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.

ForPythonversion3ThisbookwillteachyoutousePythonversion3.TherewillalsobeguidanceforyoutoadapttotheolderandmorecommonPythonversion2inthebook.

WhoreadsAByteofPython?Herearewhatpeoplearesayingaboutthebook:

Thisisthebestbeginner'stutorialI'veeverseen!Thankyouforyoureffort.--WaltMichalik

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

Excellentgentleintroductiontoprogramming#Pythonforbeginners--ShanRajasekaran

Bestnewbieguidetopython--NicksonKaigi

starttolovepythonwitheverysinglepageread--HerbertFeutl

perfectbeginnersguideforpython,willgiveukeytounlockmagicalworldofpython--Dilip

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

RecentlystartedreadingaByteofpython.Awesomework.Andthattooforfree.Highlyrecommendedforaspiringpythonistas.--Mangesh

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

Introduction

2

EnjoyingReading#ByteOfPythonby@swaroopchbestbookever--YuvrajSharma

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

Introduction

3

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

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

Introduction

4

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

Introduction

5

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.YourbookwassowellputtogetherthatIfoundverylittleinitIcouldn'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

Introduction

6

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.

AcademicCoursesThisbookis/wasbeingusedasinstructionalmaterialinvariouseducationalinstitutions:

'PrinciplesofProgrammingLanguages'courseatVrijeUniversiteit,Amsterdam'BasicConceptsofComputing'courseatUniversityofCalifornia,Davis'ProgrammingWithPython'courseatHarvardUniversity'IntroductiontoProgramming'courseatUniversityofLeeds

Introduction

7

'IntroductiontoApplicationProgramming'courseatBostonUniversity'InformationTechnologySkillsforMeteorology'courseatUniversityofOklahoma'Geoprocessing'courseatMichiganStateUniversity'MultiAgentSemanticWebSystems'courseattheUniversityofEdinburgh'IntroductiontoComputerScienceandProgramming'atMITOpenCourseWare'BasicprogrammingattheFacultyofSocialSciences,UniversityofLjubljana,Slovenia'--AlešŽibernasays"I(andmypredecessor)havebeenusingyourbookasthemainliteratureforthiscourse"'Introductiontoprogramming',DepartmentofInformationSciences,UniversityofZadar,Croatia--KrešimirZaudersays"IwouldliketoinformyouthatAByteofPythonisamandatoryreadatmycourse"

LicenseThisbookislicensedunderaCreativeCommonsAttribution-ShareAlike4.0InternationalLicense.

Thismeans:

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

Pleasenote:

Pleasedonotsellelectronicorprintedcopiesofthebookunlessyouhaveclearlyandprominentlymentionedinthedescriptionthatthesecopiesarenotfromtheoriginalauthorofthisbook.Attributionmustbeshownintheintroductorydescriptionandfrontpageofthedocumentbylinkingbacktohttp://python.swaroopch.com/andclearlyindicatingthattheoriginaltextcanbefetchedfromthislocation.Allthecode/scriptsprovidedinthisbookislicensedunderthe3-clauseBSDLicenseunlessotherwisenoted.

ReadNowYoucanreaditonlineathttp://python.swaroopch.com/

BuyTheBook

Introduction

8

Aprintedhardcopyofthebookcanbepurchasedathttp://www.swaroopch.com/buybook/foryourofflinereadingpleasure,andtosupportthecontinueddevelopmentandimprovementofthisbook.

DownloadVisithttps://www.gitbook.com/book/swaroopch/byte-of-python/detailsforthefollowingtypesofdownloads:

PDF(fordesktopreading,etc.)EPUB(foriPhone/iPad,ebookreaders,etc.)Mobi(forKindle)

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

ReadthebookinyournativelanguageIfyouareinterestedinreadingorcontributingtranslationsofthisbooktootherhumanlanguages,pleaseseeTranslations.

Introduction

9

DedicationToKalyanVarmaandmanyotherseniorsatPESITwhointroducedustoGNU/Linuxandtheworldofopensource.

TothememoryofAtulChitnis,afriendandguidewhoshallbemissedgreatly.

TothepioneerswhomadetheInternethappen.Thisbookwasfirstwrittenin2003.Itstillremainspopular,thankstothenatureofsharingknowledgeontheInternetasenvisionedbythepioneers.

Dedication

10

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

WhoThisBookIsForThisbookservesasaguideortutorialtothePythonprogramminglanguage.Itismainlytargetedatnewbies.Itisusefulforexperiencedprogrammersaswell.

Theaimisthatifallyouknowaboutcomputersishowtosavetextfiles,thenyoucanlearnPythonfromthisbook.Ifyouhavepreviousprogrammingexperience,thenyoucanalsolearnPythonfromthisbook.

