacleanniceoutputandtheprogramismuchmorereadablethisway(sincewedon'tneedtoworryaboutspacinginthestringsweuseforoutput).ThisisanexampleofhowPythonmakeslifeeasyfortheprogrammer.

Summary

Wehaveseenhowtouseoperators,operandsandexpressions-thesearethebasicbuildingblocksofanyprogram.Next,wewillseehowtomakeuseoftheseinourprogramsusingstatements.

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38

ControlFlowIntheprogramswehaveseentillnow,therehasalwaysbeenaseriesofstatementsfaithfullyexecutedbyPythoninexacttop-downorder.Whatifyouwantedtochangetheflowofhowitworks?Forexample,youwanttheprogramtotakesomedecisionsanddodifferentthingsdependingondifferentsituations,suchasprinting'GoodMorning'or'GoodEvening'dependingonthetimeoftheday?

Asyoumighthaveguessed,thisisachievedusingcontrolflowstatements.TherearethreecontrolflowstatementsinPython-if, forand while.

The ifstatement

The ifstatementisusedtocheckacondition:iftheconditionistrue,werunablockofstatements(calledtheif-block),elseweprocessanotherblockofstatements(calledtheelse-block).Theelseclauseisoptional.

Example(saveas if.py):

number=23

guess=int(input('Enteraninteger:'))

ifguess==number:

#Newblockstartshere

print('Congratulations,youguessedit.')

print('(butyoudonotwinanyprizes!)')

#Newblockendshere

elifguess<number:

#Anotherblock

print('No,itisalittlehigherthanthat')

#Youcandowhateveryouwantinablock...

else:

print('No,itisalittlelowerthanthat')

#youmusthaveguessed>numbertoreachhere

print('Done')

#Thislaststatementisalwaysexecuted,

#aftertheifstatementisexecuted.

Output:

$pythonif.py

Enteraninteger:50

No,itisalittlelowerthanthat

Done

$pythonif.py

Enteraninteger:22

No,itisalittlehigherthanthat

Done

$pythonif.py

Enteraninteger:23

Congratulations,youguessedit.

(butyoudonotwinanyprizes!)

Done

HowItWorks

Controlflow

39

Inthisprogram,wetakeguessesfromtheuserandcheckifitisthenumberthatwehave.Wesetthevariable numbertoanyintegerwewant,say 23.Then,wetaketheuser'sguessusingthe input()function.Functionsarejustreusablepiecesofprograms.We'llreadmoreabouttheminthenextchapter.

Wesupplyastringtothebuilt-in inputfunctionwhichprintsittothescreenandwaitsforinputfromtheuser.Onceweentersomethingandpresskbd:[enter]key,the input()functionreturnswhatweentered,asastring.Wethenconvertthisstringtoanintegerusing intandthenstoreitinthevariable guess.Actually,the intisaclassbutallyouneedtoknowrightnowisthatyoucanuseittoconvertastringtoaninteger(assumingthestringcontainsavalidintegerinthetext).

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

Noticehowthe ifstatementcontainsacolonattheend-weareindicatingtoPythonthatablockofstatementsfollows.

Then,wecheckiftheguessislessthanthenumber,andifso,weinformtheuserthattheymustguessalittlehigherthanthat.Whatwehaveusedhereisthe elifclausewhichactuallycombinestworelated ifelse-ifelsestatementsintoonecombined if-elif-elsestatement.Thismakestheprogrameasierandreducestheamountofindentationrequired.

The elifand elsestatementsmustalsohaveacolonattheendofthelogicallinefollowedbytheircorrespondingblockofstatements(withproperindentation,ofcourse)

Youcanhaveanother ifstatementinsidetheif-blockofan ifstatementandsoon-thisiscalledanested ifstatement.

Rememberthatthe elifand elsepartsareoptional.Aminimalvalid ifstatementis:

ifTrue:

print('Yes,itistrue')

AfterPythonhasfinishedexecutingthecomplete ifstatementalongwiththeassociated elifand elseclauses,itmovesontothenextstatementintheblockcontainingthe ifstatement.Inthiscase,itisthemainblock(whereexecutionoftheprogramstarts),andthenextstatementisthe print('Done')statement.Afterthis,Pythonseestheendsoftheprogramandsimplyfinishesup.

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

NoteforC/C++Programmers

Thereisno switchstatementinPython.Youcanusean if..elif..elsestatementtodothesamething(andinsomecases,useadictionarytodoitquickly)

ThewhileStatementThe whilestatementallowsyoutorepeatedlyexecuteablockofstatementsaslongasaconditionistrue.A whilestatementisanexampleofwhatiscalledaloopingstatement.A whilestatementcanhaveanoptional elseclause.

Example(saveas while.py):

number=23

running=True

whilerunning:

guess=int(input('Enteraninteger:'))

ifguess==number:

print('Congratulations,youguessedit.')

Controlflow

40

#thiscausesthewhilelooptostop

running=False

elifguess<number:

print('No,itisalittlehigherthanthat.')

else:

print('No,itisalittlelowerthanthat.')

else:

print('Thewhileloopisover.')

#Doanythingelseyouwanttodohere

print('Done')

Output:

$pythonwhile.py

Enteraninteger:50

No,itisalittlelowerthanthat.

Enteraninteger:22

No,itisalittlehigherthanthat.

Enteraninteger:23

Congratulations,youguessedit.

Thewhileloopisover.

Done

HowItWorks

Inthisprogram,wearestillplayingtheguessinggame,buttheadvantageisthattheuserisallowedtokeepguessinguntilheguessescorrectly-thereisnoneedtorepeatedlyruntheprogramforeachguess,aswehavedoneintheprevioussection.Thisaptlydemonstratestheuseofthe whilestatement.

Wemovethe inputand ifstatementstoinsidethe whileloopandsetthevariable runningto Truebeforethewhileloop.First,wecheckifthevariable runningis Trueandthenproceedtoexecutethecorrespondingwhile-block.Afterthisblockisexecuted,theconditionisagaincheckedwhichinthiscaseisthe runningvariable.Ifitistrue,weexecutethewhile-blockagain,elsewecontinuetoexecutetheoptionalelse-blockandthencontinuetothenextstatement.

The elseblockisexecutedwhenthe whileloopconditionbecomes False-thismayevenbethefirsttimethattheconditionischecked.Ifthereisan elseclausefora whileloop,itisalwaysexecutedunlessyoubreakoutoftheloopwithabreakstatement.

The Trueand FalsearecalledBooleantypesandyoucanconsiderthemtobeequivalenttothevalue 1and 0respectively.

NoteforC/C++Programmers

Rememberthatyoucanhavean elseclauseforthe whileloop.

The forloopThe for..instatementisanotherloopingstatementwhichiteratesoverasequenceofobjectsi.e.gothrougheachiteminasequence.Wewillseemoreaboutsequencesindetailinlaterchapters.Whatyouneedtoknowrightnowisthatasequenceisjustanorderedcollectionofitems.

Example(saveas for.py):

foriinrange(1,5):

print(i)

else:

print('Theforloopisover')

Controlflow

41

Output:

$pythonfor.py

1

2

3

4

Theforloopisover

HowItWorks

Inthisprogram,weareprintingasequenceofnumbers.Wegeneratethissequenceofnumbersusingthebuilt-in rangefunction.

Whatwedohereissupplyittwonumbersand rangereturnsasequenceofnumbersstartingfromthefirstnumberanduptothesecondnumber.Forexample, range(1,5)givesthesequence [1,2,3,4].Bydefault, rangetakesastepcountof1.Ifwesupplyathirdnumberto range,thenthatbecomesthestepcount.Forexample, range(1,5,2)gives [1,3].Rememberthattherangeextendsuptothesecondnumberi.e.itdoesnotincludethesecondnumber.

Notethat range()generatesonlyonenumberatatime,ifyouwantthefulllistofnumbers,call list()onthe range(),forexample, list(range(5))willresultin [0,1,2,3,4].Listsareexplainedinthedatastructureschapter.

The forlooptheniteratesoverthisrange- foriinrange(1,5)isequivalentto foriin[1,2,3,4]whichislikeassigningeachnumber(orobject)inthesequencetoi,oneatatime,andthenexecutingtheblockofstatementsforeachvalueofi.Inthiscase,wejustprintthevalueintheblockofstatements.

Rememberthatthe elsepartisoptional.Whenincluded,itisalwaysexecutedonceafterthe forloopisoverunlessabreakstatementisencountered.

Rememberthatthe for..inloopworksforanysequence.Here,wehavealistofnumbersgeneratedbythebuilt-in rangefunction,butingeneralwecanuseanykindofsequenceofanykindofobjects!Wewillexplorethisideaindetailinlaterchapters.

NoteforC/C++/Java/C#Programmers

ThePython forloopisradicallydifferentfromtheC/C++ forloop.C#programmerswillnotethatthe forloopinPythonissimilartothe foreachloopinC#.Javaprogrammerswillnotethatthesameissimilarto for(inti:IntArray)inJava1.5.

InC/C++,ifyouwanttowrite for(inti=0;i<5;i++),theninPythonyouwritejust foriinrange(0,5).Asyoucansee,the forloopissimpler,moreexpressiveandlesserrorproneinPython.

ThebreakStatement

The breakstatementisusedtobreakoutofaloopstatementi.e.stoptheexecutionofaloopingstatement,eveniftheloopconditionhasnotbecome Falseorthesequenceofitemshasnotbeencompletelyiteratedover.

Animportantnoteisthatifyoubreakoutofa foror whileloop,anycorrespondingloop elseblockisnotexecuted.

Example(saveas break.py):

whileTrue:

s=input('Entersomething:')

ifs=='quit':

break

print('Lengthofthestringis',len(s))

print('Done')

Controlflow

42

Output:

$pythonbreak.py

Entersomething:Programmingisfun

Lengthofthestringis18

Entersomething:Whentheworkisdone

Lengthofthestringis21

Entersomething:ifyouwannamakeyourworkalsofun:

Lengthofthestringis37

Entersomething:usePython!

Lengthofthestringis11

Entersomething:quit

Done

HowItWorks

Inthisprogram,werepeatedlytaketheuser'sinputandprintthelengthofeachinputeachtime.Weareprovidingaspecialconditiontostoptheprogrambycheckingiftheuserinputis 'quit'.Westoptheprogrambybreakingoutoftheloopandreachtheendoftheprogram.

Thelengthoftheinputstringcanbefoundoutusingthebuilt-in lenfunction.

Rememberthatthe breakstatementcanbeusedwiththe forloopaswell.

Swaroop'sPoeticPython

TheinputIhaveusedhereisaminipoemIhavewritten:

Programmingisfun

Whentheworkisdone

ifyouwannamakeyourworkalsofun:

usePython!

The continueStatementThe continuestatementisusedtotellPythontoskiptherestofthestatementsinthecurrentloopblockandtocontinuetothenextiterationoftheloop.

Example(saveas continue.py):

whileTrue:

s=input('Entersomething:')

ifs=='quit':

break

iflen(s)<3:

print('Toosmall')

continue

print('Inputisofsufficientlength')

#Dootherkindsofprocessinghere...

Output:

$pythoncontinue.py

Entersomething:a

Toosmall

Entersomething:12

Toosmall

Entersomething:abc

Inputisofsufficientlength

Entersomething:quit

Controlflow

43

HowItWorks

Inthisprogram,weacceptinputfromtheuser,butweprocesstheinputstringonlyifitisatleast3characterslong.So,weusethebuilt-in lenfunctiontogetthelengthandifthelengthislessthan3,weskiptherestofthestatementsintheblockbyusingthecontinuestatement.Otherwise,therestofthestatementsintheloopareexecuted,doinganykindofprocessingwewanttodohere.

Notethatthe continuestatementworkswiththe forloopaswell.

Summary

Wehaveseenhowtousethethreecontrolflowstatements- if, whileand foralongwiththeirassociated breakandcontinuestatements.ThesearesomeofthemostcommonlyusedpartsofPythonandhence,becomingcomfortablewiththemisessential.

Next,wewillseehowtocreateandusefunctions.

Controlflow

44

FunctionsFunctionsarereusablepiecesofprograms.Theyallowyoutogiveanametoablockofstatements,allowingyoutorunthatblockusingthespecifiednameanywhereinyourprogramandanynumberoftimes.Thisisknownascallingthefunction.Wehavealreadyusedmanybuilt-infunctionssuchas lenand range.

Thefunctionconceptisprobablythemostimportantbuildingblockofanynon-trivialsoftware(inanyprogramminglanguage),sowewillexplorevariousaspectsoffunctionsinthischapter.

Functionsaredefinedusingthe defkeyword.Afterthiskeywordcomesanidentifiernameforthefunction,followedbyapairofparentheseswhichmayenclosesomenamesofvariables,andbythefinalcolonthatendstheline.Nextfollowstheblockofstatementsthatarepartofthisfunction.Anexamplewillshowthatthisisactuallyverysimple:

Example(saveas function1.py):

defsay_hello():

#blockbelongingtothefunction

print('helloworld')

#Endoffunction

say_hello()#callthefunction

say_hello()#callthefunctionagain

Output:

$pythonfunction1.py

helloworld

helloworld

HowItWorks

Wedefineafunctioncalled say_hellousingthesyntaxasexplainedabove.Thisfunctiontakesnoparametersandhencetherearenovariablesdeclaredintheparentheses.Parameterstofunctionsarejustinputtothefunctionsothatwecanpassindifferentvaluestoitandgetbackcorrespondingresults.

Noticethatwecancallthesamefunctiontwicewhichmeanswedonothavetowritethesamecodeagain.

FunctionParameters

Afunctioncantakeparameters,whicharevaluesyousupplytothefunctionsothatthefunctioncandosomethingutilisingthosevalues.Theseparametersarejustlikevariablesexceptthatthevaluesofthesevariablesaredefinedwhenwecallthefunctionandarealreadyassignedvalueswhenthefunctionruns.

Parametersarespecifiedwithinthepairofparenthesesinthefunctiondefinition,separatedbycommas.Whenwecallthefunction,wesupplythevaluesinthesameway.Notetheterminologyused-thenamesgiveninthefunctiondefinitionarecalledparameterswhereasthevaluesyousupplyinthefunctioncallarecalledarguments.

Example(saveas function_param.py):

defprint_max(a,b):

ifa>b:

print(a,'ismaximum')

elifa==b:

print(a,'isequalto',b)

else:

Functions

45

print(b,'ismaximum')

#directlypassliteralvalues

print_max(3,4)

x=5

y=7

#passvariablesasarguments

print_max(x,y)

Output:

$pythonfunction_param.py

4ismaximum

7ismaximum

HowItWorks

Here,wedefineafunctioncalled print_maxthatusestwoparameterscalled aand b.Wefindoutthegreaternumberusingasimple if..elsestatementandthenprintthebiggernumber.

Thefirsttimewecallthefunction print_max,wedirectlysupplythenumbersasarguments.Inthesecondcase,wecallthefunctionwithvariablesasarguments. print_max(x,y)causesthevalueofargument xtobeassignedtoparameter aandthevalueofargument ytobeassignedtoparameter b.The print_maxfunctionworksthesamewayinbothcases.

LocalVariablesWhenyoudeclarevariablesinsideafunctiondefinition,theyarenotrelatedinanywaytoothervariableswiththesamenamesusedoutsidethefunction-i.e.variablenamesarelocaltothefunction.Thisiscalledthescopeofthevariable.Allvariableshavethescopeoftheblocktheyaredeclaredinstartingfromthepointofdefinitionofthename.

Example(saveas function_local.py):

x=50

deffunc(x):

print('xis',x)

x=2

print('Changedlocalxto',x)

func(x)

print('xisstill',x)

Output:

$pythonfunction_local.py

xis50

Changedlocalxto2

xisstill50

HowItWorks

Thefirsttimethatweprintthevalueofthenamexwiththefirstlineinthefunction'sbody,Pythonusesthevalueoftheparameterdeclaredinthemainblock,abovethefunctiondefinition.

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46

Next,weassignthevalue 2to x.Thename xislocaltoourfunction.So,whenwechangethevalueof xinthefunction,the xdefinedinthemainblockremainsunaffected.

Withthelast printstatement,wedisplaythevalueof xasdefinedinthemainblock,therebyconfirmingthatitisactuallyunaffectedbythelocalassignmentwithinthepreviouslycalledfunction.

The globalstatement

Ifyouwanttoassignavaluetoanamedefinedatthetopleveloftheprogram(i.e.notinsideanykindofscopesuchasfunctionsorclasses),thenyouhavetotellPythonthatthenameisnotlocal,butitisglobal.Wedothisusingthe globalstatement.Itisimpossibletoassignavaluetoavariabledefinedoutsideafunctionwithoutthe globalstatement.

Youcanusethevaluesofsuchvariablesdefinedoutsidethefunction(assumingthereisnovariablewiththesamenamewithinthefunction).However,thisisnotencouragedandshouldbeavoidedsinceitbecomesuncleartothereaderoftheprogramastowherethatvariable'sdefinitionis.Usingthe globalstatementmakesitamplyclearthatthevariableisdefinedinanoutermostblock.

Example(saveas function_global.py):

x=50

deffunc():

globalx

print('xis',x)

x=2

print('Changedglobalxto',x)

func()

print('Valueofxis',x)

Output:

$pythonfunction_global.py

xis50

Changedglobalxto2

Valueofxis2

HowItWorks

The globalstatementisusedtodeclarethat xisaglobalvariable-hence,whenweassignavalueto xinsidethefunction,thatchangeisreflectedwhenweusethevalueof xinthemainblock.

Youcanspecifymorethanoneglobalvariableusingthesame globalstatemente.g. globalx,y,z.

DefaultArgumentValues

Forsomefunctions,youmaywanttomakesomeparametersoptionalandusedefaultvaluesincasetheuserdoesnotwanttoprovidevaluesforthem.Thisisdonewiththehelpofdefaultargumentvalues.Youcanspecifydefaultargumentvaluesforparametersbyappendingtotheparameternameinthefunctiondefinitiontheassignmentoperator( =)followedbythedefaultvalue.

Notethatthedefaultargumentvalueshouldbeaconstant.Moreprecisely,thedefaultargumentvalueshouldbeimmutable-thisisexplainedindetailinlaterchapters.Fornow,justrememberthis.

Functions

47

Example(saveas function_default.py):

defsay(message,times=1):

print(message*times)

say('Hello')

say('World',5)

Output:

$pythonfunction_default.py

Hello

WorldWorldWorldWorldWorld

HowItWorks

Thefunctionnamed sayisusedtoprintastringasmanytimesasspecified.Ifwedon'tsupplyavalue,thenbydefault,thestringisprintedjustonce.Weachievethisbyspecifyingadefaultargumentvalueof 1totheparameter times.

Inthefirstusageof say,wesupplyonlythestringanditprintsthestringonce.Inthesecondusageof say,wesupplyboththestringandanargument 5statingthatwewanttosaythestringmessage5times.

CAUTION

Onlythoseparameterswhichareattheendoftheparameterlistcanbegivendefaultargumentvaluesi.e.youcannothaveaparameterwithadefaultargumentvalueprecedingaparameterwithoutadefaultargumentvalueinthefunction'sparameterlist.

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

KeywordArguments

Ifyouhavesomefunctionswithmanyparametersandyouwanttospecifyonlysomeofthem,thenyoucangivevaluesforsuchparametersbynamingthem-thisiscalledkeywordarguments-weusethename(keyword)insteadoftheposition(whichwehavebeenusingallalong)tospecifytheargumentstothefunction.

Therearetwoadvantages-one,usingthefunctioniseasiersincewedonotneedtoworryabouttheorderofthearguments.Two,wecangivevaluestoonlythoseparameterstowhichwewantto,providedthattheotherparametershavedefaultargumentvalues.

Example(saveas function_keyword.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)

Output:

$pythonfunction_keyword.py

ais3andbis7andcis10

ais25andbis5andcis24

ais100andbis5andcis50

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48

HowItWorks

Thefunctionnamed funchasoneparameterwithoutadefaultargumentvalue,followedbytwoparameterswithdefaultargumentvalues.

Inthefirstusage, func(3,7),theparameter agetsthevalue 3,theparameter bgetsthevalue 7and cgetsthedefaultvalueof 10.

Inthesecondusage func(25,c=24),thevariable agetsthevalueof25duetothepositionoftheargument.Then,theparameter cgetsthevalueof 24duetonamingi.e.keywordarguments.Thevariable bgetsthedefaultvalueof 5.

Inthethirdusage func(c=50,a=100),weusekeywordargumentsforallspecifiedvalues.Noticethatwearespecifyingthevalueforparameter cbeforethatfor aeventhough aisdefinedbefore cinthefunctiondefinition.

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

deftotal(a=5,*numbers,**phonebook):

print('a',a)

#iteratethroughalltheitemsintuple

forsingle_iteminnumbers:

print('single_item',single_item)

#iteratethroughalltheitemsindictionary

forfirst_part,second_partinphonebook.items():

print(first_part,second_part)

total(10,1,2,3,Jack=1123,John=2231,Inge=1560)

Output:

$pythonfunction_varargs.py

a10

single_item1

single_item2

single_item3

Inge1560

John2231

Jack1123

HowItWorks

Whenwedeclareastarredparametersuchas *param,thenallthepositionalargumentsfromthatpointtilltheendarecollectedasatuplecalled'param'.