Ifyoudohavepreviousprogrammingexperience,youwillbeinterestedinthedifferencesbetweenPythonandyourfavoriteprogramminglanguage-Ihavehighlightedmanysuchdifferences.Alittlewarningthough,Pythonissoongoingtobecomeyourfavoriteprogramminglanguage!

OfficialWebsiteTheofficialwebsiteofthebookishttp://python.swaroopch.com/whereyoucanreadthewholebookonline,downloadthelatestversionsofthebook,buyaprintedhardcopyandalsosendmefeedback.

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

Successinlifeisamatternotsomuchoftalentandopportunityasofconcentrationandperseverance.--C.W.Wendte

Preface

11

Preface

12

AboutPythonPythonisoneofthoserarelanguageswhichcanclaimtobebothsimpleandpowerful.Youwillfindyourselfpleasantlysurprisedtoseehoweasyitistoconcentrateonthesolutiontotheproblemratherthanthesyntaxandstructureofthelanguageyouareprogrammingin.

TheofficialintroductiontoPythonis:

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

Iwilldiscussmostofthesefeaturesinmoredetailinthenextsection.

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

FeaturesofPython

Simple

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

EasytoLearn

Asyouwillsee,Pythonisextremelyeasytogetstartedwith.Pythonhasanextraordinarilysimplesyntax,asalreadymentioned.

FreeandOpenSource

AboutPython

13

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

High-levelLanguage

WhenyouwriteprogramsinPython,youneverneedtobotheraboutthelow-leveldetailssuchasmanagingthememoryusedbyyourprogram,etc.

Portable

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

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!

AboutPython

14

ObjectOriented

Pythonsupportsprocedure-orientedprogrammingaswellasobject-orientedprogramming.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

15

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

FordetailsondifferencesbetweenPython2andPython3,see:

ThefutureofPython2PortingPython2CodetoPython3WritingcodethatrunsunderbothPython2and3SupportingPython3:Anin-depthguide

WhatProgrammersSayYoumayfinditinterestingtoreadwhatgreathackerslikeESRhavetosayaboutPython:

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

AboutPython

16

InstallationWhenwereferto"Python3"inthisbook,wewillbereferringtoanyversionofPythonequaltoorgreaterthanversionPython3.5.1.

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

CAUTION:MakesureyoucheckoptionAddPython3.5toPATH.

Tochangeinstalllocation,clickonCustomizeinstallation,thenNextandenterC:\python35asinstalllocation.

Ifnotchecked,checkAddPythontoenvironmentvariables.ThisdoesthesamethingasAddPython3.5toPATHonthefirstinstallscreen.

YoucanchoosetoinstallLauncherforallusersornot,itdoesnotmattermuch.LauncherisusedtoswitchbetweendifferentversionsofPythoninstalled.

Ifyourpathwasnotsetcorrectly,thenfollowthesestepstofixit.Otherwise,gotoRunningPythonpromptonWindows.

NOTE:Forpeoplewhoalreadyknowprogramming,ifyouarefamiliarwithDocker,checkoutPythoninDockerandDockeronWindows.

DOSPrompt

IfyouwanttobeabletousePythonfromtheWindowscommandlinei.e.theDOSprompt,thenyouneedtosetthePATHvariableappropriately.

ForWindows2000,XP,2003,clickonControlPanel->System->Advanced->EnvironmentVariables.ClickonthevariablenamedPATHintheSystemVariablessection,thenselectEditandadd;C:\Python35(pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatedirectoryname.

ForolderversionsofWindows,openthefileC:\AUTOEXEC.BATandaddthelinePATH=%PATH%;C:\Python35andrestartthesystem.ForWindowsNT,usetheAUTOEXEC.NTfile.

Installation

17

ForWindowsVista:

ClickStartandchooseControlPanelClickSystem,ontherightyou'llsee"Viewbasicinformationaboutyourcomputer"Ontheleftisalistoftasks,thelastofwhichisAdvancedsystemsettings.Clickthat.TheAdvancedtaboftheSystemPropertiesdialogboxisshown.ClicktheEnvironmentVariablesbuttononthebottomright.InthelowerboxtitledSystemVariablesscrolldowntoPathandclicktheEditbutton.Changeyourpathasneedbe.Restartyoursystem.Vistadidn'tpickupthesystempathenvironmentvariablechangeuntilIrestarted.

ForWindows7and8:

RightclickonComputerfromyourdesktopandselectPropertiesorclickStartandchooseControlPanel->SystemandSecurity->System.ClickonAdvancedsystemsettingsontheleftandthenclickontheAdvancedtab.AtthebottomclickonEnvironmentVariablesandunderSystemvariables,lookforthePATHvariable,selectandthenpressEdit.GototheendofthelineunderVariablevalueandappend;C:\Python35(pleaseverifythatthisfolderexists,itwillbedifferentfornewerversionsofPython)totheendofwhatisalreadythere.Ofcourse,usetheappropriatefoldername.Ifthevaluewas%SystemRoot%\system32;Itwillnowbecome%SystemRoot%\system32;C:\Python35

ClickOKandyouaredone.Norestartisrequired,howeveryoumayhavetocloseandreopenthecommandline.

RunningPythonpromptonWindows

ForWindowsusers,youcanruntheinterpreterinthecommandlineifyouhavesetthePATHvariableappropriately.

ToopentheterminalinWindows,clickthestartbuttonandclickRun.Inthedialogbox,typecmdandpress[enter]key.

Then,typepythonandensuretherearenoerrors.

InstallationonMacOSXForMacOSXusers,useHomebrew:brewinstallpython3.

Installation

18

Toverify,opentheterminalbypressing[Command+Space]keys(toopenSpotlightsearch),typeTerminalandpress[enter]key.Now,runpython3andensuretherearenoerrors.

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

Toverify,opentheterminalbyopeningtheTerminalapplicationorbypressingAlt+F2andenteringgnome-terminal.Ifthatdoesn'twork,pleasereferthedocumentationofyourparticularGNU/Linuxdistribution.Now,runpython3andensuretherearenoerrors.

YoucanseetheversionofPythononthescreenbyrunning:

$python3-V

Python3.5.1

NOTE:$isthepromptoftheshell.Itwillbedifferentforyoudependingonthesettingsoftheoperatingsystemonyourcomputer,henceIwillindicatethepromptbyjustthe$symbol.

CAUTION:Outputmaybedifferentonyourcomputer,dependingontheversionofPythonsoftwareinstalledonyourcomputer.

SummaryFromnowon,wewillassumethatyouhavePythoninstalledonyoursystem.

Next,wewillwriteourfirstPythonprogram.

Installation

19

FirstStepsWewillnowseehowtorunatraditional'HelloWorld'programinPython.Thiswillteachyouhowtowrite,saveandrunPythonprograms.

TherearetwowaysofusingPythontorunyourprogram-usingtheinteractiveinterpreterpromptorusingasourcefile.Wewillnowseehowtousebothofthesemethods.

UsingTheInterpreterPromptOpentheterminalinyouroperatingsystem(asdiscussedpreviouslyintheInstallationchapter)andthenopenthePythonpromptbytypingpython3andpressing[enter]key.

OnceyouhavestartedPython,youshouldsee>>>whereyoucanstarttypingstuff.ThisiscalledthePythoninterpreterprompt.

AtthePythoninterpreterprompt,type:

print("HelloWorld")

followedbythe[enter]key.YoushouldseethewordsHelloWorldprintedtothescreen.

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

>python3

Python3.5.1(default,Jan142016,06:54:11)

[GCC4.2.1CompatibleAppleLLVM7.0.2(clang-700.1.81)]ondarwin

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

>>>print("HelloWorld")

HelloWorld

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

HowtoQuittheInterpreterPrompt

FirstSteps

20

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

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

ChoosingAnEditorWecannottypeoutourprogramattheinterpreterprompteverytimewewanttorunsomething,sowehavetosavetheminfilesandcanrunourprogramsanynumberoftimes.

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

OneoftheverybasicrequirementsissyntaxhighlightingwhereallthedifferentpartsofyourPythonprogramarecolorizedsothatyoucanseeyourprogramandvisualizeitsrunning.

Ifyouhavenoideawheretostart,IwouldrecommendusingPyCharmEducationalEditionsoftwarewhichisavailableonWindows,MacOSXandGNU/Linux.Detailsinthenextsection.

IfyouareusingWindows,donotuseNotepad-itisabadchoicebecauseitdoesnotdosyntaxhighlightingandalsoimportantlyitdoesnotsupportindentationofthetextwhichisveryimportantinourcaseaswewillseelater.Goodeditorswillautomaticallydothis.

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

IncaseyouarewillingtotakethetimetolearnVimorEmacs,thenIhighlyrecommendthatyoudolearntouseeitherofthemasitwillbeveryusefulforyouinthelongrun.However,asImentionedbefore,beginnerscanstartwithPyCharmandfocusthelearningonPythonratherthantheeditoratthismoment.

Toreiterate,pleasechooseapropereditor-itcanmakewritingPythonprogramsmorefunandeasy.

FirstSteps

21

PyCharmPyCharmEducationalEditionisafreeeditorwhichyoucanuseforwritingPythonprograms.