Similarly,whenwedeclareadouble-starredparametersuchas **param,thenallthekeywordargumentsfromthatpointtilltheendarecollectedasadictionarycalled'param'.

Wewillexploretuplesanddictionariesinalaterchapter.

The returnstatement

The returnstatementisusedtoreturnfromafunctioni.e.breakoutofthefunction.Wecanoptionallyreturnavaluefromthefunctionaswell.

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49

Example(saveas function_return.py):

defmaximum(x,y):

ifx>y:

returnx

elifx==y:

return'Thenumbersareequal'

else:

returny

print(maximum(2,3))

Output:

$pythonfunction_return.py

3

HowItWorks

The maximumfunctionreturnsthemaximumoftheparameters,inthiscasethenumberssuppliedtothefunction.Itusesasimpleif..elsestatementtofindthegreatervalueandthenreturnsthatvalue.

Notethata returnstatementwithoutavalueisequivalentto returnNone. NoneisaspecialtypeinPythonthatrepresentsnothingness.Forexample,itisusedtoindicatethatavariablehasnovalueifithasavalueof None.

Everyfunctionimplicitlycontainsa returnNonestatementattheendunlessyouhavewrittenyourown returnstatement.Youcanseethisbyrunning print(some_function())wherethefunction some_functiondoesnotusethe returnstatementsuchas:

defsome_function():

pass

The passstatementisusedinPythontoindicateanemptyblockofstatements.

TIP:Thereisabuilt-infunctioncalled maxthatalreadyimplementsthe'findmaximum'functionality,sousethisbuilt-infunctionwheneverpossible.

DocStringsPythonhasaniftyfeaturecalleddocumentationstrings,usuallyreferredtobyitsshorternamedocstrings.DocStringsareanimportanttoolthatyoushouldmakeuseofsinceithelpstodocumenttheprogrambetterandmakesiteasiertounderstand.Amazingly,wecanevengetthedocstringbackfrom,sayafunction,whentheprogramisactuallyrunning!

Example(saveas function_docstring.py):

defprint_max(x,y):

'''Printsthemaximumoftwonumbers.

Thetwovaluesmustbeintegers.'''

#converttointegers,ifpossible

x=int(x)

y=int(y)

ifx>y:

print(x,'ismaximum')

else:

print(y,'ismaximum')

Functions

50

print_max(3,5)

print(print_max.__doc__)

Output:

$pythonfunction_docstring.py

5ismaximum

Printsthemaximumoftwonumbers.

Thetwovaluesmustbeintegers.

HowItWorks

Astringonthefirstlogicallineofafunctionisthedocstringforthatfunction.NotethatDocStringsalsoapplytomodulesandclasseswhichwewilllearnaboutintherespectivechapters.

Theconventionfollowedforadocstringisamulti-linestringwherethefirstlinestartswithacapitalletterandendswithadot.Thenthesecondlineisblankfollowedbyanydetailedexplanationstartingfromthethirdline.Youarestronglyadvisedtofollowthisconventionforallyourdocstringsforallyournon-trivialfunctions.

Wecanaccessthedocstringofthe print_maxfunctionusingthe __doc__(noticethedoubleunderscores)attribute(namebelongingto)ofthefunction.JustrememberthatPythontreatseverythingasanobjectandthisincludesfunctions.We'lllearnmoreaboutobjectsinthechapteronclasses.

Ifyouhaveused help()inPython,thenyouhavealreadyseentheusageofdocstrings!Whatitdoesisjustfetchthe __doc__attributeofthatfunctionanddisplaysitinaneatmannerforyou.Youcantryitoutonthefunctionabove-justincludehelp(print_max)inyourprogram.Remembertopressthe qkeytoexit help.

Automatedtoolscanretrievethedocumentationfromyourprograminthismanner.Therefore,Istronglyrecommendthatyouusedocstringsforanynon-trivialfunctionthatyouwrite.The pydoccommandthatcomeswithyourPythondistributionworkssimilarlyto help()usingdocstrings.

Summary

Wehaveseensomanyaspectsoffunctionsbutnotethatwestillhaven'tcoveredallaspectsofthem.However,wehavealreadycoveredmostofwhatyou'lluseregardingPythonfunctionsonaneverydaybasis.

Next,wewillseehowtouseaswellascreatePythonmodules.

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51

ModulesYouhaveseenhowyoucanreusecodeinyourprogrambydefiningfunctionsonce.Whatifyouwantedtoreuseanumberoffunctionsinotherprogramsthatyouwrite?Asyoumighthaveguessed,theanswerismodules.

Therearevariousmethodsofwritingmodules,butthesimplestwayistocreateafilewitha .pyextensionthatcontainsfunctionsandvariables.

AnothermethodistowritethemodulesinthenativelanguageinwhichthePythoninterpreteritselfwaswritten.Forexample,youcanwritemodulesintheCprogramminglanguageandwhencompiled,theycanbeusedfromyourPythoncodewhenusingthestandardPythoninterpreter.

Amodulecanbeimportedbyanotherprogramtomakeuseofitsfunctionality.ThisishowwecanusethePythonstandardlibraryaswell.First,wewillseehowtousethestandardlibrarymodules.

Example(saveas module_using_sys.py):

importsys

print('Thecommandlineargumentsare:')

foriinsys.argv:

print(i)

print('\n\nThePYTHONPATHis',sys.path,'\n')

Output:

$pythonmodule_using_sys.pywearearguments#eachargisseparatedbywhitespace

Thecommandlineargumentsare:

module_using_sys.py

we

are

arguments

ThePYTHONPATHis['/tmp/py',

#manyentrieshere,notshownhere

'/Library/Python/2.7/site-packages',

'/usr/local/lib/python2.7/site-packages']

HowItWorks

First,weimportthe sysmoduleusingthe importstatement.Basically,thistranslatestoustellingPythonthatwewanttousethismodule.The sysmodulecontainsfunctionalityrelatedtothePythoninterpreteranditsenvironmenti.e.thesystem.

WhenPythonexecutesthe importsysstatement,itlooksforthe sysmodule.Inthiscase,itisoneofthebuilt-inmodules,andhencePythonknowswheretofindit.

Ifitwasnotacompiledmodulei.e.amodulewritteninPython,thenthePythoninterpreterwillsearchforitinthedirectorieslistedinits sys.pathvariable.Ifthemoduleisfound,thenthestatementsinthebodyofthatmodulearerunandthemoduleismadeavailableforyoutouse.Notethattheinitializationisdoneonlythefirsttimethatweimportamodule.

The argvvariableinthe sysmoduleisaccessedusingthedottednotationi.e. sys.argv.Itclearlyindicatesthatthisnameispartofthe sysmodule.Anotheradvantageofthisapproachisthatthenamedoesnotclashwithany argvvariableusedinyourprogram.

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52

The sys.argvvariableisalistofstrings(listsareexplainedindetailinalaterchapter).Specifically,the sys.argvcontainsthelistofcommandlineargumentsi.e.theargumentspassedtoyourprogramusingthecommandline.

IfyouareusinganIDEtowriteandruntheseprograms,lookforawaytospecifycommandlineargumentstotheprograminthemenus.

Here,whenweexecute pythonmodule_using_sys.pywearearguments,werunthemodule module_using_sys.pywiththepythoncommandandtheotherthingsthatfollowareargumentspassedtotheprogram.Pythonstoresthecommandlineargumentsinthe sys.argvvariableforustouse.

Remember,thenameofthescriptrunningisalwaysthefirstelementinthe sys.argvlist.So,inthiscasewewillhave'module_using_sys.py'as sys.argv[0], 'we'as sys.argv[1], 'are'as sys.argv[2]and 'arguments'assys.argv[3].NoticethatPythonstartscountingfrom0andnot1.

The sys.pathcontainsthelistofdirectorynameswheremodulesareimportedfrom.Observethatthefirststringin sys.pathisempty-thisemptystringindicatesthatthecurrentdirectoryisalsopartofthe sys.pathwhichissameasthe PYTHONPATHenvironmentvariable.Thismeansthatyoucandirectlyimportmoduleslocatedinthecurrentdirectory.Otherwise,youwillhavetoplaceyourmoduleinoneofthedirectorieslistedin sys.path.

Notethatthecurrentdirectoryisthedirectoryfromwhichtheprogramislaunched.Run importos;print(os.getcwd())tofindoutthecurrentdirectoryofyourprogram.

Byte-compiled.pycfiles

Importingamoduleisarelativelycostlyaffair,soPythondoessometrickstomakeitfaster.Onewayistocreatebyte-compiledfileswiththeextension .pycwhichisanintermediateformthatPythontransformstheprograminto(remembertheintroductionsectiononhowPythonworks?).This .pycfileisusefulwhenyouimportthemodulethenexttimefromadifferentprogram-itwillbemuchfastersinceaportionoftheprocessingrequiredinimportingamoduleisalreadydone.Also,thesebyte-compiledfilesareplatform-independent.

NOTE:These .pycfilesareusuallycreatedinthesamedirectoryasthecorresponding .pyfiles.IfPythondoesnothavepermissiontowritetofilesinthatdirectory,thenthe .pycfileswillnotbecreated.

Thefrom..importstatement

Ifyouwanttodirectlyimportthe argvvariableintoyourprogram(toavoidtypingthe sys.everytimeforit),thenyoucanusethe fromsysimportargvstatement.

WARNING:Ingeneral,avoidusingthe from..importstatement,usethe importstatementinstead.Thisisbecauseyourprogramwillavoidnameclashesandwillbemorereadable.

Example:

frommathimportsqrt

print("Squarerootof16is",sqrt(16))

Amodule's __name__

Everymodulehasanameandstatementsinamodulecanfindoutthenameoftheirmodule.Thisishandyfortheparticularpurposeoffiguringoutwhetherthemoduleisbeingrunstandaloneorbeingimported.Asmentionedpreviously,whenamoduleisimportedforthefirsttime,thecodeitcontainsgetsexecuted.Wecanusethistomakethemodulebehaveindifferentways

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53

dependingonwhetheritisbeingusedbyitselforbeingimportedfromanothermodule.Thiscanbeachievedusingthe__name__attributeofthemodule.

Example(saveas module_using_name.py):

if__name__=='__main__':

print('Thisprogramisbeingrunbyitself')

else:

print('Iambeingimportedfromanothermodule')

Output:

$pythonmodule_using_name.py

Thisprogramisbeingrunbyitself

$python

>>>importmodule_using_name

Iambeingimportedfromanothermodule

>>>

HowItWorks

EveryPythonmodulehasits __name__defined.Ifthisis '__main__',thatimpliesthatthemoduleisbeingrunstandalonebytheuserandwecantakeappropriateactions.

MakingYourOwnModules

Creatingyourownmodulesiseasy,you'vebeendoingitallalong!ThisisbecauseeveryPythonprogramisalsoamodule.Youjusthavetomakesureithasa .pyextension.Thefollowingexampleshouldmakeitclear.

Example(saveas mymodule.py):

defsay_hi():

print('Hi,thisismymodulespeaking.')

__version__='0.1'

Theabovewasasamplemodule.Asyoucansee,thereisnothingparticularlyspecialaboutitcomparedtoourusualPythonprogram.WewillnextseehowtousethismoduleinourotherPythonprograms.

Rememberthatthemoduleshouldbeplacedeitherinthesamedirectoryastheprogramfromwhichweimportit,orinoneofthedirectorieslistedin sys.path.

Anothermodule(saveas mymodule_demo.py):

importmymodule

mymodule.say_hi()

print('Version',mymodule.__version__)

Output:

$pythonmymodule_demo.py

Hi,thisismymodulespeaking.

Version0.1

HowItWorks

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54

Noticethatweusethesamedottednotationtoaccessmembersofthemodule.Pythonmakesgoodreuseofthesamenotationtogivethedistinctive'Pythonic'feeltoitsothatwedon'thavetokeeplearningnewwaystodothings.

Hereisaversionutilisingthe from..importsyntax(saveas mymodule_demo2.py):

frommymoduleimportsay_hi,__version__

say_hi()

print('Version',__version__)

Theoutputof mymodule_demo2.pyissameastheoutputof mymodule_demo.py.

Noticethatiftherewasalreadya __version__namedeclaredinthemodulethatimportsmymodule,therewouldbeaclash.Thisisalsolikelybecauseitiscommonpracticeforeachmoduletodeclareit'sversionnumberusingthisname.Hence,itisalwaysrecommendedtopreferthe importstatementeventhoughitmightmakeyourprogramalittlelonger.

Youcouldalsouse:

frommymoduleimport*

Thiswillimportallpublicnamessuchas say_hibutwouldnotimport __version__becauseitstartswithdoubleunderscores.

WARNING:Rememberthatyoushouldavoidusingimport-star,i.e. frommymoduleimport*.

ZenofPython

OneofPython'sguidingprinciplesisthat"ExplicitisbetterthanImplicit".Run importthisinPythontolearnmore.

The dirfunctionThebuilt-in dir()functionreturnsthelistofnamesdefinedbyanobject.Iftheobjectisamodule,thislistincludesfunctions,classesandvariables,definedinsidethatmodule.

Thisfunctioncanacceptarguments.Iftheargumentisthenameofamodule,thefunctionreturnsthelistofnamesfromthatspecifiedmodule.Ifthereisnoargument,thefunctionreturnsthelistofnamesfromthecurrentmodule.

Example:

$python

>>>importsys

#getnamesofattributesinsysmodule

>>>dir(sys)

['__displayhook__','__doc__',

'argv','builtin_module_names',

'version','version_info']

#onlyfewentriesshownhere

#getnamesofattributesforcurrentmodule

>>>dir()

['__builtins__','__doc__',

'__name__','__package__','sys']

#createanewvariable'a'

>>>a=5

>>>dir()

['__builtins__','__doc__','__name__','__package__','sys','a']

#delete/removeaname

>>>dela

Modules

55

>>>dir()

['__builtins__','__doc__','__name__','__package__','sys']

HowItWorks

First,weseetheusageof dirontheimported sysmodule.Wecanseethehugelistofattributesthatitcontains.

Next,weusethe dirfunctionwithoutpassingparameterstoit.Bydefault,itreturnsthelistofattributesforthecurrentmodule.Noticethatthelistofimportedmodulesisalsopartofthislist.

Inordertoobserve dirinaction,wedefineanewvariable aandassignitavalueandthencheck dirandweobservethatthereisanadditionalvalueinthelistofthesamename.Weremovethevariable/attributeofthecurrentmoduleusingthe delstatementandthechangeisreflectedagainintheoutputofthe dirfunction.

Anoteon del:Thisstatementisusedtodeleteavariable/nameandafterthestatementhasrun,inthiscase dela,youcannolongeraccessthevariable a-itisasifitneverexistedbeforeatall.

Notethatthe dir()functionworksonanyobject.Forexample,run dir(str)fortheattributesofthe str(string)class.

Thereisalsoa vars()functionwhichcanpotentiallygiveyoutheattributesandtheirvalues,butitwillnotworkforallcases.

PackagesBynow,youmusthavestartedobservingthehierarchyoforganizingyourprograms.Variablesusuallygoinsidefunctions.Functionsandglobalvariablesusuallygoinsidemodules.Whatifyouwantedtoorganizemodules?That'swherepackagescomeintothepicture.

Packagesarejustfoldersofmoduleswithaspecial __init__.pyfilethatindicatestoPythonthatthisfolderisspecialbecauseitcontainsPythonmodules.

Let'ssayyouwanttocreateapackagecalled'world'withsubpackages'asia','africa',etc.andthesesubpackagesinturncontainmoduleslike'india','madagascar',etc.

Thisishowyouwouldstructurethefolders:

-<somefolderpresentinthesys.path>/

-world/

-__init__.py

-asia/

-__init__.py

-india/

-__init__.py

-foo.py

-africa/

-__init__.py

-madagascar/

-__init__.py

-bar.py

Packagesarejustaconveniencetoorganizemoduleshierarchically.Youwillseemanyinstancesofthisinthestandardlibrary.

SummaryJustlikefunctionsarereusablepartsofprograms,modulesarereusableprograms.Packagesareanotherhierarchytoorganizemodules.ThestandardlibrarythatcomeswithPythonisanexampleofsuchasetofpackagesandmodules.

Wehaveseenhowtousethesemodulesandcreateourownmodules.

Modules

56

Next,wewilllearnaboutsomeinterestingconceptscalleddatastructures.

Modules

57

DataStructuresDatastructuresarebasicallyjustthat-theyarestructureswhichcanholdsomedatatogether.Inotherwords,theyareusedtostoreacollectionofrelateddata.

Therearefourbuilt-indatastructuresinPython-list,tuple,dictionaryandset.Wewillseehowtouseeachofthemandhowtheymakelifeeasierforus.

List

A listisadatastructurethatholdsanorderedcollectionofitemsi.e.youcanstoreasequenceofitemsinalist.Thisiseasytoimagineifyoucanthinkofashoppinglistwhereyouhavealistofitemstobuy,exceptthatyouprobablyhaveeachitemonaseparatelineinyourshoppinglistwhereasinPythonyouputcommasinbetweenthem.

ThelistofitemsshouldbeenclosedinsquarebracketssothatPythonunderstandsthatyouarespecifyingalist.Onceyouhavecreatedalist,youcanadd,removeorsearchforitemsinthelist.Sincewecanaddandremoveitems,wesaythatalistisamutabledatatypei.e.thistypecanbealtered.

QuickIntroductionToObjectsAndClassesAlthoughI'vebeengenerallydelayingthediscussionofobjectsandclassestillnow,alittleexplanationisneededrightnowsothatyoucanunderstandlistsbetter.Wewillexplorethistopicindetailinalaterchapter.

Alistisanexampleofusageofobjectsandclasses.Whenweuseavariable iandassignavaluetoit,sayinteger 5toit,youcanthinkofitascreatinganobject(i.e.instance) iofclass(i.e.type) int.Infact,youcanread help(int)tounderstandthisbetter.

Aclasscanalsohavemethodsi.e.functionsdefinedforusewithrespecttothatclassonly.Youcanusethesepiecesoffunctionalityonlywhenyouhaveanobjectofthatclass.Forexample,Pythonprovidesan appendmethodforthe listclasswhichallowsyoutoaddanitemtotheendofthelist.Forexample, mylist.append('anitem')willaddthatstringtothelistmylist.Notetheuseofdottednotationforaccessingmethodsoftheobjects.

Aclasscanalsohavefieldswhicharenothingbutvariablesdefinedforusewithrespecttothatclassonly.Youcanusethesevariables/namesonlywhenyouhaveanobjectofthatclass.Fieldsarealsoaccessedbythedottednotation,forexample,mylist.field.

Example(saveas ds_using_list.py):

#Thisismyshoppinglist

shoplist=['apple','mango','carrot','banana']

print('Ihave',len(shoplist),'itemstopurchase.')

print('Theseitemsare:',end='')

foriteminshoplist:

print(item,end='')

print('\nIalsohavetobuyrice.')

shoplist.append('rice')

print('Myshoppinglistisnow',shoplist)

print('Iwillsortmylistnow')

shoplist.sort()

print('Sortedshoppinglistis',shoplist)

DataStructures

58

print('ThefirstitemIwillbuyis',shoplist[0])

olditem=shoplist[0]

delshoplist[0]

print('Iboughtthe',olditem)

print('Myshoppinglistisnow',shoplist)

Output:

$pythonds_using_list.py

Ihave4itemstopurchase.

Theseitemsare:applemangocarrotbanana

Ialsohavetobuyrice.

Myshoppinglistisnow['apple','mango','carrot','banana','rice']

Iwillsortmylistnow

Sortedshoppinglistis['apple','banana','carrot','mango','rice']

ThefirstitemIwillbuyisapple

Iboughttheapple

Myshoppinglistisnow['banana','carrot','mango','rice']

HowItWorks

Thevariable shoplistisashoppinglistforsomeonewhoisgoingtothemarket.In shoplist,weonlystorestringsofthenamesoftheitemstobuybutyoucanaddanykindofobjecttoalistincludingnumbersandevenotherlists.

Wehavealsousedthe for..inlooptoiteratethroughtheitemsofthelist.Bynow,youmusthaverealisedthatalistisalsoasequence.Thespecialityofsequenceswillbediscussedinalatersection.

Noticetheuseofthe endparameterinthecallto printfunctiontoindicatethatwewanttoendtheoutputwithaspaceinsteadoftheusuallinebreak.

Next,weaddanitemtothelistusingthe appendmethodofthelistobject,asalreadydiscussedbefore.Then,wecheckthattheitemhasbeenindeedaddedtothelistbyprintingthecontentsofthelistbysimplypassingthelisttothe printfunctionwhichprintsitneatly.

Then,wesortthelistbyusingthe sortmethodofthelist.Itisimportanttounderstandthatthismethodaffectsthelistitselfanddoesnotreturnamodifiedlist-thisisdifferentfromthewaystringswork.Thisiswhatwemeanbysayingthatlistsaremutableandthatstringsareimmutable.