WhenyouopenPyCharm,you'llseethis,clickonCreateNewProject:

SelectPurePython:

FirstSteps

22

Changeuntitledtohelloworldasthelocationoftheproject,youshouldseedetailssimilartothis:

FirstSteps

23

ClicktheCreatebutton.

Right-clickonthehelloworldinthesidebarandselectNew->PythonFile:

FirstSteps

24

Youwillbeaskedtotypethename,typehello:

Youcannowseeafileopenedforyou:

FirstSteps

25

Deletethelinesthatarealreadypresent,andnowtypethefollowing:

print("helloworld")

Nowright-clickonwhatyoutyped(withoutselectingthetext),andclickonRun'hello'.

FirstSteps

26

Youshouldnowseetheoutput(whatitprints)ofyourprogram:

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

YoucanfindmoreinformationaboutPyCharminthePyCharmQuickstartpage.

Vim1. InstallVim

MacOSXusersshouldinstallmacvimpackageviaHomeBrewWindowsusersshoulddownloadthe"self-installingexecutable"fromVimwebsiteGNU/LinuxusersshouldgetVimfromtheirdistribution'ssoftwarerepositories,e.g.DebianandUbuntuuserscaninstallthevimpackage.

2. Installjedi-vimpluginforautocompletion.3. Installcorrespondingjedipythonpackage:pipinstall-Ujedi

Emacs1. InstallEmacs24+.

MacOSXusersshouldgetEmacsfromhttp://emacsformacosx.comWindowsusersshouldgetEmacsfromhttp://ftp.gnu.org/gnu/emacs/windows/

FirstSteps

27

GNU/LinuxusersshouldgetEmacsfromtheirdistribution'ssoftwarerepositories,e.g.DebianandUbuntuuserscaninstalltheemacs24package.

2. InstallELPY

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

Startyourchoiceofeditor,enterthefollowingprogramandsaveitashello.py.

IfyouareusingPyCharm,wehavealreadydiscussedhowtorunfromasourcefile.

Forothereditors,openanewfilehello.pyandtypethis:

print("helloworld")

Whereshouldyousavethefile?Toanyfolderforwhichyouknowthelocationofthefolder.Ifyoudon'tunderstandwhatthatmeans,createanewfolderandusethatlocationtosaveandrunallyourPythonprograms:

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

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

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

TorunyourPythonprogram:

1. Openaterminalwindow(seethepreviousInstallationchapteronhowtodothat)2. Changedirectorytowhereyousavedthefile,forexample,cd/tmp/py3. Runtheprogrambyenteringthecommandpythonhello.py.Theoutputisasshown

below.

$pythonhello.py

helloworld

1

FirstSteps

28

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

Incaseyougotanerror,pleasetypetheaboveprogramexactlyasshownaboveandruntheprogramagain.NotethatPythoniscase-sensitivei.e.printisnotthesameasPrint-notethelowercasepintheformerandtheuppercasePinthelatter.Also,ensuretherearenospacesortabsbeforethefirstcharacterineachline-wewillseewhythisisimportantlater.

HowItWorks

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

GettingHelpIfyouneedquickinformationaboutanyfunctionorstatementinPython,thenyoucanusethebuilt-inhelpfunctionality.Thisisveryusefulespeciallywhenusingtheinterpreterprompt.Forexample,runhelp('len')-thisdisplaysthehelpforthelenfunctionwhichisusedtocountnumberofitems.

FirstSteps

29

TIP:Pressqtoexitthehelp.

Similarly,youcanobtaininformationaboutalmostanythinginPython.Usehelp()tolearnmoreaboutusinghelpitself!

Incaseyouneedtogethelpforoperatorslikereturn,thenyouneedtoputthoseinsidequotessuchashelp('return')sothatPythondoesn'tgetconfusedonwhatwe'retryingtodo.

SummaryYoushouldnowbeabletowrite,saveandrunPythonprogramsatease.

NowthatyouareaPythonuser,let'slearnsomemorePythonconcepts.

.theauthoroftheamazing'BeginningPerl'book↩1

FirstSteps

30

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

CommentsCommentsareanytexttotherightofthe#symbolandismainlyusefulasnotesforthereaderoftheprogram.

Forexample:

print('helloworld')#Notethatprintisafunction

or:

#Notethatprintisafunction

print('helloworld')

Useasmanyusefulcommentsasyoucaninyourprogramto:

explainassumptionsexplainimportantdecisionsexplainimportantdetailsexplainproblemsyou'retryingtosolveexplainproblemsyou'retryingtoovercomeinyourprogram,etc.

Codetellsyouhow,commentsshouldtellyouwhy.

Thisisusefulforreadersofyourprogramsothattheycaneasilyunderstandwhattheprogramisdoing.Remember,thatpersoncanbeyourselfaftersixmonths!

LiteralConstantsAnexampleofaliteralconstantisanumberlike5,1.23,orastringlike'Thisisastring'or"It'sastring!".

Basics

31

Itiscalledaliteralbecauseitisliteral-youuseitsvalueliterally.Thenumber2alwaysrepresentsitselfandnothingelse-itisaconstantbecauseitsvaluecannotbechanged.Hence,allthesearereferredtoasliteralconstants.

NumbersNumbersaremainlyoftwotypes-integersandfloats.

Anexampleofanintegeris2whichisjustawholenumber.

Examplesoffloatingpointnumbers(orfloatsforshort)are3.23and52.3E-4.TheEnotationindicatespowersof10.Inthiscase,52.3E-4means52.3*10^-4^.

NoteforExperiencedProgrammers

Thereisnoseparatelongtype.Theinttypecanbeanintegerofanysize.

StringsAstringisasequenceofcharacters.Stringsarebasicallyjustabunchofwords.

YouwillbeusingstringsinalmosteveryPythonprogramthatyouwrite,sopayattentiontothefollowingpart.

SingleQuote

Youcanspecifystringsusingsinglequotessuchas'Quotemeonthis'.

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

DoubleQuotes

Stringsindoublequotesworkexactlythesamewayasstringsinsinglequotes.Anexampleis"What'syourname?".

TripleQuotes

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

Basics

32

'''Thisisamulti-linestring.Thisisthefirstline.

Thisisthesecondline.

"What'syourname?,"Iasked.

Hesaid"Bond,JamesBond."

'''

StringsAreImmutable

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

NoteforC/C++Programmers

ThereisnoseparatechardatatypeinPython.ThereisnorealneedforitandIamsureyouwon'tmissit.

NoteforPerl/PHPProgrammers

Rememberthatsingle-quotedstringsanddouble-quotedstringsarethesame-theydonotdifferinanyway.

Theformatmethod

Sometimeswemaywanttoconstructstringsfromotherinformation.Thisiswheretheformat()methodisuseful.

Savethefollowinglinesasafilestr_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?

HowItWorks

Astringcanusecertainspecificationsandsubsequently,theformatmethodcanbecalledtosubstitutethosespecificationswithcorrespondingargumentstotheformatmethod.

Basics

33

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

Noticethatwecouldhaveachievedthesameusingstringconcatenation:

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

butthatismuchuglieranderror-prone.Second,theconversiontostringwouldbedoneautomaticallybytheformatmethodinsteadoftheexplicitconversiontostringsneededinthiscase.Third,whenusingtheformatmethod,wecanchangethemessagewithouthavingtodealwiththevariablesusedandvice-versa.

Alsonotethatthenumbersareoptional,soyoucouldhavealsowrittenas:

age=20

name='Swaroop'

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

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

whichwillgivethesameexactoutputasthepreviousprogram.

WhatPythondoesintheformatmethodisthatitsubstituteseachargumentvalueintotheplaceofthespecification.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'))

Output:

0.333

___hello___

SwaroopwroteAByteofPython

Basics

34

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

print('a',end='')

print('b',end='')

Outputis:

ab

Oryoucanendwithaspace:

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'

Basics

35

Anotherusefulescapesequencetoknowisthetab:\t.TherearemanymoreescapesequencesbutIhavementionedonlythemostusefuloneshere.

Onethingtonoteisthatinastring,asinglebackslashattheendofthelineindicatesthatthestringiscontinuedinthenextline,butnonewlineisadded.Forexample:

"Thisisthefirstsentence.\

Thisisthesecondsentence."

isequivalentto

"Thisisthefirstsentence.Thisisthesecondsentence."

RawString

Ifyouneedtospecifysomestringswherenospecialprocessingsuchasescapesequencesarehandled,thenwhatyouneedistospecifyarawstringbyprefixingrorRtothestring.Anexampleis:

r"Newlinesareindicatedby\n"

NoteforRegularExpressionUsers

Alwaysuserawstringswhendealingwithregularexpressions.Otherwise,alotofbackwhackingmayberequired.Forexample,backreferencescanbereferredtoas'\\1'orr'\1'.

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

IdentifierNaming

Basics

36

Variablesareexamplesofidentifiers.Identifiersarenamesgiventoidentifysomething.Therearesomerulesyouhavetofollowfornamingidentifiers:

Thefirstcharacteroftheidentifiermustbealetterofthealphabet(uppercaseASCIIorlowercaseASCIIorUnicodecharacter)oranunderscore(_).Therestoftheidentifiernamecanconsistofletters(uppercaseASCIIorlowercaseASCIIorUnicodecharacter),underscores(_)ordigits(0-9).Identifiernamesarecase-sensitive.Forexample,mynameandmyNamearenotthesame.NotethelowercasenintheformerandtheuppercaseNinthelatter.Examplesofvalididentifiernamesarei,name_2_3.Examplesofinvalididentifiernamesare2things,thisisspacedout,my-nameand>a1b2_c3.