Next,whenwefinishbuyinganiteminthemarket,wewanttoremoveitfromthelist.Weachievethisbyusingthe delstatement.Here,wementionwhichitemofthelistwewanttoremoveandthe delstatementremovesitfromthelistforus.Wespecifythatwewanttoremovethefirstitemfromthelistandhenceweuse delshoplist[0](rememberthatPythonstartscountingfrom0).

Ifyouwanttoknowallthemethodsdefinedbythelistobject,see help(list)fordetails.

Tuple

Tuplesareusedtoholdtogethermultipleobjects.Thinkofthemassimilartolists,butwithouttheextensivefunctionalitythatthelistclassgivesyou.Onemajorfeatureoftuplesisthattheyareimmutablelikestringsi.e.youcannotmodifytuples.

Tuplesaredefinedbyspecifyingitemsseparatedbycommaswithinanoptionalpairofparentheses.

Tuplesareusuallyusedincaseswhereastatementorauser-definedfunctioncansafelyassumethatthecollectionofvalues(i.e.thetupleofvaluesused)willnotchange.

Example(saveas ds_using_tuple.py):

#Iwouldrecommendalwaysusingparentheses

#toindicatestartandendoftuple

DataStructures

59

#eventhoughparenthesesareoptional.

#Explicitisbetterthanimplicit.

zoo=('python','elephant','penguin')

print('Numberofanimalsinthezoois',len(zoo))

new_zoo='monkey','camel',zoo#parenthesesnotrequiredbutareagoodidea

print('Numberofcagesinthenewzoois',len(new_zoo))

print('Allanimalsinnewzooare',new_zoo)

print('Animalsbroughtfromoldzooare',new_zoo[2])

print('Lastanimalbroughtfromoldzoois',new_zoo[2][2])

print('Numberofanimalsinthenewzoois',

len(new_zoo)-1+len(new_zoo[2]))

Output:

$pythonds_using_tuple.py

Numberofanimalsinthezoois3

Numberofcagesinthenewzoois3

Allanimalsinnewzooare('monkey','camel',('python','elephant','penguin'))

Animalsbroughtfromoldzooare('python','elephant','penguin')

Lastanimalbroughtfromoldzooispenguin

Numberofanimalsinthenewzoois5

HowItWorks

Thevariable zooreferstoatupleofitems.Weseethatthe lenfunctioncanbeusedtogetthelengthofthetuple.Thisalsoindicatesthatatupleisasequenceaswell.

Wearenowshiftingtheseanimalstoanewzoosincetheoldzooisbeingclosed.Therefore,the new_zootuplecontainssomeanimalswhicharealreadytherealongwiththeanimalsbroughtoverfromtheoldzoo.Backtoreality,notethatatuplewithinatupledoesnotloseitsidentity.

Wecanaccesstheitemsinthetuplebyspecifyingtheitem'spositionwithinapairofsquarebracketsjustlikewedidforlists.Thisiscalledtheindexingoperator.Weaccessthethirditemin new_zoobyspecifying new_zoo[2]andweaccessthethirditemwithinthethirditeminthe new_zootuplebyspecifying new_zoo[2][2].Thisisprettysimpleonceyou'veunderstoodtheidiom.

Tuplewith0or1items

Anemptytupleisconstructedbyanemptypairofparenthesessuchas myempty=().However,atuplewithasingleitemisnotsosimple.Youhavetospecifyitusingacommafollowingthefirst(andonly)itemsothatPythoncandifferentiatebetweenatupleandapairofparenthesessurroundingtheobjectinanexpressioni.e.youhavetospecify singleton=(2,)ifyoumeanyouwantatuplecontainingtheitem 2.

NoteforPerlprogrammers

Alistwithinalistdoesnotloseitsidentityi.e.listsarenotflattenedasinPerl.Thesameappliestoatuplewithinatuple,oratuplewithinalist,oralistwithinatuple,etc.AsfarasPythonisconcerned,theyarejustobjectsstoredusinganotherobject,that'sall.

Dictionary

Adictionaryislikeanaddress-bookwhereyoucanfindtheaddressorcontactdetailsofapersonbyknowingonlyhis/hernamei.e.weassociatekeys(name)withvalues(details).Notethatthekeymustbeuniquejustlikeyoucannotfindoutthecorrectinformationifyouhavetwopersonswiththeexactsamename.

Notethatyoucanuseonlyimmutableobjects(likestrings)forthekeysofadictionarybutyoucanuseeitherimmutableormutableobjectsforthevaluesofthedictionary.Thisbasicallytranslatestosaythatyoushoulduseonlysimpleobjectsforkeys.

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60

Pairsofkeysandvaluesarespecifiedinadictionarybyusingthenotation d={key1:value1,key2:value2}.Noticethatthekey-valuepairsareseparatedbyacolonandthepairsareseparatedthemselvesbycommasandallthisisenclosedinapairofcurlybraces.

Rememberthatkey-valuepairsinadictionaryarenotorderedinanymanner.Ifyouwantaparticularorder,thenyouwillhavetosortthemyourselfbeforeusingit.

Thedictionariesthatyouwillbeusingareinstances/objectsofthe dictclass.

Example(saveas ds_using_dict.py):

#'ab'isshortfor'a'ddress'b'ook

ab={

'Swaroop':'swaroop@swaroopch.com',

'Larry':'larry@wall.org',

'Matsumoto':'matz@ruby-lang.org',

'Spammer':'spammer@hotmail.com'

}

print("Swaroop'saddressis",ab['Swaroop'])

#Deletingakey-valuepair

delab['Spammer']

print('\nThereare{}contactsintheaddress-book\n'.format(len(ab)))

forname,addressinab.items():

print('Contact{}at{}'.format(name,address))

#Addingakey-valuepair

ab['Guido']='guido@python.org'

if'Guido'inab:

print("\nGuido'saddressis",ab['Guido'])

Output:

$pythonds_using_dict.py

Swaroop'saddressisswaroop@swaroopch.com

Thereare3contactsintheaddress-book

ContactSwaroopatswaroop@swaroopch.com

ContactMatsumotoatmatz@ruby-lang.org

ContactLarryatlarry@wall.org

Guido'saddressisguido@python.org

HowItWorks

Wecreatethedictionary abusingthenotationalreadydiscussed.Wethenaccesskey-valuepairsbyspecifyingthekeyusingtheindexingoperatorasdiscussedinthecontextoflistsandtuples.Observethesimplesyntax.

Wecandeletekey-valuepairsusingouroldfriend-the delstatement.Wesimplyspecifythedictionaryandtheindexingoperatorforthekeytoberemovedandpassittothe delstatement.Thereisnoneedtoknowthevaluecorrespondingtothekeyforthisoperation.

Next,weaccesseachkey-valuepairofthedictionaryusingthe itemsmethodofthedictionarywhichreturnsalistoftupleswhereeachtuplecontainsapairofitems-thekeyfollowedbythevalue.Weretrievethispairandassignittothevariables nameand addresscorrespondinglyforeachpairusingthe for..inloopandthenprintthesevaluesinthefor-block.

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61

Wecanaddnewkey-valuepairsbysimplyusingtheindexingoperatortoaccessakeyandassignthatvalue,aswehavedoneforGuidointheabovecase.

Wecancheckifakey-valuepairexistsusingthe inoperator.

Forthelistofmethodsofthe dictclass,see help(dict).

KeywordArgumentsandDictionaries

Ifyouhaveusedkeywordargumentsinyourfunctions,youhavealreadyuseddictionaries!Justthinkaboutit-thekey-valuepairisspecifiedbyyouintheparameterlistofthefunctiondefinitionandwhenyouaccessvariableswithinyourfunction,itisjustakeyaccessofadictionary(whichiscalledthesymboltableincompilerdesignterminology).

SequenceLists,tuplesandstringsareexamplesofsequences,butwhataresequencesandwhatissospecialaboutthem?

Themajorfeaturesaremembershiptests,(i.e.the inand notinexpressions)andindexingoperations,whichallowustofetchaparticulariteminthesequencedirectly.

Thethreetypesofsequencesmentionedabove-lists,tuplesandstrings,alsohaveaslicingoperationwhichallowsustoretrieveasliceofthesequencei.e.apartofthesequence.

Example(saveas ds_seq.py):

shoplist=['apple','mango','carrot','banana']

name='swaroop'

#Indexingor'Subscription'operation#

print('Item0is',shoplist[0])

print('Item1is',shoplist[1])

print('Item2is',shoplist[2])

print('Item3is',shoplist[3])

print('Item-1is',shoplist[-1])

print('Item-2is',shoplist[-2])

print('Character0is',name[0])

#Slicingonalist#

print('Item1to3is',shoplist[1:3])

print('Item2toendis',shoplist[2:])

print('Item1to-1is',shoplist[1:-1])

print('Itemstarttoendis',shoplist[:])

#Slicingonastring#

print('characters1to3is',name[1:3])

print('characters2toendis',name[2:])

print('characters1to-1is',name[1:-1])

print('charactersstarttoendis',name[:])

Output:

$pythonds_seq.py

Item0isapple

Item1ismango

Item2iscarrot

Item3isbanana

Item-1isbanana

Item-2iscarrot

Character0iss

Item1to3is['mango','carrot']

Item2toendis['carrot','banana']

Item1to-1is['mango','carrot']

DataStructures

62

Itemstarttoendis['apple','mango','carrot','banana']

characters1to3iswa

characters2toendisaroop

characters1to-1iswaroo

charactersstarttoendisswaroop

HowItWorks

First,weseehowtouseindexestogetindividualitemsofasequence.Thisisalsoreferredtoasthesubscriptionoperation.Wheneveryouspecifyanumbertoasequencewithinsquarebracketsasshownabove,Pythonwillfetchyoutheitemcorrespondingtothatpositioninthesequence.RememberthatPythonstartscountingnumbersfrom0.Hence, shoplist[0]fetchesthefirstitemand shoplist[3]fetchesthefourthiteminthe shoplistsequence.

Theindexcanalsobeanegativenumber,inwhichcase,thepositioniscalculatedfromtheendofthesequence.Therefore,shoplist[-1]referstothelastiteminthesequenceand shoplist[-2]fetchesthesecondlastiteminthesequence.

Theslicingoperationisusedbyspecifyingthenameofthesequencefollowedbyanoptionalpairofnumbersseparatedbyacolonwithinsquarebrackets.Notethatthisisverysimilartotheindexingoperationyouhavebeenusingtillnow.Rememberthenumbersareoptionalbutthecolonisn't.

Thefirstnumber(beforethecolon)intheslicingoperationreferstothepositionfromwheretheslicestartsandthesecondnumber(afterthecolon)indicateswheretheslicewillstopat.Ifthefirstnumberisnotspecified,Pythonwillstartatthebeginningofthesequence.Ifthesecondnumberisleftout,Pythonwillstopattheendofthesequence.Notethattheslicereturnedstartsatthestartpositionandwillendjustbeforetheendpositioni.e.thestartpositionisincludedbuttheendpositionisexcludedfromthesequenceslice.

Thus, shoplist[1:3]returnsasliceofthesequencestartingatposition1,includesposition2butstopsatposition3andthereforeasliceoftwoitemsisreturned.Similarly, shoplist[:]returnsacopyofthewholesequence.

Youcanalsodoslicingwithnegativepositions.Negativenumbersareusedforpositionsfromtheendofthesequence.Forexample, shoplist[:-1]willreturnasliceofthesequencewhichexcludesthelastitemofthesequencebutcontainseverythingelse.

Youcanalsoprovideathirdargumentfortheslice,whichisthestepfortheslicing(bydefault,thestepsizeis1):

>>>shoplist=['apple','mango','carrot','banana']

>>>shoplist[::1]

['apple','mango','carrot','banana']

>>>shoplist[::2]

['apple','carrot']

>>>shoplist[::3]

['apple','banana']

>>>shoplist[::-1]

['banana','carrot','mango','apple']

Noticethatwhenthestepis2,wegettheitemswithposition0,2,...Whenthestepsizeis3,wegettheitemswithposition0,3,etc.

TryvariouscombinationsofsuchslicespecificationsusingthePythoninterpreterinteractivelyi.e.thepromptsothatyoucanseetheresultsimmediately.Thegreatthingaboutsequencesisthatyoucanaccesstuples,listsandstringsallinthesameway!

Set

Setsareunorderedcollectionsofsimpleobjects.Theseareusedwhentheexistenceofanobjectinacollectionismoreimportantthantheorderorhowmanytimesitoccurs.

Usingsets,youcantestformembership,whetheritisasubsetofanotherset,findtheintersectionbetweentwosets,andsoon.

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63

>>>bri=set(['brazil','russia','india'])

>>>'india'inbri

True

>>>'usa'inbri

False

>>>bric=bri.copy()

>>>bric.add('china')

>>>bric.issuperset(bri)

True

>>>bri.remove('russia')

>>>bri&bric#ORbri.intersection(bric)

{'brazil','india'}

HowItWorks

Ifyourememberbasicsettheorymathematicsfromschool,thenthisexampleisfairlyself-explanatory.Butifnot,youcangoogle"settheory"and"Venndiagram"tobetterunderstandouruseofsetsinPython.

ReferencesWhenyoucreateanobjectandassignittoavariable,thevariableonlyreferstotheobjectanddoesnotrepresenttheobjectitself!Thatis,thevariablenamepointstothatpartofyourcomputer'smemorywheretheobjectisstored.Thisiscalledbindingthenametotheobject.

Generally,youdon'tneedtobeworriedaboutthis,butthereisasubtleeffectduetoreferenceswhichyouneedtobeawareof:

Example(saveas ds_reference.py):

print('SimpleAssignment')

shoplist=['apple','mango','carrot','banana']

#mylistisjustanothernamepointingtothesameobject!

mylist=shoplist

#Ipurchasedthefirstitem,soIremoveitfromthelist

delshoplist[0]

print('shoplistis',shoplist)

print('mylistis',mylist)

#Noticethatbothshoplistandmylistbothprint

#thesamelistwithoutthe'apple'confirmingthat

#theypointtothesameobject

print('Copybymakingafullslice')

#Makeacopybydoingafullslice

mylist=shoplist[:]

#Removefirstitem

delmylist[0]

print('shoplistis',shoplist)

print('mylistis',mylist)

#Noticethatnowthetwolistsaredifferent

Output:

$pythonds_reference.py

SimpleAssignment

shoplistis['mango','carrot','banana']

mylistis['mango','carrot','banana']

Copybymakingafullslice

shoplistis['mango','carrot','banana']

mylistis['carrot','banana']

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64

HowItWorks

Mostoftheexplanationisavailableinthecomments.

Rememberthatifyouwanttomakeacopyofalistorsuchkindsofsequencesorcomplexobjects(notsimpleobjectssuchasintegers),thenyouhavetousetheslicingoperationtomakeacopy.Ifyoujustassignthevariablenametoanothername,bothofthemwill''refer''tothesameobjectandthiscouldbetroubleifyouarenotcareful.

NoteforPerlprogrammers

Rememberthatanassignmentstatementforlistsdoesnotcreateacopy.Youhavetouseslicingoperationtomakeacopyofthesequence.

MoreAboutStringsWehavealreadydiscussedstringsindetailearlier.Whatmorecantherebetoknow?Well,didyouknowthatstringsarealsoobjectsandhavemethodswhichdoeverythingfromcheckingpartofastringtostrippingspaces?Infact,you'vealreadybeenusingastringmethod...the formatmethod!

Thestringsthatyouuseinprogramsareallobjectsoftheclass str.Someusefulmethodsofthisclassaredemonstratedinthenextexample.Foracompletelistofsuchmethods,see help(str).

Example(saveas ds_str_methods.py):

#Thisisastringobject

name='Swaroop'

ifname.startswith('Swa'):

print('Yes,thestringstartswith"Swa"')

if'a'inname:

print('Yes,itcontainsthestring"a"')

ifname.find('war')!=-1:

print('Yes,itcontainsthestring"war"')

delimiter='_*_'

mylist=['Brazil','Russia','India','China']

print(delimiter.join(mylist))

Output:

$pythonds_str_methods.py

Yes,thestringstartswith"Swa"

Yes,itcontainsthestring"a"

Yes,itcontainsthestring"war"

Brazil_*_Russia_*_India_*_China

HowItWorks

Here,weseealotofthestringmethodsinaction.The startswithmethodisusedtofindoutwhetherthestringstartswiththegivenstring.The inoperatorisusedtocheckifagivenstringisapartofthestring.

The findmethodisusedtolocatethepositionofthegivensubstringwithinthestring; findreturns-1ifitisunsuccessfulinfindingthesubstring.The strclassalsohasaneatmethodto jointheitemsofasequencewiththestringactingasadelimiterbetweeneachitemofthesequenceandreturnsabiggerstringgeneratedfromthis.

Summary

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65

Wehaveexploredthevariousbuilt-indatastructuresofPythonindetail.Thesedatastructureswillbeessentialforwritingprogramsofreasonablesize.

NowthatwehavealotofthebasicsofPythoninplace,wewillnextseehowtodesignandwriteareal-worldPythonprogram.

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66

ProblemSolvingWehaveexploredvariouspartsofthePythonlanguageandnowwewilltakealookathowallthesepartsfittogether,bydesigningandwritingaprogramwhichdoessomethinguseful.TheideaistolearnhowtowriteaPythonscriptonyourown.

TheProblem

Theproblemwewanttosolveis:

Iwantaprogramwhichcreatesabackupofallmyimportantfiles.

Although,thisisasimpleproblem,thereisnotenoughinformationforustogetstartedwiththesolution.Alittlemoreanalysisisrequired.Forexample,howdowespecifywhichfilesaretobebackedup?Howaretheystored?Wherearetheystored?

Afteranalyzingtheproblemproperly,wedesignourprogram.Wemakealistofthingsabouthowourprogramshouldwork.Inthiscase,IhavecreatedthefollowinglistonhowIwantittowork.Ifyoudothedesign,youmaynotcomeupwiththesamekindofanalysissinceeverypersonhastheirownwayofdoingthings,sothatisperfectlyokay.

Thefilesanddirectoriestobebackeduparespecifiedinalist.Thebackupmustbestoredinamainbackupdirectory.Thefilesarebackedupintoazipfile.Thenameoftheziparchiveisthecurrentdateandtime.Weusethestandard zipcommandavailablebydefaultinanystandardGNU/LinuxorUnixdistribution.Notethatyoucanuseanyarchivingcommandyouwantaslongasithasacommandlineinterface.

ForWindowsusers

Windowsuserscaninstallthe zipcommandfromtheGnuWin32projectpageandadd C:\ProgramFiles\GnuWin32\bintoyoursystem PATHenvironmentvariable,similartowhatwedidforrecognizingthepythoncommanditself.

TheSolution

Asthedesignofourprogramisnowreasonablystable,wecanwritethecodewhichisanimplementationofoursolution.

Saveas backup_ver1.py:

importos

importtime

#1.Thefilesanddirectoriestobebackedupare

#specifiedinalist.

#ExampleonWindows:

#source=['"C:\\MyDocuments"']

#ExampleonMacOSXandLinux:

source=['/Users/swa/notes']

#Noticewehavetousedoublequotesinsideastring

#fornameswithspacesinit.Wecouldhavealsoused

#arawstringbywriting[r'C:\MyDocuments'].

#2.Thebackupmustbestoredina

#mainbackupdirectory

#ExampleonWindows:

#target_dir='E:\\Backup'

#ExampleonMacOSXandLinux:

target_dir='/Users/swa/backup'

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67

#Remembertochangethistowhichfolderyouwillbeusing

#3.Thefilesarebackedupintoazipfile.

#4.Thenameoftheziparchiveisthecurrentdateandtime

target=target_dir+os.sep+\

time.strftime('%Y%m%d%H%M%S')+'.zip'

#Createtargetdirectoryifitisnotpresent

ifnotos.path.exists(target_dir):

os.mkdir(target_dir)#makedirectory

#5.Weusethezipcommandtoputthefilesinaziparchive

zip_command='zip-r{0}{1}'.format(target,

''.join(source))

#Runthebackup

print('Zipcommandis:')

print(zip_command)

print('Running:')

ifos.system(zip_command)==0:

print('Successfulbackupto',target)

else:

print('BackupFAILED')

Output:

$pythonbackup_ver1.py

Zipcommandis:

zip-r/Users/swa/backup/20140328084844.zip/Users/swa/notes

Running:

adding:Users/swa/notes/(stored0%)

adding:Users/swa/notes/blah1.txt(stored0%)

adding:Users/swa/notes/blah2.txt(stored0%)

adding:Users/swa/notes/blah3.txt(stored0%)

Successfulbackupto/Users/swa/backup/20140328084844.zip

Now,weareinthetestingphasewherewetestthatourprogramworksproperly.Ifitdoesn'tbehaveasexpected,thenwehavetodebugourprogrami.e.removethebugs(errors)fromtheprogram.