DataTypesVariablescanholdvaluesofdifferenttypescalleddatatypes.Thebasictypesarenumbersandstrings,whichwehavealreadydiscussed.Inlaterchapters,wewillseehowtocreateourowntypesusingclasses.

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

NoteforObjectOrientedProgrammingusers:

Pythonisstronglyobject-orientedinthesensethateverythingisanobjectincludingnumbers,stringsandfunctions.

Wewillnowseehowtousevariablesalongwithliteralconstants.Savethefollowingexampleandruntheprogram.

HowtowritePythonprogramsHenceforth,thestandardproceduretosaveandrunaPythonprogramisasfollows:

ForPyCharm

1. OpenPyCharm.2. Createnewfilewiththefilenamementioned.3. Typetheprogramcodegivenintheexample.

Basics

37

4. Right-clickandrunthecurrentfile.

NOTE:Wheneveryouhavetoprovidecommandlinearguments,clickonRun->EditConfigurationsandtypetheargumentsintheScriptparameters:sectionandclicktheOKbutton:

Forothereditors

1. Openyoureditorofchoice.2. Typetheprogramcodegivenintheexample.3. Saveitasafilewiththefilenamementioned.4. Runtheinterpreterwiththecommandpythonprogram.pytoruntheprogram.

Example:UsingVariablesAndLiteralConstants

Typeandrunthefollowingprogram:

#Filename:var.py

i=5

print(i)

i=i+1

print(i)

s='''Thisisamulti-linestring.

Thisisthesecondline.'''

print(s)

Basics

38

Output:

5

6

Thisisamulti-linestring.

Thisisthesecondline.

HowItWorks

Here'showthisprogramworks.First,weassigntheliteralconstantvalue5tothevariableiusingtheassignmentoperator(=).Thislineiscalledastatementbecauseitstatesthatsomethingshouldbedoneandinthiscase,weconnectthevariablenameitothevalue5.Next,weprintthevalueofiusingtheprintstatementwhich,unsurprisingly,justprintsthevalueofthevariabletothescreen.

Thenweadd1tothevaluestorediniandstoreitback.Wethenprintitandexpectedly,wegetthevalue6.

Similarly,weassigntheliteralstringtothevariablesandthenprintit.

Noteforstaticlanguageprogrammers

Variablesareusedbyjustassigningthemavalue.Nodeclarationordatatypedefinitionisneeded/used.

LogicalAndPhysicalLineAphysicallineiswhatyouseewhenyouwritetheprogram.AlogicallineiswhatPythonseesasasinglestatement.Pythonimplicitlyassumesthateachphysicallinecorrespondstoalogicalline.

Anexampleofalogicallineisastatementlikeprint'helloworld'-ifthiswasonalinebyitself(asyouseeitinaneditor),thenthisalsocorrespondstoaphysicalline.

Implicitly,Pythonencouragestheuseofasinglestatementperlinewhichmakescodemorereadable.

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

i=5

print(i)

Basics

39

iseffectivelysameas

i=5;

print(i);

whichisalsosameas

i=5;print(i);

andsameas

i=5;print(i)

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.Youcansee

Basics

40

thisinactionwhenwewriteprogramsusinglistinlaterchapters.

IndentationWhitespaceisimportantinPython.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).Caseswhereyoucanusenewblockswillbedetailedinlaterchapterssuchasthecontrolflow.

Howtoindent

Usefourspacesforindentation.ThisistheofficialPythonlanguagerecommendation.Goodeditorswillautomaticallydothisforyou.Makesureyouuseaconsistentnumberofspacesforindentation,otherwiseyourprogramwillnotrunorwillhaveunexpectedbehavior.

Basics

41

Notetostaticlanguageprogrammers

Pythonwillalwaysuseindentationforblocksandwillneverusebraces.Runfrom__future__importbracestolearnmore.

SummaryNowthatwehavegonethroughmanynitty-grittydetails,wecanmoveontomoreinterestingstuffsuchascontrolflowstatements.Besuretobecomecomfortablewithwhatyouhavereadinthischapter.

Basics

42

OperatorsandExpressionsMoststatements(logicallines)thatyouwritewillcontainexpressions.Asimpleexampleofanexpressionis2+3.Anexpressioncanbebrokendownintooperatorsandoperands.