Iftheaboveprogramdoesnotworkforyou,copythelineprintedafterthe Zipcommandislineintheoutput,pasteitintheshell(onGNU/LinuxandMacOSX)/ cmd(onWindows),seewhattheerrorisandtrytofixit.Alsocheckthezipcommandmanualonwhatcouldbewrong.Ifthiscommandsucceeds,thentheproblemmightbeinthePythonprogramitself,socheckifitexactlymatchestheprogramwrittenabove.

HowItWorks

Youwillnoticehowwehaveconvertedourdesignintocodeinastep-by-stepmanner.

Wemakeuseofthe osand timemodulesbyfirstimportingthem.Then,wespecifythefilesanddirectoriestobebackedupinthe sourcelist.Thetargetdirectoryiswherewestoreallthebackupfilesandthisisspecifiedinthe target_dirvariable.Thenameoftheziparchivethatwearegoingtocreateisthecurrentdateandtimewhichwegenerateusingthetime.strftime()function.Itwillalsohavethe .zipextensionandwillbestoredinthe target_dirdirectory.

Noticetheuseofthe os.sepvariable-thisgivesthedirectoryseparatoraccordingtoyouroperatingsystem,i.e.itwillbe '/'inGNU/Linux,Unix,macOS,andwillbe '\\'inWindows.Using os.sepinsteadofthesecharactersdirectlywillmakeourprogramportableandworkacrossallofthesesystems.

The time.strftime()functiontakesaspecificationsuchastheonewehaveusedintheaboveprogram.The %Yspecificationwillbereplacedbytheyearwiththecentury.The %mspecificationwillbereplacedbythemonthasadecimalnumberbetween01and 12andsoon.ThecompletelistofsuchspecificationscanbefoundinthePythonReferenceManual.

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Wecreatethenameofthetargetzipfileusingtheadditionoperatorwhichconcatenatesthestringsi.e.itjoinsthetwostringstogetherandreturnsanewone.Then,wecreateastring zip_commandwhichcontainsthecommandthatwearegoingtoexecute.Youcancheckifthiscommandworksbyrunningitintheshell(GNU/LinuxterminalorDOSprompt).

The zipcommandthatweareusinghassomeoptionsavailable,andoneoftheseoptionsis -r.The -roptionspecifiesthatthezipcommandshouldworkrecursivelyfordirectories,i.e.itshouldincludeallthesubdirectoriesandfiles.Optionsarefollowedbythenameoftheziparchivetocreate,followedbythelistoffilesanddirectoriestobackup.Weconvertthe sourcelistintoastringusingthe joinmethodofstringswhichwehavealreadyseenhowtouse.

Then,wefinallyrunthecommandusingthe os.systemfunctionwhichrunsthecommandasifitwasrunfromthesystemi.e.intheshell-itreturns 0ifthecommandwassuccessfully,elseitreturnsanerrornumber.

Dependingontheoutcomeofthecommand,weprinttheappropriatemessagethatthebackuphasfailedorsucceeded.

That'sit,wehavecreatedascripttotakeabackupofourimportantfiles!

NotetoWindowsUsers

Insteadofdoublebackslashescapesequences,youcanalsouserawstrings.Forexample,use 'C:\\Documents'orr'C:\Documents'.However,donotuse 'C:\Documents'sinceyouendupusinganunknownescapesequence \D.

Nowthatwehaveaworkingbackupscript,wecanuseitwheneverwewanttotakeabackupofthefiles.Thisiscalledtheoperationphaseorthedeploymentphaseofthesoftware.

Theaboveprogramworksproperly,but(usually)firstprogramsdonotworkexactlyasyouexpect.Forexample,theremightbeproblemsifyouhavenotdesignedtheprogramproperlyorifyouhavemadeamistakewhentypingthecode,etc.Appropriately,youwillhavetogobacktothedesignphaseoryouwillhavetodebugyourprogram.

SecondVersion

Thefirstversionofourscriptworks.However,wecanmakesomerefinementstoitsothatitcanworkbetteronadailybasis.Thisiscalledthemaintenancephaseofthesoftware.

OneoftherefinementsIfeltwasusefulisabetterfile-namingmechanism-usingthetimeasthenameofthefilewithinadirectorywiththecurrentdateasadirectorywithinthemainbackupdirectory.Thefirstadvantageisthatyourbackupsarestoredinahierarchicalmannerandthereforeitismucheasiertomanage.Thesecondadvantageisthatthefilenamesaremuchshorter.Thethirdadvantageisthatseparatedirectorieswillhelpyoucheckifyouhavemadeabackupforeachdaysincethedirectorywouldbecreatedonlyifyouhavemadeabackupforthatday.

Saveas backup_ver2.py:

importos

importtime

#1.Thefilesanddirectoriestobebackedupare

#specifiedinalist.

#ExampleonWindows:

#source=['"C:\\MyDocuments"','C:\\Code']

#ExampleonMacOSXandLinux:

source=['/Users/swa/notes']

#Noticewehadtousedoublequotesinsidethestring

#fornameswithspacesinit.

#2.Thebackupmustbestoredina

#mainbackupdirectory

#ExampleonWindows:

#target_dir='E:\\Backup'

#ExampleonMacOSXandLinux:

target_dir='/Users/swa/backup'

#Remembertochangethistowhichfolderyouwillbeusing

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69

#Createtargetdirectoryifitisnotpresent

ifnotos.path.exists(target_dir):

os.mkdir(target_dir)#makedirectory

#3.Thefilesarebackedupintoazipfile.

#4.Thecurrentdayisthenameofthesubdirectory

#inthemaindirectory.

today=target_dir+os.sep+time.strftime('%Y%m%d')

#Thecurrenttimeisthenameoftheziparchive.

now=time.strftime('%H%M%S')

#Thenameofthezipfile

target=today+os.sep+now+'.zip'

#Createthesubdirectoryifitisn'talreadythere

ifnotos.path.exists(today):

os.mkdir(today)

print('Successfullycreateddirectory',today)

#5.Weusethezipcommandtoputthefilesinaziparchive

zip_command='zip-r{0}{1}'.format(target,

''.join(source))

#Runthebackup

print('Zipcommandis:')

print(zip_command)

print('Running:')

ifos.system(zip_command)==0:

print('Successfulbackupto',target)

else:

print('BackupFAILED')

Output:

$pythonbackup_ver2.py

Successfullycreateddirectory/Users/swa/backup/20140329

Zipcommandis:

zip-r/Users/swa/backup/20140329/073201.zip/Users/swa/notes

Running:

adding:Users/swa/notes/(stored0%)

adding:Users/swa/notes/blah1.txt(stored0%)

adding:Users/swa/notes/blah2.txt(stored0%)

adding:Users/swa/notes/blah3.txt(stored0%)

Successfulbackupto/Users/swa/backup/20140329/073201.zip

HowItWorks

Mostoftheprogramremainsthesame.Thechangesarethatwecheckifthereisadirectorywiththecurrentdayasitsnameinsidethemainbackupdirectoryusingthe os.path.existsfunction.Ifitdoesn'texist,wecreateitusingthe os.mkdirfunction.

ThirdVersion

ThesecondversionworksfinewhenIdomanybackups,butwhentherearelotsofbackups,Iamfindingithardtodifferentiatewhatthebackupswerefor!Forexample,Imighthavemadesomemajorchangestoaprogramorpresentation,thenIwanttoassociatewhatthosechangesarewiththenameoftheziparchive.Thiscanbeeasilyachievedbyattachingauser-suppliedcommenttothenameoftheziparchive.

WARNING:Thefollowingprogramdoesnotwork,sodonotbealarmed,pleasefollowalongbecausethere'salessoninhere.

Saveas backup_ver3.py:

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70

importos

importtime

#1.Thefilesanddirectoriestobebackedupare

#specifiedinalist.

#ExampleonWindows:

#source=['"C:\\MyDocuments"','C:\\Code']

#ExampleonMacOSXandLinux:

source=['/Users/swa/notes']

#Noticewehadtousedoublequotesinsidethestring

#fornameswithspacesinit.

#2.Thebackupmustbestoredina

#mainbackupdirectory

#ExampleonWindows:

#target_dir='E:\\Backup'

#ExampleonMacOSXandLinux:

target_dir='/Users/swa/backup'

#Remembertochangethistowhichfolderyouwillbeusing

#Createtargetdirectoryifitisnotpresent

ifnotos.path.exists(target_dir):

os.mkdir(target_dir)#makedirectory

#3.Thefilesarebackedupintoazipfile.

#4.Thecurrentdayisthenameofthesubdirectory

#inthemaindirectory.

today=target_dir+os.sep+time.strftime('%Y%m%d')

#Thecurrenttimeisthenameoftheziparchive.

now=time.strftime('%H%M%S')

#Takeacommentfromtheuserto

#createthenameofthezipfile

comment=input('Enteracomment-->')

#Checkifacommentwasentered

iflen(comment)==0:

target=today+os.sep+now+'.zip'

else:

target=today+os.sep+now+'_'+

comment.replace('','_')+'.zip'

#Createthesubdirectoryifitisn'talreadythere

ifnotos.path.exists(today):

os.mkdir(today)

print('Successfullycreateddirectory',today)

#5.Weusethezipcommandtoputthefilesinaziparchive

zip_command="zip-r{0}{1}".format(target,

''.join(source))

#Runthebackup

print('Zipcommandis:')

print(zip_command)

print('Running:')

ifos.system(zip_command)==0:

print('Successfulbackupto',target)

else:

print('BackupFAILED')

Output:

$pythonbackup_ver3.py

File"backup_ver3.py",line39

target=today+os.sep+now+'_'+

^

SyntaxError:invalidsyntax

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71

HowThis(doesnot)Work

Thisprogramdoesnotwork!PythonsaysthereisasyntaxerrorwhichmeansthatthescriptdoesnotsatisfythestructurethatPythonexpectstosee.WhenweobservetheerrorgivenbyPython,italsotellsustheplacewhereitdetectedtheerroraswell.Sowestartdebuggingourprogramfromthatline.

Oncarefulobservation,weseethatthesinglelogicallinehasbeensplitintotwophysicallinesbutwehavenotspecifiedthatthesetwophysicallinesbelongtogether.Basically,Pythonhasfoundtheadditionoperator( +)withoutanyoperandinthatlogicallineandhenceitdoesn'tknowhowtocontinue.Rememberthatwecanspecifythatthelogicallinecontinuesinthenextphysicallinebytheuseofabackslashattheendofthephysicalline.So,wemakethiscorrectiontoourprogram.Thiscorrectionoftheprogramwhenwefinderrorsiscalledbugfixing.

FourthVersionSaveas backup_ver4.py:

importos

importtime

#1.Thefilesanddirectoriestobebackedupare

#specifiedinalist.

#ExampleonWindows:

#source=['"C:\\MyDocuments"','C:\\Code']

#ExampleonMacOSXandLinux:

source=['/Users/swa/notes']

#Noticewehadtousedoublequotesinsidethestring

#fornameswithspacesinit.

#2.Thebackupmustbestoredina

#mainbackupdirectory

#ExampleonWindows:

#target_dir='E:\\Backup'

#ExampleonMacOSXandLinux:

target_dir='/Users/swa/backup'

#Remembertochangethistowhichfolderyouwillbeusing

#Createtargetdirectoryifitisnotpresent

ifnotos.path.exists(target_dir):

os.mkdir(target_dir)#makedirectory

#3.Thefilesarebackedupintoazipfile.

#4.Thecurrentdayisthenameofthesubdirectory

#inthemaindirectory.

today=target_dir+os.sep+time.strftime('%Y%m%d')

#Thecurrenttimeisthenameoftheziparchive.

now=time.strftime('%H%M%S')

#Takeacommentfromtheuserto

#createthenameofthezipfile

comment=input('Enteracomment-->')

#Checkifacommentwasentered

iflen(comment)==0:

target=today+os.sep+now+'.zip'

else:

target=today+os.sep+now+'_'+\

comment.replace('','_')+'.zip'

#Createthesubdirectoryifitisn'talreadythere

ifnotos.path.exists(today):

os.mkdir(today)

print('Successfullycreateddirectory',today)

#5.Weusethezipcommandtoputthefilesinaziparchive

zip_command='zip-r{0}{1}'.format(target,

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72

''.join(source))

#Runthebackup

print('Zipcommandis:')

print(zip_command)

print('Running:')

ifos.system(zip_command)==0:

print('Successfulbackupto',target)

else:

print('BackupFAILED')

Output:

$pythonbackup_ver4.py

Enteracomment-->addednewexamples

Zipcommandis:

zip-r/Users/swa/backup/20140329/074122_added_new_examples.zip/Users/swa/notes

Running:

adding:Users/swa/notes/(stored0%)

adding:Users/swa/notes/blah1.txt(stored0%)

adding:Users/swa/notes/blah2.txt(stored0%)

adding:Users/swa/notes/blah3.txt(stored0%)

Successfulbackupto/Users/swa/backup/20140329/074122_added_new_examples.zip

HowItWorks

Thisprogramnowworks!Letusgothroughtheactualenhancementsthatwehadmadeinversion3.Wetakeintheuser'scommentsusingthe inputfunctionandthencheckiftheuseractuallyenteredsomethingbyfindingoutthelengthoftheinputusingthe lenfunction.Iftheuserhasjustpressed enterwithoutenteringanything(maybeitwasjustaroutinebackupornospecialchangesweremade),thenweproceedaswehavedonebefore.

However,ifacommentwassupplied,thenthisisattachedtothenameoftheziparchivejustbeforethe .zipextension.Noticethatwearereplacingspacesinthecommentwithunderscores-thisisbecausemanagingfilenameswithoutspacesismucheasier.

MoreRefinementsThefourthversionisasatisfactorilyworkingscriptformostusers,butthereisalwaysroomforimprovement.Forexample,youcanincludeaverbositylevelforthezipcommandbyspecifyinga -voptiontomakeyourprogrambecomemoretalkativeora-qoptiontomakeitquiet.

Anotherpossibleenhancementwouldbetoallowextrafilesanddirectoriestobepassedtothescriptatthecommandline.Wecangetthesenamesfromthe sys.argvlistandwecanaddthemtoour sourcelistusingthe extendmethodprovidedbythelistclass.

Themostimportantrefinementwouldbetonotusethe os.systemwayofcreatingarchivesandinsteadusingthezipfileortarfilebuilt-inmodulestocreatethesearchives.Theyarepartofthestandardlibraryandavailablealreadyforyoutousewithoutexternaldependenciesonthezipprogramtobeavailableonyourcomputer.

However,Ihavebeenusingthe os.systemwayofcreatingabackupintheaboveexamplespurelyforpedagogicalpurposes,sothattheexampleissimpleenoughtobeunderstoodbyeverybodybutrealenoughtobeuseful.

Canyoutrywritingthefifthversionthatusesthezipfilemoduleinsteadofthe os.systemcall?

TheSoftwareDevelopmentProcessWehavenowgonethroughthevariousphasesintheprocessofwritingasoftware.Thesephasescanbesummarisedasfollows:

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73

1. What(Analysis)2. How(Design)3. DoIt(Implementation)4. Test(TestingandDebugging)5. Use(OperationorDeployment)6. Maintain(Refinement)

Arecommendedwayofwritingprogramsistheprocedurewehavefollowedincreatingthebackupscript:Dotheanalysisanddesign.Startimplementingwithasimpleversion.Testanddebugit.Useittoensurethatitworksasexpected.Now,addanyfeaturesthatyouwantandcontinuetorepeattheDoIt-Test-Usecycleasmanytimesasrequired.

Remember:

Softwareisgrown,notbuilt.--BilldehÓra

Summary

WehaveseenhowtocreateourownPythonprograms/scriptsandthevariousstagesinvolvedinwritingsuchprograms.YoumayfinditusefultocreateyourownprogramjustlikewedidinthischaptersothatyoubecomecomfortablewithPythonaswellasproblem-solving.

Next,wewilldiscussobject-orientedprogramming.

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74

ObjectOrientedProgrammingInalltheprogramswewrotetillnow,wehavedesignedourprogramaroundfunctionsi.e.blocksofstatementswhichmanipulatedata.Thisiscalledtheprocedure-orientedwayofprogramming.Thereisanotherwayoforganizingyourprogramwhichistocombinedataandfunctionalityandwrapitinsidesomethingcalledanobject.Thisiscalledtheobjectorientedprogrammingparadigm.Mostofthetimeyoucanuseproceduralprogramming,butwhenwritinglargeprogramsorhaveaproblemthatisbettersuitedtothismethod,youcanuseobjectorientedprogrammingtechniques.

Classesandobjectsarethetwomainaspectsofobjectorientedprogramming.Aclasscreatesanewtypewhereobjectsareinstancesoftheclass.Ananalogyisthatyoucanhavevariablesoftype intwhichtranslatestosayingthatvariablesthatstoreintegersarevariableswhichareinstances(objects)ofthe intclass.

NoteforStaticLanguageProgrammers

Notethatevenintegersaretreatedasobjects(ofthe intclass).ThisisunlikeC++andJava(beforeversion1.5)whereintegersareprimitivenativetypes.

See help(int)formoredetailsontheclass.

C#andJava1.5programmerswillfindthissimilartotheboxingandunboxingconcept.

Objectscanstoredatausingordinaryvariablesthatbelongtotheobject.Variablesthatbelongtoanobjectorclassarereferredtoasfields.Objectscanalsohavefunctionalitybyusingfunctionsthatbelongtoaclass.Suchfunctionsarecalledmethodsoftheclass.Thisterminologyisimportantbecauseithelpsustodifferentiatebetweenfunctionsandvariableswhichareindependentandthosewhichbelongtoaclassorobject.Collectively,thefieldsandmethodscanbereferredtoastheattributesofthatclass.

Fieldsareoftwotypes-theycanbelongtoeachinstance/objectoftheclassortheycanbelongtotheclassitself.Theyarecalledinstancevariablesandclassvariablesrespectively.

Aclassiscreatedusingthe classkeyword.Thefieldsandmethodsoftheclassarelistedinanindentedblock.

The self

Classmethodshaveonlyonespecificdifferencefromordinaryfunctions-theymusthaveanextrafirstnamethathastobeaddedtothebeginningoftheparameterlist,butyoudonotgiveavalueforthisparameterwhenyoucallthemethod,Pythonwillprovideit.Thisparticularvariablereferstotheobjectitself,andbyconvention,itisgiventhename self.

Although,youcangiveanynameforthisparameter,itisstronglyrecommendedthatyouusethename self-anyothernameisdefinitelyfrownedupon.Therearemanyadvantagestousingastandardname-anyreaderofyourprogramwillimmediatelyrecognizeitandevenspecializedIDEs(IntegratedDevelopmentEnvironments)canhelpyouifyouuse self.

NoteforC++/Java/C#Programmers

The selfinPythonisequivalenttothe thispointerinC++andthe thisreferenceinJavaandC#.

YoumustbewonderinghowPythongivesthevaluefor selfandwhyyoudon'tneedtogiveavalueforit.Anexamplewillmakethisclear.Sayyouhaveaclasscalled MyClassandaninstanceofthisclasscalled myobject.Whenyoucallamethodofthisobjectas myobject.method(arg1,arg2),thisisautomaticallyconvertedbyPythoninto MyClass.method(myobject,arg1,arg2)-thisisallthespecial selfisabout.

Thisalsomeansthatifyouhaveamethodwhichtakesnoarguments,thenyoustillhavetohaveoneargument-the self.

Classes

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75

Thesimplestclasspossibleisshowninthefollowingexample(saveas oop_simplestclass.py).

classPerson:

pass#Anemptyblock

p=Person()

print(p)

Output:

$pythonoop_simplestclass.py

<__main__.Personinstanceat0x10171f518>

HowItWorks

Wecreateanewclassusingthe classstatementandthenameoftheclass.Thisisfollowedbyanindentedblockofstatementswhichformthebodyoftheclass.Inthiscase,wehaveanemptyblockwhichisindicatedusingthe passstatement.

Next,wecreateanobject/instanceofthisclassusingthenameoftheclassfollowedbyapairofparentheses.(Wewilllearnmoreaboutinstantiationinthenextsection).Forourverification,weconfirmthetypeofthevariablebysimplyprintingit.Ittellsusthatwehaveaninstanceofthe Personclassinthe __main__module.

Noticethattheaddressofthecomputermemorywhereyourobjectisstoredisalsoprinted.TheaddresswillhaveadifferentvalueonyourcomputersincePythoncanstoretheobjectwhereveritfindsspace.

MethodsWehavealreadydiscussedthatclasses/objectscanhavemethodsjustlikefunctionsexceptthatwehaveanextra selfvariable.Wewillnowseeanexample(saveas oop_method.py).

classPerson:

defsay_hi(self):

print('Hello,howareyou?')

p=Person()

p.say_hi()

#Theprevious2linescanalsobewrittenas

#Person().say_hi()

Output:

$pythonoop_method.py

Hello,howareyou?

HowItWorks

Hereweseethe selfinaction.Noticethatthe say_himethodtakesnoparametersbutstillhasthe selfinthefunctiondefinition.

The __init__method

TherearemanymethodnameswhichhavespecialsignificanceinPythonclasses.Wewillseethesignificanceofthe __init__methodnow.

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The __init__methodisrunassoonasanobjectofaclassisinstantiated(i.e.created).Themethodisusefultodoanyinitialization(i.e.passinginitialvaluestoyourobject)youwanttodowithyourobject.Noticethedoubleunderscoresbothatthebeginningandattheendofthename.

Example(saveas oop_init.py):

classPerson:

def__init__(self,name):

self.name=name

defsay_hi(self):

print('Hello,mynameis',self.name)

p=Person('Swaroop')

p.say_hi()

#Theprevious2linescanalsobewrittenas

#Person('Swaroop').say_hi()

Output:

$pythonoop_init.py

Hello,mynameisSwaroop

HowItWorks

Here,wedefinethe __init__methodastakingaparameter name(alongwiththeusual self).Here,wejustcreateanewfieldalsocalled name.Noticethesearetwodifferentvariableseventhoughtheyarebothcalled'name'.Thereisnoproblembecausethedottednotation self.namemeansthatthereissomethingcalled"name"thatispartoftheobjectcalled"self"andtheother nameisalocalvariable.Sinceweexplicitlyindicatewhichnamewearereferringto,thereisnoconfusion.

Whencreatingnewinstance p,oftheclass Person,wedosobyusingtheclassname,followedbytheargumentsintheparentheses:p=Person('Swaroop').

Wedonotexplicitlycallthe __init__method.Thisisthespecialsignificanceofthismethod.

Now,weareabletousethe self.namefieldinourmethodswhichisdemonstratedinthe say_himethod.

ClassAndObjectVariables

Wehavealreadydiscussedthefunctionalitypartofclassesandobjects(i.e.methods),nowletuslearnaboutthedatapart.Thedatapart,i.e.fields,arenothingbutordinaryvariablesthatareboundtothenamespacesoftheclassesandobjects.Thismeansthatthesenamesarevalidwithinthecontextoftheseclassesandobjectsonly.That'swhytheyarecallednamespaces.

Therearetwotypesoffields-classvariablesandobjectvariableswhichareclassifieddependingonwhethertheclassortheobjectownsthevariablesrespectively.

Classvariablesareshared-theycanbeaccessedbyallinstancesofthatclass.Thereisonlyonecopyoftheclassvariableandwhenanyoneobjectmakesachangetoaclassvariable,thatchangewillbeseenbyalltheotherinstances.

Objectvariablesareownedbyeachindividualobject/instanceoftheclass.Inthiscase,eachobjecthasitsowncopyofthefieldi.e.theyarenotsharedandarenotrelatedinanywaytothefieldbythesamenameinadifferentinstance.Anexamplewillmakethiseasytounderstand(saveas oop_objvar.py):

classRobot:

"""Representsarobot,withaname."""

#Aclassvariable,countingthenumberofrobots

population=0

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def__init__(self,name):

"""Initializesthedata."""

self.name=name

print("(Initializing{})".format(self.name))

#Whenthispersoniscreated,therobot

#addstothepopulation

Robot.population+=1

defdie(self):

"""Iamdying."""

print("{}isbeingdestroyed!".format(self.name))

Robot.population-=1

ifRobot.population==0:

print("{}wasthelastone.".format(self.name))

else:

print("Therearestill{:d}robotsworking.".format(

Robot.population))

defsay_hi(self):

"""Greetingbytherobot.

Yeah,theycandothat."""

print("Greetings,mymasterscallme{}.".format(self.name))

@classmethod

defhow_many(cls):

"""Printsthecurrentpopulation."""

print("Wehave{:d}robots.".format(cls.population))

droid1=Robot("R2-D2")

droid1.say_hi()

Robot.how_many()

droid2=Robot("C-3PO")

droid2.say_hi()

Robot.how_many()

print("\nRobotscandosomeworkhere.\n")

print("Robotshavefinishedtheirwork.Solet'sdestroythem.")

droid1.die()

droid2.die()

Robot.how_many()

Output:

$pythonoop_objvar.py

(InitializingR2-D2)

Greetings,mymasterscallmeR2-D2.

Wehave1robots.

(InitializingC-3PO)

Greetings,mymasterscallmeC-3PO.

Wehave2robots.

Robotscandosomeworkhere.

Robotshavefinishedtheirwork.Solet'sdestroythem.

R2-D2isbeingdestroyed!

Therearestill1robotsworking.

C-3POisbeingdestroyed!

C-3POwasthelastone.

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Wehave0robots.

HowItWorks

Thisisalongexamplebuthelpsdemonstratethenatureofclassandobjectvariables.Here, populationbelongstothe Robotclassandhenceisaclassvariable.The namevariablebelongstotheobject(itisassignedusing self)andhenceisanobjectvariable.

Thus,werefertothe populationclassvariableas Robot.populationandnotas self.population.Werefertotheobjectvariable nameusing self.namenotationinthemethodsofthatobject.Rememberthissimpledifferencebetweenclassandobjectvariables.Alsonotethatanobjectvariablewiththesamenameasaclassvariablewillhidetheclassvariable!

Insteadof Robot.population,wecouldhavealsoused self.__class__.populationbecauseeveryobjectreferstoitsclassviathe self.__class__attribute.

The how_manyisactuallyamethodthatbelongstotheclassandnottotheobject.Thismeanswecandefineitaseitheraclassmethodora staticmethoddependingonwhetherweneedtoknowwhichclasswearepartof.Sincewerefertoaclassvariable,let'suse classmethod.

Wehavemarkedthe how_manymethodasaclassmethodusingadecorator.

Decoratorscanbeimaginedtobeashortcuttocallingawrapperfunction(i.e.afunctionthat"wraps"aroundanotherfunctionsothatitcandosomethingbeforeoraftertheinnerfunction),soapplyingthe @classmethoddecoratoristhesameascalling:

how_many=classmethod(how_many)

Observethatthe __init__methodisusedtoinitializethe Robotinstancewithaname.Inthismethod,weincreasethepopulationcountby1sincewehaveonemorerobotbeingadded.Alsoobservethatthevaluesof self.nameisspecifictoeachobjectwhichindicatesthenatureofobjectvariables.

Remember,thatyoumustrefertothevariablesandmethodsofthesameobjectusingthe selfonly.Thisiscalledanattributereference.

Inthisprogram,wealsoseetheuseofdocstringsforclassesaswellasmethods.Wecanaccesstheclassdocstringatruntimeusing Robot.__doc__andthemethoddocstringas Robot.say_hi.__doc__

Inthe diemethod,wesimplydecreasethe Robot.populationcountby1.

Allclassmembersarepublic.Oneexception:Ifyouusedatamemberswithnamesusingthedoubleunderscoreprefixsuchas__privatevar,Pythonusesname-manglingtoeffectivelymakeitaprivatevariable.

Thus,theconventionfollowedisthatanyvariablethatistobeusedonlywithintheclassorobjectshouldbeginwithanunderscoreandallothernamesarepublicandcanbeusedbyotherclasses/objects.RememberthatthisisonlyaconventionandisnotenforcedbyPython(exceptforthedoubleunderscoreprefix).

NoteforC++/Java/C#Programmers

Allclassmembers(includingthedatamembers)arepublicandallthemethodsarevirtualinPython.

Inheritance

Oneofthemajorbenefitsofobjectorientedprogrammingisreuseofcodeandoneofthewaysthisisachievedisthroughtheinheritancemechanism.Inheritancecanbebestimaginedasimplementingatypeandsubtyperelationshipbetweenclasses.

Supposeyouwanttowriteaprogramwhichhastokeeptrackoftheteachersandstudentsinacollege.Theyhavesomecommoncharacteristicssuchasname,ageandaddress.Theyalsohavespecificcharacteristicssuchassalary,coursesandleavesforteachersand,marksandfeesforstudents.

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Youcancreatetwoindependentclassesforeachtypeandprocessthembutaddinganewcommoncharacteristicwouldmeanaddingtobothoftheseindependentclasses.Thisquicklybecomesunwieldy.

Abetterwaywouldbetocreateacommonclasscalled SchoolMemberandthenhavetheteacherandstudentclassesinheritfromthisclass,i.e.theywillbecomesub-typesofthistype(class)andthenwecanaddspecificcharacteristicstothesesub-types.

Therearemanyadvantagestothisapproach.Ifweadd/changeanyfunctionalityin SchoolMember,thisisautomaticallyreflectedinthesubtypesaswell.Forexample,youcanaddanewIDcardfieldforbothteachersandstudentsbysimplyaddingittotheSchoolMemberclass.However,changesinthesubtypesdonotaffectothersubtypes.Anotheradvantageisthatyoucanrefertoateacherorstudentobjectasa SchoolMemberobjectwhichcouldbeusefulinsomesituationssuchascountingofthenumberofschoolmembers.Thisiscalledpolymorphismwhereasub-typecanbesubstitutedinanysituationwhereaparenttypeisexpected,i.e.theobjectcanbetreatedasaninstanceoftheparentclass.

Alsoobservethatwereusethecodeoftheparentclassandwedonotneedtorepeatitinthedifferentclassesaswewouldhavehadtoincasewehadusedindependentclasses.

The SchoolMemberclassinthissituationisknownasthebaseclassorthesuperclass.The Teacherand Studentclassesarecalledthederivedclassesorsubclasses.

Wewillnowseethisexampleasaprogram(saveas oop_subclass.py):

classSchoolMember:

'''Representsanyschoolmember.'''

def__init__(self,name,age):

self.name=name

self.age=age

print('(InitializedSchoolMember:{})'.format(self.name))

deftell(self):

'''Tellmydetails.'''

print('Name:"{}"Age:"{}"'.format(self.name,self.age),end="")

classTeacher(SchoolMember):

'''Representsateacher.'''

def__init__(self,name,age,salary):

SchoolMember.__init__(self,name,age)

self.salary=salary

print('(InitializedTeacher:{})'.format(self.name))

deftell(self):

SchoolMember.tell(self)

print('Salary:"{:d}"'.format(self.salary))

classStudent(SchoolMember):

'''Representsastudent.'''

def__init__(self,name,age,marks):

SchoolMember.__init__(self,name,age)

self.marks=marks

print('(InitializedStudent:{})'.format(self.name))

deftell(self):

SchoolMember.tell(self)

print('Marks:"{:d}"'.format(self.marks))

t=Teacher('Mrs.Shrividya',40,30000)

s=Student('Swaroop',25,75)

#printsablankline

print()

members=[t,s]

formemberinmembers:

#WorksforbothTeachersandStudents

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80

member.tell()

Output:

$pythonoop_subclass.py

(InitializedSchoolMember:Mrs.Shrividya)

(InitializedTeacher:Mrs.Shrividya)

(InitializedSchoolMember:Swaroop)

(InitializedStudent:Swaroop)

Name:"Mrs.Shrividya"Age:"40"Salary:"30000"

Name:"Swaroop"Age:"25"Marks:"75"

HowItWorks

Touseinheritance,wespecifythebaseclassnamesinatuplefollowingtheclassnameintheclassdefinition(forexample, classTeacher(SchoolMember)).Next,weobservethatthe __init__methodofthebaseclassisexplicitlycalledusingthe selfvariablesothatwecaninitializethebaseclasspartofaninstanceinthesubclass.Thisisveryimportanttoremember-Sincewearedefininga __init__methodin Teacherand Studentsubclasses,Pythondoesnotautomaticallycalltheconstructorofthebaseclass SchoolMember,youhavetoexplicitlycallityourself.

Incontrast,ifwehavenotdefinedan __init__methodinasubclass,Pythonwillcalltheconstructorofthebaseclassautomatically.

Whilewecouldtreatinstancesof Teacheror Studentaswewouldaninstanceof SchoolMemberandaccessthe tellmethodof SchoolMemberbysimplytyping Teacher.tellor Student.tell,weinsteaddefineanother tellmethodineachsubclass(usingthe tellmethodof SchoolMemberforpartofit)totailoritforthatsubclass.Becausewehavedonethis,whenwewrite Teacher.tellPythonusesthe tellmethodforthatsubclassvsthesuperclass.However,ifwedidnothavea tellmethodinthesubclass,Pythonwouldusethe tellmethodinthesuperclass.Pythonalwaysstartslookingformethodsintheactualsubclasstypefirst,andifitdoesn�tfindanything,itstartslookingatthemethodsinthesubclass�sbaseclasses,onebyoneintheordertheyarespecifiedinthetuple(hereweonlyhave1baseclass,butyoucanhavemultiplebaseclasses)intheclassdefinition.

Anoteonterminology-ifmorethanoneclassislistedintheinheritancetuple,thenitiscalledmultipleinheritance.

The endparameterisusedinthe printfunctioninthesuperclass's tell()methodtoprintalineandallowthenextprinttocontinueonthesameline.Thisisatricktomake printnotprinta \n(newline)symbolattheendoftheprinting.

Summary

Wehavenowexploredthevariousaspectsofclassesandobjectsaswellasthevariousterminologiesassociatedwithit.Wehavealsoseenthebenefitsandpitfallsofobject-orientedprogramming.Pythonishighlyobject-orientedandunderstandingtheseconceptscarefullywillhelpyoualotinthelongrun.

Next,wewilllearnhowtodealwithinput/outputandhowtoaccessfilesinPython.

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InputandOutputTherewillbesituationswhereyourprogramhastointeractwiththeuser.Forexample,youwouldwanttotakeinputfromtheuserandthenprintsomeresultsback.Wecanachievethisusingthe input()functionand printfunctionrespectively.

Foroutput,wecanalsousethevariousmethodsofthe str(string)class.Forexample,youcanusethe rjustmethodtogetastringwhichisrightjustifiedtoaspecifiedwidth.See help(str)formoredetails.

Anothercommontypeofinput/outputisdealingwithfiles.Theabilitytocreate,readandwritefilesisessentialtomanyprogramsandwewillexplorethisaspectinthischapter.

InputfromuserSavethisprogramas io_input.py:

defreverse(text):

returntext[::-1]

defis_palindrome(text):

returntext==reverse(text)

something=input("Entertext:")

ifis_palindrome(something):

print("Yes,itisapalindrome")

else:

print("No,itisnotapalindrome")

Output:

$python3io_input.py

Entertext:sir

No,itisnotapalindrome

$python3io_input.py

Entertext:madam

Yes,itisapalindrome

$python3io_input.py

Entertext:racecar

Yes,itisapalindrome

HowItWorks

Weusetheslicingfeaturetoreversethetext.We'vealreadyseenhowwecanmakeslicesfromsequencesusingthe seq[a:b]codestartingfromposition atoposition b.Wecanalsoprovideathirdargumentthatdeterminesthestepbywhichtheslicingisdone.Thedefaultstepis 1becauseofwhichitreturnsacontinuouspartofthetext.Givinganegativestep,i.e., -1willreturnthetextinreverse.

The input()functiontakesastringasargumentanddisplaysittotheuser.Thenitwaitsfortheusertotypesomethingandpressthereturnkey.Oncetheuserhasenteredandpressedthereturnkey,the input()functionwillthenreturnthattexttheuserhasentered.

Wetakethattextandreverseit.Iftheoriginaltextandreversedtextareequal,thenthetextisapalindrome.

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82

Homeworkexercise

Checkingwhetheratextisapalindromeshouldalsoignorepunctuation,spacesandcase.Forexample,"Risetovote,sir."isalsoapalindromebutourcurrentprogramdoesn'tsayitis.Canyouimprovetheaboveprogramtorecognizethispalindrome?

Ifyouneedahint,theideaisthat...

FilesYoucanopenandusefilesforreadingorwritingbycreatinganobjectofthe fileclassandusingits read, readlineorwritemethodsappropriatelytoreadfromorwritetothefile.Theabilitytoreadorwritetothefiledependsonthemodeyouhavespecifiedforthefileopening.Thenfinally,whenyouarefinishedwiththefile,youcallthe closemethodtotellPythonthatwearedoneusingthefile.

Example(saveas io_using_file.py):

poem='''\

Programmingisfun

Whentheworkisdone

ifyouwannamakeyourworkalsofun:

usePython!

'''

#Openfor'w'riting

f=open('poem.txt','w')

#Writetexttofile

f.write(poem)

#Closethefile

f.close()

#Ifnomodeisspecified,

#'r'eadmodeisassumedbydefault

f=open('poem.txt')

whileTrue:

line=f.readline()

#ZerolengthindicatesEOF

iflen(line)==0:

break

#The`line`alreadyhasanewline

#attheendofeachline

#sinceitisreadingfromafile.

print(line,end='')

#closethefile

f.close()

Output:

$python3io_using_file.py

Programmingisfun

Whentheworkisdone

ifyouwannamakeyourworkalsofun:

usePython!

HowItWorks

Notethatwecancreateanewfileobjectsimplybyusingthe openmethod.Weopen(orcreateitifitdoesn'talreadyexist)thisfilebyusingthebuilt-in openfunctionandspecifyingthenameofthefileandthemodeinwhichwewanttoopenthefile.Themodecanbeareadmode( 'r'),writemode( 'w')orappendmode( 'a').Wecanalsospecifywhetherwearereading,

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83

writing,orappendingintextmode( 't')orbinarymode( 'b').Thereareactuallymanymoremodesavailableandhelp(open)willgiveyoumoredetailsaboutthem.Bydefault, open()considersthefiletobea't'extfileandopensitin'r'eadmode.

Inourexample,wefirstopen/createthefileinwritetextmodeandusethe writemethodofthefileobjecttowriteourstringvariable poemtothefileandthenwefinally closethefile.

Next,weopenthesamefileagainforreading.Wedon'tneedtospecifyamodebecause'readtextfile'isthedefaultmode.Wereadineachlineofthefileusingthe readlinemethodinaloop.Thismethodreturnsacompletelineincludingthenewlinecharacterattheendoftheline.Whenanemptystringisreturned,itmeansthatwehavereachedtheendofthefileandwe'break'outoftheloop.

Intheend,wefinally closethefile.

Wecanseefromour readlineoutputthatthisprogramhasindeedwrittentoandreadfromournew poem.txtfile.

PicklePythonprovidesastandardmodulecalled picklewhichyoucanusetostoreanyplainPythonobjectinafileandthengetitbacklater.Thisiscalledstoringtheobjectpersistently.

Example(saveas io_pickle.py):

importpickle

#Thenameofthefilewherewewillstoretheobject

shoplistfile='shoplist.data'

#Thelistofthingstobuy

shoplist=['apple','mango','carrot']

#Writetothefile

f=open(shoplistfile,'wb')

#Dumptheobjecttoafile

pickle.dump(shoplist,f)

f.close()

#Destroytheshoplistvariable

delshoplist

#Readbackfromthestorage

f=open(shoplistfile,'rb')

#Loadtheobjectfromthefile

storedlist=pickle.load(f)

print(storedlist)

f.close()

Output:

$pythonio_pickle.py

['apple','mango','carrot']

HowItWorks

Tostoreanobjectinafile,wehavetofirst openthefileinwritebinarymodeandthencallthe dumpfunctionofthe picklemodule.Thisprocessiscalledpickling.

Next,weretrievetheobjectusingthe loadfunctionofthe picklemodulewhichreturnstheobject.Thisprocessiscalledunpickling.

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84

Unicode

Sofar,whenwehavebeenwritingandusingstrings,orreadingandwritingtoafile,wehaveusedsimpleEnglishcharactersonly.BothEnglishandnon-EnglishcharacterscanberepresentedinUnicode(pleaseseethearticlesattheendofthissectionformoreinfo),andPython3bydefaultstoresstringvariables(thinkofallthattextwewroteusingsingleordoubleortriplequotes)inUnicode.

NOTE:IfyouareusingPython2,andwewanttobeabletoreadandwriteothernon-Englishlanguages,weneedtousethe unicodetype,anditallstartswiththecharacter u,e.g. u"helloworld"

>>>"helloworld"

'helloworld'

>>>type("helloworld")

<class'str'>

>>>u"helloworld"

'helloworld'

>>>type(u"helloworld")

<class'str'>

WhendataissentovertheInternet,weneedtosenditinbytes...somethingyourcomputereasilyunderstands.TherulesfortranslatingUnicode(whichiswhatPythonuseswhenitstoresastring)tobytesiscalledencoding.ApopularencodingtouseisUTF-8.WecanreadandwriteinUTF-8byusingasimplekeywordargumentinour openfunction.

#encoding=utf-8

importio

f=io.open("abc.txt","wt",encoding="utf-8")

f.write(u"Imaginenon-Englishlanguagehere")

f.close()

text=io.open("abc.txt",encoding="utf-8").read()

print(text)

HowItWorks

Weuse io.openandthenusethe encodingargumentinthefirstopenstatementtoencodethemessage,andthenagaininthesecondopenstatementwhendecodingthemessage.Notethatweshouldonlyuseencodingintheopenstatementwhenintextmode.

WheneverwewriteaprogramthatusesUnicodeliterals(byputtinga ubeforethestring)likewehaveusedabove,wehavetomakesurethatPythonitselfistoldthatourprogramusesUTF-8,andwehavetoput #encoding=utf-8commentatthetopofourprogram.