Operatorsarefunctionalitythatdosomethingandcanberepresentedbysymbolssuchas+orbyspecialkeywords.Operatorsrequiresomedatatooperateonandsuchdataiscalledoperands.Inthiscase,2and3aretheoperands.

OperatorsWewillbrieflytakealookattheoperatorsandtheirusage.

Notethatyoucanevaluatetheexpressionsgivenintheexamplesusingtheinterpreterinteractively.Forexample,totesttheexpression2+3,usetheinteractivePythoninterpreterprompt:

>>>2+3

5

>>>3*5

15

>>>

Hereisaquickoverviewoftheavailableoperators:

+(plus)

Addstwoobjects3+5gives8.'a'+'b'gives'ab'.

-(minus)

Givesthesubtractionofonenumberfromtheother;ifthefirstoperandisabsentitisassumedtobezero.-5.2givesanegativenumberand50-24gives26.

*(multiply)

Givesthemultiplicationofthetwonumbersorreturnsthestringrepeatedthatmanytimes.2*3gives6.'la'*3gives'lalala'.

**(power)

Returnsxtothepowerofy

OperatorsandExpressions

43

3**4gives81(i.e.3*3*3*3)/(divide)

Dividexbyy13/3gives4.333333333333333

//(divideandfloor)

Dividexbyyandroundtheanswerdowntothenearestwholenumber13//3gives4-13//3gives-5

%(modulo)

Returnstheremainderofthedivision13%3gives1.-25.5%2.25gives1.5.

<<(leftshift)

Shiftsthebitsofthenumbertotheleftbythenumberofbitsspecified.(Eachnumberisrepresentedinmemorybybitsorbinarydigitsi.e.0and1)2<<2gives8.2isrepresentedby10inbits.Leftshiftingby2bitsgives1000whichrepresentsthedecimal8.

>>(rightshift)

Shiftsthebitsofthenumbertotherightbythenumberofbitsspecified.11>>1gives5.11isrepresentedinbitsby1011whichwhenrightshiftedby1bitgives101whichisthedecimal5.

&(bit-wiseAND)

Bit-wiseANDofthenumbers5&3gives1.

|(bit-wiseOR)

BitwiseORofthenumbers5|3gives7

(bit-wiseXOR)

BitwiseXORofthenumbers5^3gives6

~(bit-wiseinvert)

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

<(lessthan)

Returnswhetherxislessthany.AllcomparisonoperatorsreturnTrueorFalse.

OperatorsandExpressions

44

Notethecapitalizationofthesenames.5<3givesFalseand3<5givesTrue.Comparisonscanbechainedarbitrarily:3<5<7givesTrue.

>(greaterthan)

Returnswhetherxisgreaterthany5>3returnsTrue.Ifbothoperandsarenumbers,theyarefirstconvertedtoacommontype.Otherwise,italwaysreturnsFalse.

<=(lessthanorequalto)

Returnswhetherxislessthanorequaltoyx=3;y=6;x<=yreturnsTrue

>=(greaterthanorequalto)

Returnswhetherxisgreaterthanorequaltoyx=4;y=3;x>=3returnsTrue

==(equalto)

Comparesiftheobjectsareequalx=2;y=2;x==yreturnsTruex='str';y='stR';x==yreturnsFalsex='str';y='str';x==yreturnsTrue

!=(notequalto)

Comparesiftheobjectsarenotequalx=2;y=3;x!=yreturnsTrue

not(booleanNOT)

IfxisTrue,itreturnsFalse.IfxisFalse,itreturnsTrue.x=True;notxreturnsFalse.

and(booleanAND)

xandyreturnsFalseifxisFalse,elseitreturnsevaluationofyx=False;y=True;xandyreturnsFalsesincexisFalse.Inthiscase,Pythonwillnotevaluateysinceitknowsthatthelefthandsideofthe'and'expressionisFalsewhichimpliesthatthewholeexpressionwillbeFalseirrespectiveoftheothervalues.Thisiscalledshort-circuitevaluation.

or(booleanOR)

IfxisTrue,itreturnsTrue,elseitreturnsevaluationofyx=True;y=False;xoryreturnsTrue.Short-circuitevaluationapplieshereaswell.

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45

ShortcutformathoperationandassignmentItiscommontorunamathoperationonavariableandthenassigntheresultoftheoperationbacktothevariable,hencethereisashortcutforsuchexpressions:

a=2

a=a*3

canbewrittenas:

a=2

a*=3

Noticethatvar=varoperationexpressionbecomesvaroperation=expression.

EvaluationOrderIfyouhadanexpressionsuchas2+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

OperatorsandExpressions

46

<<,>>:Shifts+,-: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)isdefinitelyeasiertounderstandthan2+3*4whichrequiresknowledgeoftheoperatorprecedences.Aswitheverythingelse,theparenthesesshouldbeusedreasonably(donotoverdoit)andshouldnotberedundant,asin(2+(3*4)).