Youshouldlearnmoreaboutthistopicbyreading:

"TheAbsoluteMinimumEverySoftwareDeveloperAbsolutely,PositivelyMustKnowAboutUnicodeandCharacterSets"PythonUnicodeHowtoPragmaticUnicodetalkbyNatBatchelder

SummaryWehavediscussedvarioustypesofinput/output,aboutfilehandling,aboutthepicklemoduleandaboutUnicode.

Next,wewillexploretheconceptofexceptions.

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85

.Useatuple(youcanfindalistofallpunctuationmarkshere)toholdalltheforbiddencharacters,thenusethemembershiptesttodeterminewhetheracharactershouldberemovedornot,i.e.forbidden=( !, ?, .,...).↩

1

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86

ExceptionsExceptionsoccurwhenexceptionalsituationsoccurinyourprogram.Forexample,whatifyouaregoingtoreadafileandthefiledoesnotexist?Orwhatifyouaccidentallydeleteditwhentheprogramwasrunning?Suchsituationsarehandledusingexceptions.

Similarly,whatifyourprogramhadsomeinvalidstatements?ThisishandledbyPythonwhichraisesitshandsandtellsyouthereisanerror.

Errors

Considerasimple printfunctioncall.Whatifwemisspelt printas Print?Notethecapitalization.Inthiscase,Pythonraisesasyntaxerror.

>>>Print("HelloWorld")

Traceback(mostrecentcalllast):

File"<stdin>",line1,in<module>

NameError:name'Print'isnotdefined

>>>print("HelloWorld")

HelloWorld

Observethata NameErrorisraisedandalsothelocationwheretheerrorwasdetectedisprinted.Thisiswhatanerrorhandlerforthiserrordoes.

Exceptions

Wewilltrytoreadinputfromtheuser.Enterthefirstlinebelowandhitthe Enterkey.Whenyourcomputerpromptsyouforinput,insteadpress [ctrl-d]onaMacor [ctrl-z]withWindowsandseewhathappens.(Ifyou'reusingWindowsandneitheroptionworks,youcantry [ctrl-c]intheCommandPrompttogenerateaKeyboardInterrupterrorinstead).

>>>s=input('Entersomething-->')

Entersomething-->Traceback(mostrecentcalllast):

File"<stdin>",line1,in<module>

EOFError

Pythonraisesanerrorcalled EOFErrorwhichbasicallymeansitfoundanendoffilesymbol(whichisrepresentedby ctrl-d)whenitdidnotexpecttoseeit.

HandlingExceptionsWecanhandleexceptionsusingthe try..exceptstatement.Webasicallyputourusualstatementswithinthetry-blockandputallourerrorhandlersintheexcept-block.

Example(saveas exceptions_handle.py):

try:

text=input('Entersomething-->')

exceptEOFError:

print('WhydidyoudoanEOFonme?')

exceptKeyboardInterrupt:

print('Youcancelledtheoperation.')

Exceptions

87

else:

print('Youentered{}'.format(text))

Output:

#Pressctrl+d

$pythonexceptions_handle.py

Entersomething-->WhydidyoudoanEOFonme?

#Pressctrl+c

$pythonexceptions_handle.py

Entersomething-->^CYoucancelledtheoperation.

$pythonexceptions_handle.py

Entersomething-->Noexceptions

YouenteredNoexceptions

HowItWorks

Weputallthestatementsthatmightraiseexceptions/errorsinsidethe tryblockandthenputhandlersfortheappropriateerrors/exceptionsinthe exceptclause/block.The exceptclausecanhandleasinglespecifiederrororexception,oraparenthesizedlistoferrors/exceptions.Ifnonamesoferrorsorexceptionsaresupplied,itwillhandleallerrorsandexceptions.

Notethattherehastobeatleastone exceptclauseassociatedwithevery tryclause.Otherwise,what'sthepointofhavingatryblock?

Ifanyerrororexceptionisnothandled,thenthedefaultPythonhandleriscalledwhichjuststopstheexecutionoftheprogramandprintsanerrormessage.Wehavealreadyseenthisinactionabove.

Youcanalsohavean elseclauseassociatedwitha try..exceptblock.The elseclauseisexecutedifnoexceptionoccurs.

Inthenextexample,wewillalsoseehowtogettheexceptionobjectsothatwecanretrieveadditionalinformation.

RaisingExceptions

Youcanraiseexceptionsusingthe raisestatementbyprovidingthenameoftheerror/exceptionandtheexceptionobjectthatistobethrown.

Theerrororexceptionthatyoucanraiseshouldbeaclasswhichdirectlyorindirectlymustbeaderivedclassofthe Exceptionclass.

Example(saveas exceptions_raise.py):

classShortInputException(Exception):

'''Auser-definedexceptionclass.'''

def__init__(self,length,atleast):

Exception.__init__(self)

self.length=length

self.atleast=atleast

try:

text=input('Entersomething-->')

iflen(text)<3:

raiseShortInputException(len(text),3)

#Otherworkcancontinueasusualhere

exceptEOFError:

print('WhydidyoudoanEOFonme?')

exceptShortInputExceptionasex:

print(('ShortInputException:Theinputwas'+

'{0}long,expectedatleast{1}')

.format(ex.length,ex.atleast))

Exceptions

88

else:

print('Noexceptionwasraised.')

Output:

$pythonexceptions_raise.py

Entersomething-->a

ShortInputException:Theinputwas1long,expectedatleast3

$pythonexceptions_raise.py

Entersomething-->abc

Noexceptionwasraised.

HowItWorks

Here,wearecreatingourownexceptiontype.Thisnewexceptiontypeiscalled ShortInputException.Ithastwofields-lengthwhichisthelengthofthegiveninput,and atleastwhichistheminimumlengththattheprogramwasexpecting.

Inthe exceptclause,wementiontheclassoferrorwhichwillbestored asthevariablenametoholdthecorrespondingerror/exceptionobject.Thisisanalogoustoparametersandargumentsinafunctioncall.Withinthisparticular exceptclause,weusethe lengthand atleastfieldsoftheexceptionobjecttoprintanappropriatemessagetotheuser.

Try...FinallySupposeyouarereadingafileinyourprogram.Howdoyouensurethatthefileobjectisclosedproperlywhetherornotanexceptionwasraised?Thiscanbedoneusingthe finallyblock.

Savethisprogramas exceptions_finally.py:

importsys

importtime

f=None

try:

f=open("poem.txt")

#Ourusualfile-readingidiom

whileTrue:

line=f.readline()

iflen(line)==0:

break

print(line,end='')

sys.stdout.flush()

print("Pressctrl+cnow")

#Tomakesureitrunsforawhile

time.sleep(2)

exceptIOError:

print("Couldnotfindfilepoem.txt")

exceptKeyboardInterrupt:

print("!!Youcancelledthereadingfromthefile.")

finally:

iff:

f.close()

print("(Cleaningup:Closedthefile)")

Output:

$pythonexceptions_finally.py

Programmingisfun

Pressctrl+cnow

^C!!Youcancelledthereadingfromthefile.

Exceptions

89

(Cleaningup:Closedthefile)

HowItWorks

Wedotheusualfile-readingstuff,butwehavearbitrarilyintroducedsleepingfor2secondsafterprintingeachlineusingthetime.sleepfunctionsothattheprogramrunsslowly(Pythonisveryfastbynature).Whentheprogramisstillrunning,pressctrl+ctointerrupt/canceltheprogram.

Observethatthe KeyboardInterruptexceptionisthrownandtheprogramquits.However,beforetheprogramexits,thefinallyclauseisexecutedandthefileobjectisalwaysclosed.

Noticethatavariableassignedavalueof0or Noneoravariablewhichisanemptysequenceorcollectionisconsidered FalsebyPython.Thisiswhywecanuse iff:inthecodeabove.

Alsonotethatweuse sys.stdout.flush()after printsothatitprintstothescreenimmediately.

Thewithstatement

Acquiringaresourceinthe tryblockandsubsequentlyreleasingtheresourceinthe finallyblockisacommonpattern.Hence,thereisalsoa withstatementthatenablesthistobedoneinacleanmanner:

Saveas exceptions_using_with.py:

withopen("poem.txt")asf:

forlineinf:

print(line,end='')

HowItWorks

Theoutputshouldbesameasthepreviousexample.Thedifferencehereisthatweareusingthe openfunctionwiththe withstatement-weleavetheclosingofthefiletobedoneautomaticallyby withopen.

Whathappensbehindthescenesisthatthereisaprotocolusedbythe withstatement.Itfetchestheobjectreturnedbytheopenstatement,let'scallit"thefile"inthiscase.

Italwayscallsthe thefile.__enter__functionbeforestartingtheblockofcodeunderitandalwayscalls thefile.__exit__afterfinishingtheblockofcode.

Sothecodethatwewouldhavewrittenina finallyblockshouldbetakencareofautomaticallybythe __exit__method.Thisiswhathelpsustoavoidhavingtouseexplicit try..finallystatementsrepeatedly.

Morediscussiononthistopicisbeyondscopeofthisbook,sopleasereferPEP343foracomprehensiveexplanation.

Summary

Wehavediscussedtheusageofthe try..exceptand try..finallystatements.Wehaveseenhowtocreateourownexceptiontypesandhowtoraiseexceptionsaswell.

Next,wewillexplorethePythonStandardLibrary.

Exceptions

90

StandardLibraryThePythonStandardLibrarycontainsahugenumberofusefulmodulesandispartofeverystandardPythoninstallation.ItisimportanttobecomefamiliarwiththePythonStandardLibrarysincemanyproblemscanbesolvedquicklyifyouarefamiliarwiththerangeofthingsthattheselibrariescando.

Wewillexploresomeofthecommonlyusedmodulesinthislibrary.YoucanfindcompletedetailsforallofthemodulesinthePythonStandardLibraryinthe'LibraryReference'sectionofthedocumentationthatcomeswithyourPythoninstallation.

Letusexploreafewusefulmodules.

CAUTION:Ifyoufindthetopicsinthischaptertooadvanced,youmayskipthischapter.However,IhighlyrecommendcomingbacktothischapterwhenyouaremorecomfortablewithprogrammingusingPython.

sysmodule

The sysmodulecontainssystem-specificfunctionality.Wehavealreadyseenthatthe sys.argvlistcontainsthecommand-linearguments.

SupposewewanttochecktheversionofthePythonsoftwarebeingused,the sysmodulegivesusthatinformation.

>>>importsys

>>>sys.version_info

sys.version_info(major=3,minor=6,micro=0,releaselevel='final',serial=0)

>>>sys.version_info.major==3

True

HowItWorks

The sysmodulehasa version_infotuplethatgivesustheversioninformation.Thefirstentryisthemajorversion.Wecanpulloutthisinformationtouseit.

loggingmoduleWhatifyouwantedtohavesomedebuggingmessagesorimportantmessagestobestoredsomewheresothatyoucancheckwhetheryourprogramhasbeenrunningasyouwouldexpectit?Howdoyou"storesomewhere"thesemessages?Thiscanbeachievedusingthe loggingmodule.

Saveas stdlib_logging.py:

importos

importplatform

importlogging

ifplatform.platform().startswith('Windows'):

logging_file=os.path.join(os.getenv('HOMEDRIVE'),

os.getenv('HOMEPATH'),

'test.log')

else:

logging_file=os.path.join(os.getenv('HOME'),

'test.log')

print("Loggingto",logging_file)

logging.basicConfig(

StandardLibrary

91

level=logging.DEBUG,

format='%(asctime)s:%(levelname)s:%(message)s',

filename=logging_file,

filemode='w',

)

logging.debug("Startoftheprogram")

logging.info("Doingsomething")

logging.warning("Dyingnow")

Output:

$pythonstdlib_logging.py

Loggingto/Users/swa/test.log

$cat/Users/swa/test.log

2014-03-2909:27:36,660:DEBUG:Startoftheprogram

2014-03-2909:27:36,660:INFO:Doingsomething

2014-03-2909:27:36,660:WARNING:Dyingnow

The catcommandisusedinthecommandlinetoreadthe'test.log'file.Ifthe catcommandisnotavailable,youcanopenthetest.logfileinatexteditorinstead.

HowItWorks

Weusethreemodulesfromthestandardlibrary-the osmoduleforinteractingwiththeoperatingsystem,the platformmoduleforinformationabouttheplatformi.e.theoperatingsystemandthe loggingmoduletologinformation.

First,wecheckwhichoperatingsystemweareusingbycheckingthestringreturnedby platform.platform()(formoreinformation,see importplatform;help(platform)).IfitisWindows,wefigureoutthehomedrive,thehomefolderandthefilenamewherewewanttostoretheinformation.Puttingthesethreepartstogether,wegetthefulllocationofthefile.Forotherplatforms,weneedtoknowjustthehomefolderoftheuserandwegetthefulllocationofthefile.

Weusethe os.path.join()functiontoputthesethreepartsofthelocationtogether.Thereasontouseaspecialfunctionratherthanjustaddingthestringstogetherisbecausethisfunctionwillensurethefulllocationmatchestheformatexpectedbytheoperatingsystem.Note:the join()methodweuseherethat'spartofthe osmoduleisdifferentfromthestringmethodjoin()thatwe'veusedelsewhereinthisbook.

Weconfigurethe loggingmoduletowriteallthemessagesinaparticularformattothefilewehavespecified.

Finally,wecanputmessagesthatareeithermeantfordebugging,information,warningorevencriticalmessages.Oncetheprogramhasrun,wecancheckthisfileandwewillknowwhathappenedintheprogram,eventhoughnoinformationwasdisplayedtotheuserrunningtheprogram.

ModuleoftheWeekSeriesThereismuchmoretobeexploredinthestandardlibrarysuchasdebugging,handlingcommandlineoptions,regularexpressionsandsoon.

ThebestwaytofurtherexplorethestandardlibraryistoreadDougHellmann'sexcellentPythonModuleoftheWeekseries(alsoavailableasabook)andreadingthePythondocumentation.

Summary

WehaveexploredsomeofthefunctionalityofmanymodulesinthePythonStandardLibrary.ItishighlyrecommendedtobrowsethroughthePythonStandardLibrarydocumentationtogetanideaofallthemodulesthatareavailable.

StandardLibrary

92

Next,wewillcovervariousaspectsofPythonthatwillmakeourtourofPythonmorecomplete.

StandardLibrary

93

MoreSofarwehavecoveredamajorityofthevariousaspectsofPythonthatyouwilluse.Inthischapter,wewillcoversomemoreaspectsthatwillmakeourknowledgeofPythonmorewell-rounded.

Passingtuplesaround

Everwishedyoucouldreturntwodifferentvaluesfromafunction?Youcan.Allyouhavetodoisuseatuple.

>>>defget_error_details():

...return(2,'details')

...

>>>errnum,errstr=get_error_details()

>>>errnum

2

>>>errstr

'details'

Noticethattheusageof a,b=<someexpression>interpretstheresultoftheexpressionasatuplewithtwovalues.

ThisalsomeansthefastestwaytoswaptwovariablesinPythonis:

>>>a=5;b=8

>>>a,b

(5,8)

>>>a,b=b,a

>>>a,b

(8,5)

SpecialMethods

Therearecertainmethodssuchasthe __init__and __del__methodswhichhavespecialsignificanceinclasses.

Specialmethodsareusedtomimiccertainbehaviorsofbuilt-intypes.Forexample,ifyouwanttousethe x[key]indexingoperationforyourclass(justlikeyouuseitforlistsandtuples),thenallyouhavetodoisimplementthe __getitem__()methodandyourjobisdone.Ifyouthinkaboutit,thisiswhatPythondoesforthe listclassitself!

Someusefulspecialmethodsarelistedinthefollowingtable.Ifyouwanttoknowaboutallthespecialmethods,seethemanual.

__init__(self,...)

Thismethodiscalledjustbeforethenewlycreatedobjectisreturnedforusage.__del__(self)

Calledjustbeforetheobjectisdestroyed(whichhasunpredictabletiming,soavoidusingthis)__str__(self)

Calledwhenweusethe printfunctionorwhen str()isused.__lt__(self,other)

Calledwhenthelessthanoperator(<)isused.Similarly,therearespecialmethodsforalltheoperators(+,>,etc.)__getitem__(self,key)

Calledwhen x[key]indexingoperationisused.

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94

__len__(self)

Calledwhenthebuilt-in len()functionisusedforthesequenceobject.

SingleStatementBlocks

Wehaveseenthateachblockofstatementsissetapartfromtherestbyitsownindentationlevel.Well,thereisonecaveat.Ifyourblockofstatementscontainsonlyonesinglestatement,thenyoucanspecifyitonthesamelineof,say,aconditionalstatementorloopingstatement.Thefollowingexampleshouldmakethisclear:

>>>flag=True

>>>ifflag:print('Yes')

...

Yes

Noticethatthesinglestatementisusedin-placeandnotasaseparateblock.Although,youcanusethisformakingyourprogramsmaller,Istronglyrecommendavoidingthisshort-cutmethod,exceptforerrorchecking,mainlybecauseitwillbemucheasiertoaddanextrastatementifyouareusingproperindentation.

LambdaForms

A lambdastatementisusedtocreatenewfunctionobjects.Essentially,the lambdatakesaparameterfollowedbyasingleexpression.Lambdabecomesthebodyofthefunction.Thevalueofthisexpressionisreturnedbythenewfunction.

Example(saveas more_lambda.py):

points=[{'x':2,'y':3},

{'x':4,'y':1}]

points.sort(key=lambdai:i['y'])

print(points)

Output:

$pythonmore_lambda.py

[{'y':1,'x':4},{'y':3,'x':2}]

HowItWorks

Noticethatthe sortmethodofa listcantakea keyparameterwhichdetermineshowthelistissorted(usuallyweknowonlyaboutascendingordescendingorder).Inourcase,wewanttodoacustomsort,andforthatweneedtowriteafunction.Insteadofwritingaseparate defblockforafunctionthatwillgetusedinonlythisoneplace,weusealambdaexpressiontocreateanewfunction.

ListComprehension

Listcomprehensionsareusedtoderiveanewlistfromanexistinglist.Supposeyouhavealistofnumbersandyouwanttogetacorrespondinglistwithallthenumbersmultipliedby2onlywhenthenumberitselfisgreaterthan2.Listcomprehensionsareidealforsuchsituations.

Example(saveas more_list_comprehension.py):

listone=[2,3,4]

listtwo=[2*iforiinlistoneifi>2]

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95

print(listtwo)

Output:

$pythonmore_list_comprehension.py

[6,8]

HowItWorks

Here,wederiveanewlistbyspecifyingthemanipulationtobedone( 2*i)whensomeconditionissatisfied( ifi>2).Notethattheoriginallistremainsunmodified.

Theadvantageofusinglistcomprehensionsisthatitreducestheamountofboilerplatecoderequiredwhenweuseloopstoprocesseachelementofalistandstoreitinanewlist.

ReceivingTuplesandDictionariesinFunctionsThereisaspecialwayofreceivingparameterstoafunctionasatupleoradictionaryusingthe *or **prefixrespectively.Thisisusefulwhentakingvariablenumberofargumentsinthefunction.

>>>defpowersum(power,*args):

...'''Returnthesumofeachargumentraisedtothespecifiedpower.'''

...total=0

...foriinargs:

...total+=pow(i,power)

...returntotal

...

>>>powersum(2,3,4)

25

>>>powersum(2,10)

100

Becausewehavea *prefixonthe argsvariable,allextraargumentspassedtothefunctionarestoredin argsasatuple.Ifa**prefixhadbeenusedinstead,theextraparameterswouldbeconsideredtobekey/valuepairsofadictionary.

TheassertstatementThe assertstatementisusedtoassertthatsomethingistrue.Forexample,ifyouareverysurethatyouwillhaveatleastoneelementinalistyouareusingandwanttocheckthis,andraiseanerrorifitisnottrue,then assertstatementisidealinthissituation.Whentheassertstatementfails,an AssertionErrorisraised.The pop()methodremovesandreturnsthelastitemfromthelist.

>>>mylist=['item']

>>>assertlen(mylist)>=1

>>>mylist.pop()

'item'

>>>assertlen(mylist)>=1

Traceback(mostrecentcalllast):

File"<stdin>",line1,in<module>

AssertionError

The assertstatementshouldbeusedjudiciously.Mostofthetime,itisbettertocatchexceptions,eitherhandletheproblemordisplayanerrormessagetotheuserandthenquit.

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96

Decorators

Decoratorsareashortcuttoapplyingwrapperfunctions.Thisishelpfulto"wrap"functionalitywiththesamecodeoverandoveragain.Forexample,Icreateda retrydecoratorformyselfthatIcanjustapplytoanyfunctionandifanyexceptionisthrownduringarun,itisretriedagain,tillamaximumof5timesandwithadelaybetweeneachretry.Thisisespeciallyusefulforsituationswhereyouaretryingtomakeanetworkcalltoaremotecomputer:

fromtimeimportsleep

fromfunctoolsimportwraps

importlogging

logging.basicConfig()

log=logging.getLogger("retry")

defretry(f):

@wraps(f)

defwrapper_function(*args,**kwargs):

MAX_ATTEMPTS=5

forattemptinrange(1,MAX_ATTEMPTS+1):

try:

returnf(*args,**kwargs)

exceptException:

log.exception("Attempt%s/%sfailed:%s",

attempt,

MAX_ATTEMPTS,

(args,kwargs))

sleep(10*attempt)

log.critical("All%sattemptsfailed:%s",

MAX_ATTEMPTS,

(args,kwargs))

returnwrapper_function

counter=0

@retry

defsave_to_database(arg):

print("Writetoadatabaseormakeanetworkcalloretc.")

print("Thiswillbeautomaticallyretriedifexceptionisthrown.")

globalcounter

counter+=1

#Thiswillthrowanexceptioninthefirstcall

#Andwillworkfineinthesecondcall(i.e.aretry)

ifcounter<2:

raiseValueError(arg)

if__name__=='__main__':

save_to_database("Somebadvalue")

Output:

$pythonmore_decorator.py

Writetoadatabaseormakeanetworkcalloretc.

Thiswillbeautomaticallyretriedifexceptionisthrown.

ERROR:retry:Attempt1/5failed:(('Somebadvalue',),{})

Traceback(mostrecentcalllast):

File"more_decorator.py",line14,inwrapper_function

returnf(*args,**kwargs)

File"more_decorator.py",line39,insave_to_database

raiseValueError(arg)

ValueError:Somebadvalue

Writetoadatabaseormakeanetworkcalloretc.

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97

Thiswillbeautomaticallyretriedifexceptionisthrown.

HowItWorks

See:

Video:PythonDecoratorsMadeEasyhttp://www.ibm.com/developerworks/linux/library/l-cpdecor.htmlhttp://toumorokoshi.github.io/dry-principles-through-python-decorators.html

DifferencesbetweenPython2andPython3

See:

"Six"libraryPortingtoPython3ReduxbyArminPython3experiencebyPyDannyOfficialDjangoGuidetoPortingtoPython3DiscussiononWhataretheadvantagestopython3.x?

SummaryWehavecoveredsomemorefeaturesofPythoninthischapterandyetwehaven'tcoveredallthefeaturesofPython.However,atthisstage,wehavecoveredmostofwhatyouareevergoingtouseinpractice.Thisissufficientforyoutogetstartedwithwhateverprogramsyouaregoingtocreate.

Next,wewilldiscusshowtoexplorePythonfurther.

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98

WhatNextIfyouhavereadthisbookthoroughlytillnowandpracticedwritingalotofprograms,thenyoumusthavebecomecomfortableandfamiliarwithPython.YouhaveprobablycreatedsomePythonprogramstotryoutstuffandtoexerciseyourPythonskillsaswell.Ifyouhavenotdoneitalready,youshould.Thequestionnowis'WhatNext?'.

Iwouldsuggestthatyoutacklethisproblem:

Createyourowncommand-lineaddress-bookprogramusingwhichyoucanbrowse,add,modify,deleteorsearchforyourcontactssuchasfriends,familyandcolleaguesandtheirinformationsuchasemailaddressand/orphonenumber.Detailsmustbestoredforlaterretrieval.

Thisisfairlyeasyifyouthinkaboutitintermsofallthevariousstuffthatwehavecomeacrosstillnow.Ifyoustillwantdirectionsonhowtoproceed,thenhere'sahint .

Onceyouareabletodothis,youcanclaimtobeaPythonprogrammer.Now,immediatelysendmeanemailthankingmeforthisgreatbook;-).Thisstepisoptionalbutrecommended.Also,pleaseconsiderbuyingaprintedcopytosupportthecontinueddevelopmentofthisbook.

Ifyoufoundthatprogrameasy,here'sanotherone:

Implementthereplacecommand.Thiscommandwillreplaceonestringwithanotherinthelistoffilesprovided.

Thereplacecommandcanbeassimpleorassophisticatedasyouwish,fromsimplestringsubstitutiontolookingforpatterns(regularexpressions).

NextProjectsIfyoufoundaboveprogramseasytocreate,thenlookatthiscomprehensivelistofprojectsandtrywritingyourownprograms:https://github.com/thekarangoel/Projects#numbers(thesamelistisalsoatMartyr2'sMegaProjectList).

Alsosee:

ExercisesforProgrammers:57ChallengestoDevelopYourCodingSkillsIntermediatePythonProjects.

ExampleCodeThebestwaytolearnaprogramminglanguageistowritealotofcodeandreadalotofcode:

PythonCookbookisanextremelyvaluablecollectionofrecipesortipsonhowtosolvecertainkindsofproblemsusingPython.Thisisamust-readforeveryPythonuser.PythonModuleoftheWeekisanotherexcellentmust-readguidetotheStandardLibrary.

Advice

TheHitchhiker'sGuidetoPython!TheElementsofPythonStylePythonBigPicture"WritingIdiomaticPython"ebook(paid)

1

WhatNext

99

Videos

FullStackWebDevelopmentwithFlaskPyVideo

QuestionsandAnswers

OfficialPythonDosandDon'tsOfficialPythonFAQNorvig'slistofInfrequentlyAskedQuestionsPythonInterviewQ&AStackOverflowquestionstaggedwithpython

Tutorials

HiddenfeaturesofPythonWhat'stheonecodesnippet/pythontrick/etcdidyouwishyouknewwhenyoulearnedpython?Awaretek'scomprehensivelistofPythontutorials

Discussion

IfyouarestuckwithaPythonproblem,anddon'tknowwhomtoask,thenthepython-tutorlististhebestplacetoaskyourquestion.

Makesureyoudoyourhomeworkbytryingtosolvingtheproblemyourselffirstandasksmartquestions.

NewsIfyouwanttolearnwhatisthelatestintheworldofPython,thenfollowtheOfficialPythonPlanet.

InstallinglibrariesThereareahugenumberofopensourcelibrariesatthePythonPackageIndexwhichyoucanuseinyourownprograms.

Toinstallandusetheselibraries,youcanusepip.

CreatingaWebsite

LearnFlasktocreateyourownwebsite.Someresourcestogetstarted:

FlaskOfficialQuickstartTheFlaskMega-TutorialExampleFlaskProjects

GraphicalSoftware

WhatNext

100

SupposeyouwanttocreateyourowngraphicalprogramsusingPython.ThiscanbedoneusingaGUI(GraphicalUserInterface)librarywiththeirPythonbindings.BindingsarewhatallowyoutowriteprogramsinPythonandusethelibrarieswhicharethemselveswritteninCorC++orotherlanguages.

TherearelotsofchoicesforGUIusingPython:

Kivy

http://kivy.orgPyGTK

ThisisthePythonbindingfortheGTK+toolkitwhichisthefoundationuponwhichGNOMEisbuilt.GTK+hasmanyquirksinusagebutonceyoubecomecomfortable,youcancreateGUIappsfast.TheGladegraphicalinterfacedesignerisindispensable.Thedocumentationisyettoimprove.GTK+workswellonGNU/LinuxbutitsporttoWindowsisincomplete.YoucancreatebothfreeaswellasproprietarysoftwareusingGTK+.Togetstarted,readthePyGTKtutorial.

PyQt

ThisisthePythonbindingfortheQttoolkitwhichisthefoundationuponwhichtheKDEisbuilt.QtisextremelyeasytouseandverypowerfulespeciallyduetotheQtDesignerandtheamazingQtdocumentation.PyQtisfreeifyouwanttocreateopensource(GPL'ed)softwareandyouneedtobuyitifyouwanttocreateproprietaryclosedsourcesoftware.StartingwithQt4.5youcanuseittocreatenon-GPLsoftwareaswell.Togetstarted,readaboutPySide.

wxPython

ThisisthePythonbindingsforthewxWidgetstoolkit.wxPythonhasalearningcurveassociatedwithit.However,itisveryportableandrunsonGNU/Linux,Windows,Macandevenembeddedplatforms.TherearemanyIDEsavailableforwxPythonwhichincludeGUIdesignersaswellsuchasSPE(Stani'sPythonEditor)andthewxGladeGUIbuilder.YoucancreatefreeaswellasproprietarysoftwareusingwxPython.Togetstarted,readthewxPythontutorial.

SummaryofGUITools

Formorechoices,seetheGuiProgrammingwikipageattheofficialpythonwebsite.

Unfortunately,thereisnoonestandardGUItoolforPython.Isuggestthatyouchooseoneoftheabovetoolsdependingonyoursituation.ThefirstfactoriswhetheryouarewillingtopaytouseanyoftheGUItools.ThesecondfactoriswhetheryouwanttheprogramtorunonlyonWindowsoronMacandGNU/Linuxorallofthem.Thethirdfactor,ifGNU/Linuxisachosenplatform,iswhetheryouareaKDEorGNOMEuseronGNU/Linux.

Foramoredetailedandcomprehensiveanalysis,seePage26ofthe'ThePythonPapers,Volume3,Issue1'(PDF).

VariousImplementations

Thereareusuallytwopartsaprogramminglanguage-thelanguageandthesoftware.Alanguageishowyouwritesomething.Thesoftwareiswhatactuallyrunsourprograms.

WehavebeenusingtheCPythonsoftwaretorunourprograms.ItisreferredtoasCPythonbecauseitiswrittenintheClanguageandistheClassicalPythoninterpreter.

TherearealsoothersoftwarethatcanrunyourPythonprograms:

Jython

APythonimplementationthatrunsontheJavaplatform.ThismeansyoucanuseJavalibrariesandclassesfromwithinPythonlanguageandvice-versa.

IronPython

APythonimplementationthatrunsonthe.NETplatform.Thismeansyoucanuse.NETlibrariesandclassesfrom

WhatNext

101

withinPythonlanguageandvice-versa.PyPy

APythonimplementationwritteninPython!Thisisaresearchprojecttomakeitfastandeasytoimprovetheinterpretersincetheinterpreteritselfiswritteninadynamiclanguage(asopposedtostaticlanguagessuchasC,JavaorC#intheabovethreeimplementations)

TherearealsootherssuchasCLPython-aPythonimplementationwritteninCommonLispandBrythonwhichisanimplementationontopofaJavaScriptinterpreterwhichcouldmeanthatyoucanusePython(insteadofJavaScript)towriteyourweb-browser("Ajax")programs.

Eachoftheseimplementationshavetheirspecializedareaswheretheyareuseful.

FunctionalProgramming(foradvancedreaders)

Whenyoustartwritinglargerprograms,youshoulddefinitelylearnmoreaboutafunctionalapproachtoprogrammingasopposedtotheclass-basedapproachtoprogrammingthatwelearnedintheobject-orientedprogrammingchapter:

FunctionalProgrammingHowtobyA.M.KuchlingFunctionalprogrammingchapterin'DiveIntoPython'bookFunctionalProgrammingwithPythonpresentationFuncylibraryPyToolzlibrary

SummaryWehavenowcometotheendofthisbookbut,astheysay,thisisthethebeginningoftheend!.YouarenowanavidPythonuserandyouarenodoubtreadytosolvemanyproblemsusingPython.Youcanstartautomatingyourcomputertodoallkindsofpreviouslyunimaginablethingsorwriteyourowngamesandmuchmuchmore.So,getstarted!

.Createaclasstorepresenttheperson'sinformation.Useadictionarytostorepersonobjectswiththeirnameasthekey.Usethepicklemoduletostoretheobjectspersistentlyonyourharddisk.Usethedictionarybuilt-inmethodstoadd,deleteandmodifythepersons.↩

1

WhatNext

102

Appendix:FLOSSNOTE:Pleasenotethatthissectionwaswrittenin2003,sosomeofthiswillsoundquainttoyou:-)

"Free/LibreandOpenSourceSoftware",inshort,FLOSSisbasedontheconceptofacommunity,whichitselfisbasedontheconceptofsharing,andparticularlythesharingofknowledge.FLOSSarefreeforusage,modificationandredistribution.

Ifyouhavealreadyreadthisbook,thenyouarealreadyfamiliarwithFLOSSsinceyouhavebeenusingPythonallalongandPythonisanopensourcesoftware!

HerearesomeexamplesofFLOSStogiveanideaofthekindofthingsthatcommunitysharingandbuildingcancreate:

Linux:ThisisaFLOSSOSkernelusedintheGNU/Linuxoperatingsystem.Linux,thekernel,wasstartedbyLinusTorvaldsasastudent.AndroidisbasedonLinux.AnywebsiteyouusethesedayswillmostlyberunningonLinux.

Ubuntu:Thisisacommunity-drivendistribution,sponsoredbyCanonicalanditisthemostpopularGNU/Linuxdistributiontoday.ItallowsyoutoinstallaplethoraofFLOSSavailableandallthisinaneasy-to-useandeasy-to-installmanner.Bestofall,youcanjustrebootyourcomputerandrunGNU/LinuxofftheCD!ThisallowsyoutocompletelytryoutthenewOSbeforeinstallingitonyourcomputer.However,Ubuntuisnotentirelyfreesoftware;itcontainsproprietarydrivers,firmware,andapplications.

LibreOffice:Thisisanexcellentcommunity-drivenanddevelopedofficesuitewithawriter,presentation,spreadsheetanddrawingcomponentsamongotherthings.ItcanevenopenandeditMSWordandMSPowerPointfileswithease.Itrunsonalmostallplatformsandisentirelyfree,libreandopensourcesoftware.

MozillaFirefox:Thisisthebestwebbrowser.Itisblazinglyfastandhasgainedcriticalacclaimforitssensibleandimpressivefeatures.Theextensionsconceptallowsanykindofpluginstobeused.

Mono:ThisisanopensourceimplementationoftheMicrosoft.NETplatform.Itallows.NETapplicationstobecreatedandrunonGNU/Linux,Windows,FreeBSD,MacOSandmanyotherplatformsaswell.

Apachewebserver:Thisisthepopularopensourcewebserver.Infact,itisthemostpopularwebserverontheplanet!Itrunsnearlymorethanhalfofthewebsitesoutthere.Yes,that'sright-Apachehandlesmorewebsitesthanallthecompetition(includingMicrosoftIIS)combined.

VLCPlayer:ThisisavideoplayerthatcanplayanythingfromDivXtoMP3toOggtoVCDsandDVDsto...whosaysopensourceain'tfun?;-)

Thislistisjustintendedtogiveyouabriefidea-therearemanymoreexcellentFLOSSoutthere,suchasthePerllanguage,PHPlanguage,Drupalcontentmanagementsystemforwebsites,PostgreSQLdatabaseserver,TORCSracinggame,KDevelopIDE,Xine-themovieplayer,VIMeditor,Quanta+editor,Bansheeaudioplayer,GIMPimageeditingprogram,...Thislistcouldgoonforever.

TogetthelatestbuzzintheFLOSSworld,checkoutthefollowingwebsites:

OMG!Ubuntu!WebUpd8DistroWatchPlanetDebian

VisitthefollowingwebsitesformoreinformationonFLOSS:

GitHubExploreCodeTriageSourceForgeFreshMeat

Appendix:FLOSS

103

So,goaheadandexplorethevast,freeandopenworldofFLOSS!

Appendix:FLOSS

104

Appendix:ColophonAlmostallofthesoftwarethatIhaveusedinthecreationofthisbookareFLOSS.

BirthoftheBook

Inthefirstdraftofthisbook,IhadusedRedHat9.0Linuxasthefoundationofmysetupandinthesixthdraft,IusedFedoraCore3Linuxasthebasisofmysetup.

Initially,IwasusingKWordtowritethebook(asexplainedinthehistorylesson).

TeenageYears

Later,IswitchedtoDocBookXMLusingKatebutIfoundittootedious.So,IswitchedtoOpenOfficewhichwasjustexcellentwiththelevelofcontrolitprovidedforformattingaswellasthePDFgeneration,butitproducedverysloppyHTMLfromthedocument.

Finally,IdiscoveredXEmacsandIrewrotethebookfromscratchinDocBookXML(again)afterIdecidedthatthisformatwasthelongtermsolution.

Inthesixthdraft,IdecidedtouseQuanta+todoalltheediting.ThestandardXSLstylesheetsthatcamewithFedoraCore3Linuxwerebeingused.However,IhadwrittenaCSSdocumenttogivecolorandstyletotheHTMLpages.Ihadalsowrittenacrudelexicalanalyzer,inPythonofcourse,whichautomaticallyprovidessyntaxhighlightingtoalltheprogramlistings.

Fortheseventhdraft,IwasusingMediaWikiasthebasisofmysetup.Iusedtoediteverythingonlineandthereaderscandirectlyread/edit/discusswithinthewikiwebsite,butIendedupspendingmoretimefightingspamthanwriting.

Fortheeightdraft,IusedVim,Pandoc,andMacOSX.

Fortheninthdraft,IswitchedtoAsciiDocformatandusedEmacs24.3,tomorrowtheme,FiraMonofontandadoc-modetowrite.

Now

2016:IgottiredofseveralminorrenderingissuesinAsciiDoctor,likethe ++in C/C++woulddisappearanditwashardtokeeptrackofescapingsuchminorthings.Plus,IhadbecomereluctanttoeditthetextbecauseofthecomplexAsciidocformat.

Forthetenthdraft,IswitchedtowritinginMarkdown+GitBookformat,usingtheSpacemacseditor.

Nov2020:SinceGitbookabandonedtheiropensourcesoftware,migratedtoHonkit,acommunity-maintainedforkofGitbooklegacy.

AbouttheAuthor

Seehttps://swaroopch.com/about

Appendix:About

105

Appendix:HistoryLessonIfirststartedwithPythonwhenIneededtowriteaninstallerforsoftwareIhadwrittencalled'Diamond'sothatIcouldmaketheinstallationeasy.IhadtochoosebetweenPythonandPerlbindingsfortheQtlibrary.IdidsomeresearchonthewebandIcameacrossanarticlebyEricS.Raymond,afamousandrespectedhacker,wherehetalkedabouthowPythonhadbecomehisfavoriteprogramminglanguage.IalsofoundoutthatthePyQtbindingsweremorematurecomparedtoPerl-Qt.So,IdecidedthatPythonwasthelanguageforme.

Then,IstartedsearchingforagoodbookonPython.Icouldn'tfindany!IdidfindsomeO'Reillybooksbuttheywereeithertooexpensiveorweremorelikeareferencemanualthanaguide.So,IsettledforthedocumentationthatcamewithPython.However,itwastoobriefandsmall.ItdidgiveagoodideaaboutPythonbutwasnotcomplete.ImanagedwithitsinceIhadpreviousprogrammingexperience,butitwasunsuitablefornewbies.

AboutsixmonthsaftermyfirstbrushwithPython,Iinstalledthe(then)latestRedHat9.0LinuxandIwasplayingaroundwithKWord.IgotexcitedaboutitandsuddenlygottheideaofwritingsomestuffonPython.Istartedwritingafewpagesbutitquicklybecame30pageslong.Then,Ibecameseriousaboutmakingitmoreusefulinabookform.Afteralotofrewrites,ithasreachedastagewhereithasbecomeausefulguidetolearningthePythonlanguage.Iconsiderthisbooktobemycontributionandtributetotheopensourcecommunity.

ThisbookstartedoutasmypersonalnotesonPythonandIstillconsideritinthesameway,althoughI'vetakenalotofefforttomakeitmorepalatabletoothers:)

Inthetruespiritofopensource,Ihavereceivedlotsofconstructivesuggestions,criticismsandfeedbackfromenthusiasticreaderswhichhashelpedmeimprovethisbookalot.

StatusOfTheBookThebookneedsthehelpofitsreaderssuchasyourselvestopointoutanypartsofthebookwhicharenotgood,notcomprehensibleoraresimplywrong.Pleasewritetothemainauthorortherespectivetranslatorswithyourcommentsandsuggestions.

Appendix:RevisionHistoryNoversionchange

06Nov2020MigratedfromabandonedGitBooktocommunity-maintainedHonkit,aforkofGitBooklegacy

4.0

19Jan2016SwitchedbacktoPython3SwitchedbacktoMarkdown,usingGitBookandSpacemacs

3.0

31Mar2014RewrittenforPython2usingAsciiDocandadoc-mode.

2.1

03Aug2013RewrittenusingMarkdownandJasonBlevins'MarkdownMode

2.0

Appendix:RevisionHistory

106

20Oct2012RewritteninPandocformat,thankstomywifewhodidmostoftheconversionfromtheMediawikiformatSimplifyingtext,removingnon-essentialsectionssuchas nonlocalandmetaclasses

1.90

04Sep2008andstillinprogressRevivalafteragapof3.5years!RewritingforPython3.0Rewriteusinghttp://www.mediawiki.org[MediaWiki](again)

1.20

13Jan2005CompleterewriteusingQuanta+onFedoraCore3withlotofcorrectionsandupdates.Manynewexamples.RewrotemyDocBooksetupfromscratch.

1.15

28Mar2004Minorrevisions

1.12

16Mar2004Additionsandcorrections

1.10

09Mar2004Moretypocorrections,thankstomanyenthusiasticandhelpfulreaders.

1.00

08Mar2004Aftertremendousfeedbackandsuggestionsfromreaders,Ihavemadesignificantrevisionstothecontentalongwithtypocorrections.

0.99

22Feb2004Addedanewchapteronmodules.Addeddetailsaboutvariablenumberofargumentsinfunctions.

0.98

16Feb2004WroteaPythonscriptandCSSstylesheettoimproveXHTMLoutput,includingacrude-yet-functionallexicalanalyzerforautomaticVIM-likesyntaxhighlightingoftheprogramlistings.

0.97

13Feb2004Anothercompletelyrewrittendraft,inDocBookXML(again).Bookhasimprovedalot-itismorecoherentandreadable.

0.93

25Jan2004AddedIDLEtalkandmoreWindows-specificstuff

0.92

05Jan2004Changestofewexamples.

0.91

30Dec2003Correctedtypos.Improvisedmanytopics.

Appendix:RevisionHistory

107

0.90

18Dec2003Added2morechapters.OpenOfficeformatwithrevisions.

0.60

21Nov2003Fullyrewrittenandexpanded.

0.20

20Nov2003Correctedsometyposanderrors.

0.15

20Nov2003ConvertedtoDocBookXMLwithXEmacs.

0.10

14Nov2003InitialdraftusingKWord.

Appendix:RevisionHistory

108

TranslationsTherearemanytranslationsofthebookavailableindifferenthumanlanguages,thankstomanytirelessvolunteers!

Ifyouwanttohelpwiththesetranslations,pleaseseethelistofvolunteersandlanguagesbelowanddecideifyouwanttostartanewtranslationorhelpinexistingtranslationprojects.

Ifyouplantostartanewtranslation,pleasereadtheTranslationhow-to.

ArabicBelowisthelinkfortheArabicversion.ThankstoAshrafAliKhalaffortranslatingthebook,youcanreadthewholebookonlineathttp://www.khaledhosny.org/byte-of-python/index.htmloryoucandownloaditfromsourceforge.netformoreinfoseehttp://itwadi.com/byteofpython_arabi.

AzerbaijaniJahangirShabiyev(c.shabiev@gmail.com)hasvolunteeredtotranslatethebooktoAzerbaijani.Thetranslationisinprogressathttps://www.gitbook.com/book/jahangir-sh/piton-sancmasi

BrazilianPortugueseTherearetwotranslationsinvariouslevelsofcompletionandaccessibility.Theoldertranslationisnowmissing/lost,andnewertranslationisincomplete.

SamuelDiasNeto(samuel.arataca@gmail.com)madethefirstBrazilianPortuguesetranslation(oldertranslation)ofthisbookwhenPythonwasin2.3.5version.Thisisnolongerpubliclyaccessible.

RodrigoAmaral(rodrigoamaral@gmail.com)hasvolunteeredtotranslatethebooktoBrazilianPortuguese,(newertranslation)whichstillremainstobecompleted.

CatalanMoisesGomez(moisesgomezgiron@gmail.com)hasvolunteeredtotranslatethebooktoCatalan.Thetranslationisinprogress.

MoisèsGómez-Iamadeveloperandalsoateacherofprogramming(normallyforpeoplewithoutanypreviousexperience).

SometimeagoIneededtolearnhowtoprograminPython,andSwaroop'sworkwasreallyhelpful.Clear,concise,andcompleteenough.JustwhatIneeded.

Afterthisexperience,Ithoughtsomeotherpeopleinmycountrycouldtakebenefitfromittoo.ButEnglishlanguagecanbeabarrier.

So,whynottrytotranslateit?AndIdidforapreviousversionofBoP.

Imycountrytherearetwoofficiallanguages.IselectedtheCatalanlanguageassumingthatotherswilltranslateittothemorewidespreadSpanish.

Chinese

Appendix:Translations

109

In2017whichisafter11years,MoLun(i@molun.net)re-translatedthebookfromthebeginningbasedonVersion4.0.AndthetranslationisstoragedinGitHubandGitbook.HeiskeepingfollowthistranslatededitionandreadytofixitifthereisanywrongormistakeinthetranslatedBoP.

The2017translationeditionisavailableinhttps://bop.molun.net.

MoLunSays:

IamacommonjournalismstudentfromCYU,Beijing.Andactually,IamanabsolutenewbieinPythonprogrammingwhenIstarttotranslatethisbook.Initially,itwasjustawhim,butwhenIdonethiswork,Irealizedthatadecisiontriggeredbyinteresthadpromptedmetogosofar.

Withthehelpofmypredecessors’translationsandthevastamountofinformationprovidedbythedevelopedInternet,andwiththehelpofmyfriends,Iprudentlypresentedthistranslationedition.IjusthopemytranslationworkwillhelpothernewcomersinlearningPython.

Atthesametime,Iamalwayswaitingformytranslationofthecommentsandsuggestions,andreadytochangeorimprovethissuperficialwork.

EarlierChinesetranslation

In2005,ShenJieyuantranslatedthisbookwithversion1.20toChineseandpublishedittotheInternet.ThisisthefirstChineseedition.InBoPofficialsite,hewascalledJuanShen,withEmailadderssorion_val@163.comThiseditionhasbeendisseminatedonthenetworkwidely,andthelinksprovidedbyBoPofficialsitearenotavailableanymore,sothatitsoriginalsourceisunabletofind.Thereforeinherecan’tprovideacertainaddress.Butyoucantrytosearchkeywordslike“Python”tofindacopy.

JuanShensays:

IamapostgraduateatWirelessTelecommunicationGraduateSchool,BeijingUniversityofTechnology,ChinaPR.Mycurrentresearchinterestisonthesynchronization,channelestimationandmulti-userdetectionofmulticarrierCDMAsystem.Pythonismymajorprogramminglanguagefordailysimulationandresearchjob,withthehelpofPythonNumeric,actually.IlearnedPythonjusthalfayearbefore,butasyoucansee,it'sreallyeasy-understanding,easy-to-useandproductive.JustaswhatisensuredinSwaroop'sbook,'It'smyfavoriteprogramminglanguagenow'.

'AByteofPython'ismytutorialtolearnPython.It'sclearandeffectivetoleadyouintoaworldofPythonintheshortesttime.It'snottoolong,butefficientlycoversalmostallimportantthingsinPython.Ithink'AByteofPython'shouldbestronglyrecommendablefornewbiesastheirfirstPythontutorial.JustdedicatemytranslationtothepotentialmillionsofPythonusersinChina.

ChineseTraditional

FredLin(gasolin@gmail.com)hasvolunteeredtotranslatethebooktoChineseTraditional.

Itisavailableathttp://code.google.com/p/zhpy/wiki/ByteOfZhpy.

Anexcitingfeatureofthistranslationisthatitalsocontainstheexecutablechinesepythonsourcessidebysidewiththeoriginalpythonsources.

FredLin-I'mworkingasanetworkfirmwareengineeratDeltaNetwork,andI'malsoacontributorofTurboGearswebframework.

Asapythonevangelist(:-p),Ineedsomematerialtopromotepythonlanguage.Ifound'AByteofPython'hitthesweetpointforbothnewbiesandexperiencedprogrammers.'AByteofPython'elaboratesthepythonessentialswithaffordablesize.

Thetranslationareoriginallybasedonsimplifiedchineseversion,andsoonalotofrewriteweremadetofitthecurrentwikiversionandthequalityofreading.

Appendix:Translations

110

Therecentchinesetraditionalversionalsofeaturedwithexecutablechinesepythonsources,whichareachievedbymynew'zhpy'(pythoninchinese)project(launchfromAug07).

zhpy(pronounce(Z.H.?,orzippy)buildalayeruponpythontotranslateorinteractwithpythoninchinese(TraditionalorSimplified).Thisprojectismainlyaimedforeducation.

French

Gregory(coulix@ozforces.com.au)hasvolunteeredtotranslatethebooktoFrench.

GérardLabadie(gerard.labadie@gmail.com)hascompletedtotranslatethebooktoFrench.

Thistranslationwaslaterportedtothemarkdownformat,updatedtomatchthelastversionofthebook,andpublishedonGitBookbyRomainGilliotte(rgilliotte@gmail.com).

Itcanbefoundathttps://rgilliotte.gitbook.io/byte-of-python/

GermanLutzHorn(lutz.horn@gmx.de),BerndHengelein(bernd.hengelein@gmail.com)andChristophZwerschke(cito@online.de)havevolunteeredtotranslatethebooktoGerman.

Translationcanbefoundathttp://cito.github.io/byte_of_python/

LutzHornsays:

I'm32yearsoldandhaveadegreeofMathematicsfromUniversityofHeidelberg,Germany.CurrentlyI'mworkingasasoftwareengineeronapubliclyfundedprojecttobuildawebportalforallthingsrelatedtocomputerscienceinGermany.ThemainlanguageIuseasaprofessionalisJava,butItrytodoasmuchaspossiblewithPythonbehindthescenes.EspeciallytextanalysisandconversionisveryeasywithPython.I'mnotveryfamiliarwithGUItoolkits,sincemostofmyprogrammingisaboutwebapplications,wheretheuserinterfaceisbuildusingJavaframeworkslikeStruts.CurrentlyItrytomakemoreuseofthefunctionalprogrammingfeaturesofPythonandofgenerators.AftertakingashortlookintoRuby,Iwasveryimpressedwiththeuseofblocksinthislanguage.GenerallyIlikethedynamicnatureoflanguageslikePythonandRubysinceitallowsmetodothingsnotpossibleinmorestaticlanguageslikeJava.I'vesearchedforsomekindofintroductiontoprogramming,suitabletoteachacompletenon-programmer.I'vefoundthebook'HowtoThinkLikeaComputerScientist:LearningwithPython',and'DiveintoPython'.Thefirstisgoodforbeginnersbuttolongtotranslate.Thesecondisnotsuitableforbeginners.Ithink'AByteofPython'fallsnicelybetweenthese,sinceitisnottoolong,writtentothepoint,andatthesametimeverboseenoughtoteachanewbie.Besidesthis,IlikethesimpleDocBookstructure,whichmakestranslatingthetextagenerationtheoutputinvariousformatsacharm.

BerndHengeleinsays:

Lutzandmearegoingtodothegermantranslationtogether.Wejuststartedwiththeintroandprefacebutwewillkeepyouinformedabouttheprogresswemake.Ok,nowsomepersonalthingsaboutme.Iam34yearsoldandplayingwithcomputerssincethe1980's,whenthe"CommodoreC64"ruledthenurseries.AfterstudyingcomputerscienceIstartedworkingasasoftwareengineer.CurrentlyIamworkinginthefieldofmedicalimagingforamajorgermancompany.AlthoughC++isthemainlanguageI(haveto)useformydailywork,Iamconstantlylookingfornewthingstolearn.LastyearIfellinlovewithPython,whichisawonderfullanguage,bothforitspossibilitiesanditsbeauty.Ireadsomewhereinthenetaboutaguywhosaidthathelikespython,becausethecodelookssobeautiful.Inmyopinionhe'sabsolutlyright.AtthetimeIdecidedtolearnpython,Inoticedthatthereisverylittlegooddocumentationingermanavailable.WhenIcameacrossyourbookthespontaneousideaofagermantranslationcrossedmymind.Luckily,Lutzhadthesameideaandwecannowdividethework.Iamlookingforwardtoagoodcooperation!

Appendix:Translations

111

Greek

TheGreekUbuntuCommunitytranslatedthebookinGreek,foruseinouron-lineasynchronousPythonlessonsthattakeplaceinourforums.Contact@savvasradevicformoreinformation.

Indonesian

Daniel(daniel.mirror@gmail.com)istranslatingthebooktoIndonesianathttp://python.or.id/moin.cgi/ByteofPython.

WisnuPriyambodo(cibermen@gmail.com)alsohasvolunteeredtotranslatethebooktoIndonesian.

Also,BagusAjiSantoso(baguzzzaji@gmail.com)hasvolunteered.

Italian(first)

EnricoMorelli(mr.mlucci@gmail.com)andMassimoLucci(morelli@cerm.unifi.it)havevolunteeredtotranslatethebooktoItalian.

TheItaliantranslationispresentathttp://www.gentoo.it/Programmazione/byteofpython.

MassimoLucciandEnricoMorelli-weareworkingattheUniversityofFlorence(Italy)-ChemistryDepartment.I(Massimo)asserviceengineerandsystemadministratorforNuclearMagneticResonanceSpectrometers;EnricoasserviceengineerandsystemadministratorforourCEDandparallel/clusteredsystems.Weareprogrammingonpythonsinceaboutsevenyears,wehadexperienceworkingwithLinuxplatformssincetenyears.InItalyweareresponsibleandadministratorforwww.gentoo.itwebsiteforGentoo/Linuxdistrubutionandwww.nmr.it(nowunderconstruction)forNuclearMagneticResonanceapplicationsandCongressOrganizationandManagements.That'sall!WeareimpressedbythesmartlanguageusedonyourBookandwethinkthisisessentialforapproachingthePythontonewusers(wearethinkingabouthundredofstudentsandresearcherworkingonourlabs).

Italian(second)

AnItaliantranslationhasbeencreatedbyCalvinaBice&colleaguesathttp://besthcgdropswebsite.com/translate/a-byte-of-python/.

Japanese

ShunroDozono(dozono@gmail.com)istranslatingthebooktoJapanese.

Korean

Epsimatt(2019)

EpsimatthasstartedanewKoreantranslation:

Readonlineathttps://epsimatt.gitbook.io/byte-of-python/Followprogressathttps://github.com/epsimatt/byte-of-python/issues/16

Older

JeongbinPark(pjb7687@gmail.com)hastranslatedthebooktoKorean-https://github.com/pjb7687/byte_of_python

Appendix:Translations

112

IamJeongbinPark,currentlyworkingasaBiophysics&BioinformaticsresearcherinKorea.

Ayearago,Iwaslookingforagoodtutorial/guideforPythontointroduceittomycolleagues,becauseusingPythoninsuchresearchfieldsisbecominginevitableduetotheuserbaseisgrowingmoreandmore.

ButatthattimeonlyfewPythonbooksareavailableinKorean,soIdecidedtotranslateyourebookbecauseitlookslikeoneofthebestguidesthatIhaveeverread!

Currently,thebookisalmostcompletelytranslatedinKorean,exceptsomeofthetextinintroductionchapterandtheappendixes.

Thankyouagainforwritingsuchagoodguide!

MongolianAriunsanaaTunjin(luftballons2010@gmail.com)hasvolunteeredtotranslatethebooktoMongolian.

UpdateonNov22,2009:Ariunsanaaisonthevergeofcompletingthetranslation.

Norwegian(bokmål)

EirikVågeskarisahighschoolstudentatSandvikavideregåendeskoleinNorway,abloggerandcurrentlytranslatingthebooktoNorwegian(bokmål).

EirikVågeskar:Ihavealwayswantedtoprogram,butbecauseIspeakasmalllanguage,thelearningprocesswasmuchharder.MosttutorialsandbooksarewritteninverytechnicalEnglish,somosthighschoolgraduateswillnotevenhavethevocabularytounderstandwhatthetutorialisabout.WhenIdiscoveredthisbook,allmyproblemsweresolved."AByteofPython"usedsimplenon-technicallanguagetoexplainaprogramminglanguagethatisjustassimple,andthesetwothingsmakelearningPythonfun.Afterreadinghalfofthebook,Idecidedthatthebookwasworthtranslating.Ihopethetranslationwillhelppeoplewhohavefoundthemselfinthesamesituationasme(especiallyyoungpeople),andmaybehelpspreadinterestforthelanguageamongpeoplewithlesstechnicalknowledge.

Polish

DominikKozaczko(dominik@kozaczko.info)hasvolunteeredtotranslatethebooktoPolish.Translationisinprogressandit'smainpageisavailablehere:UkąśPythona.

Update:ThetranslationiscompleteandreadyasofOct2,2009.ThankstoDominik,histwostudentsandtheirfriendfortheirtimeandeffort!

DominikKozaczko-I'maComputerScienceandInformationTechnologyteacher.

Portuguese

ArturWeber(arturweberguimaraes@gmail.com)hascompletedatranslationofthisbooktoPortuguese(asofFeb21,2018)athttps://www.homeyou.com/~edu/introducao.

ArturWeber:MystudentsstudyatthePolytechnicfacultyinEcologicalUniversityinthecityofCuritiba(Brazil)andsomeofthemareinterestedindifferentpapers.

Astheyarewritingcourseandacademicpapers,theyalwayslookforinterestingarticlesandpages.Ialsodomybesttofindinterestingmaterialswhichcanbethesourcesfortheiruniversityworks.

Appendix:Translations

113

IfoundthematerialsfromyoursiteusefulforsomeofmystudentswhoarewritingpapersbasedonprogrammingonPython.Actually,thatiswhyImadeadecisiontoperformaPortuguesetranslationtoletmystudentswhodon'tknowEnglishreadexcitingarticlesintheirmothertongue(inPortuguese).

Romanian

Paul-SebastianManole(brokenthorn@gmail.com)hasvolunteeredtotranslatethisbooktoRomanian.

Paul-SebastianManole-I'masecondyearComputerSciencestudentatSpiruHaretUniversity,hereinRomania.I'mmoreofaself-taughtprogrammeranddecidedtolearnanewlanguage,Python.Thewebtoldmetherewasnobetterwaytodosobutread''AByteofPython''.That'showpopularthisbookis(congratulationstotheauthorforwritingsuchaneasytoreadbook).IstartedlikingPythonsoIdecidedtohelptranslatethelatestversionofSwaroop'sbookinRomanian.AlthoughIcouldbetheonewiththefirstinitiative,I'mjustonevolunteersoifyoucanhelp,pleasejoinme.

Russian

VladimirSmolyar(v_2e@ukr.net)hascompletedaRussiantranslationathttp://wombat.org.ua/AByteOfPython/.

Ukranian

AverkievAndrey(averkiyev@ukr.net)hasvolunteeredtotranslatethebooktoRussian,andperhapsUkranian(timepermitting).

Serbian

"BugSpice"(amortizerka@gmail.com)hascompletedaSerbiantranslation:

Thisdownloadlinkisnolongeraccessible.

Moredetailsathttp://forum.ubuntu-rs.org/Thread-zagrljaj-pitona.

Slovak

AlbertioWard(albertioward@gmail.com)hastranslatedthebooktoSlovakathttp://www.fatcow.com/edu/python-swaroopch-sl/:

Weareanon-profitorganizationcalled"Translationforeducation".Werepresentagroupofpeople,mainlystudentsandprofessors,oftheSlavonicUniversity.Herearestudentsfromdifferentdepartments:linguistics,chemistry,biology,etc.WetrytofindinterestingpublicationsontheInternetthatcanberelevantforusandouruniversitycolleagues.Sometimeswefindarticlesbyourselves;othertimesourprofessorshelpuschoosethematerialfortranslation.Afterobtainingpermissionfromauthorswetranslatearticlesandposttheminourblogwhichisavailableandaccessibletoourcolleaguesandfriends.Thesetranslatedpublicationsoftenhelpstudentsintheirdailystudyroutine.

SpanishAlfonsodelaGuardaReyes(alfonsodg@ictechperu.net),GustavoEcheverria(gustavo.echeverria@gmail.com),DavidCrespoArroyo(davidcrespoarroyo@hotmail.com)andCristianBermudezSerna(crisbermud@hotmail.com)havevolunteeredtotranslatethebooktoSpanish.

GustavoEcheverriasays:

Appendix:Translations

114

IworkasasoftwareengineerinArgentina.IusemostlyC#and.NettechnologiesatworkbutstrictlyPythonorRubyinmypersonalprojects.IknewPythonmanyyearsagoandIgotstuckinmediately.NotsolongafterknowingPythonIdiscoveredthisbookandithelpedmetolearnthelanguage.ThenIvolunteeredtotranslatethebooktoSpanish.Now,afterreceivingsomerequests,I'vebeguntotranslate"AByteofPython"withthehelpofMaximilianoSoler.

CristianBermudezSernasays:

IamstudentofTelecommunicationsengineeringattheUniversityofAntioquia(Colombia).Monthsago,istartedtolearnPythonandfoundthiswonderfulbook,soivolunteeredtogettheSpanishtranslation.

SwedishMikaelJacobsson(leochingkwake@gmail.com)hasvolunteeredtotranslatethebooktoSwedish.

TurkishTürkerSEZER(tsezer@btturk.net)andBugraCakir(bugracakir@gmail.com)havevolunteeredtotranslatethebooktoTurkish."WhereisTurkishversion?Bitsedeokusak."

Appendix:Translations

115

TranslationHow-to1. Thefullsourceofthebookisavailablefromhttps://github.com/swaroopch/byte-of-python.2. Pleaseforktherepository.3. Then,fetchtherepositorytoyourcomputer.YouneedtoknowhowtouseGittodothat.4. ReadtheHonkitdocumentation,esp.theMarkdownsection.5. Starteditingthe .mdfilestotranslatetoyourlocallanguage.6. SeeINSTALL.mdonhowtogeneratethewebsite,PDF,EPUB.

Appendix:TranslationHow-to

116

FeedbackThebookneedsthehelpofitsreaderssuchasyourselvestopointoutanypartsofthebookwhicharenotgood,notcomprehensibleoraresimplywrong.Pleasewritetothemainauthorortherespectivetranslatorswithyourcommentsandsuggestions.

Feedback

117