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

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

ExpressionsExample(saveasexpression.py):

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47

length=5

breadth=2

area=length*breadth

print('Areais',area)

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

Output:

$pythonexpression.py

Areais10

Perimeteris14

HowItWorks

Thelengthandbreadthoftherectanglearestoredinvariablesbythesamename.Weusethesetocalculatetheareaandperimeteroftherectanglewiththehelpofexpressions.Westoretheresultoftheexpressionlength*breadthinthevariableareaandthenprintitusingtheprintfunction.Inthesecondcase,wedirectlyusethevalueoftheexpression2*(length+breadth)intheprintfunction.

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

SummaryWehaveseenhowtouseoperators,operandsandexpressions-thesearethebasicbuildingblocksofanyprogram.Next,wewillseehowtomakeuseoftheseinourprogramsusingstatements.

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48

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

Asyoumighthaveguessed,thisisachievedusingcontrolflowstatements.TherearethreecontrolflowstatementsinPython-if,forandwhile.

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

Example(saveasif.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:

Controlflow

49

$pythonif.py

Enteraninteger:50

No,itisalittlelowerthanthat

Done

$pythonif.py

Enteraninteger:22

No,itisalittlehigherthanthat

Done

$pythonif.py

Enteraninteger:23

Congratulations,youguessedit.

(butyoudonotwinanyprizes!)

Done

HowItWorks

Inthisprogram,wetakeguessesfromtheuserandcheckifitisthenumberthatwehave.Wesetthevariablenumbertoanyintegerwewant,say23.Then,wetaketheuser'sguessusingtheinput()function.Functionsarejustreusablepiecesofprograms.We'llreadmoreabouttheminthenextchapter.

Wesupplyastringtothebuilt-ininputfunctionwhichprintsittothescreenandwaitsforinputfromtheuser.Onceweentersomethingandpresskbd:[enter]key,theinput()functionreturnswhatweentered,asastring.Wethenconvertthisstringtoanintegerusingintandthenstoreitinthevariableguess.Actually,theintisaclassbutallyouneedtoknowrightnowisthatyoucanuseittoconvertastringtoaninteger(assumingthestringcontainsavalidintegerinthetext).

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

Noticehowtheifstatementcontainsacolonattheend-weareindicatingtoPythonthatablockofstatementsfollows.

Then,wecheckiftheguessislessthanthenumber,andifso,weinformtheuserthattheymustguessalittlehigherthanthat.Whatwehaveusedhereistheelifclausewhichactuallycombinestworelatedifelse-ifelsestatementsintoonecombinedif-elif-elsestatement.Thismakestheprogrameasierandreducestheamountofindentationrequired.

Theelifandelsestatementsmustalsohaveacolonattheendofthelogicallinefollowedbytheircorrespondingblockofstatements(withproperindentation,ofcourse)

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50

Youcanhaveanotherifstatementinsidetheif-blockofanifstatementandsoon-thisiscalledanestedifstatement.

Rememberthattheelifandelsepartsareoptional.Aminimalvalidifstatementis:

ifTrue:

print('Yes,itistrue')

AfterPythonhasfinishedexecutingthecompleteifstatementalongwiththeassociatedelifandelseclauses,itmovesontothenextstatementintheblockcontainingtheifstatement.Inthiscase,itisthemainblock(whereexecutionoftheprogramstarts),andthenextstatementistheprint('Done')statement.Afterthis,Pythonseestheendsoftheprogramandsimplyfinishesup.

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

NoteforC/C++Programmers

ThereisnoswitchstatementinPython.Youcanuseanif..elif..elsestatementtodothesamething(andinsomecases,useadictionarytodoitquickly)

ThewhileStatementThewhilestatementallowsyoutorepeatedlyexecuteablockofstatementsaslongasaconditionistrue.Awhilestatementisanexampleofwhatiscalledaloopingstatement.Awhilestatementcanhaveanoptionalelseclause.

Example(saveaswhile.py):

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51

number=23

running=True

whilerunning:

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

ifguess==number:

print('Congratulations,youguessedit.')

#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.Thisaptlydemonstratestheuseofthewhilestatement.

WemovetheinputandifstatementstoinsidethewhileloopandsetthevariablerunningtoTruebeforethewhileloop.First,wecheckifthevariablerunningisTrueandthenproceedtoexecutethecorrespondingwhile-block.Afterthisblockisexecuted,theconditionisagaincheckedwhichinthiscaseistherunningvariable.Ifitistrue,weexecutethewhile-blockagain,elsewecontinuetoexecutetheoptionalelse-blockandthencontinuetothenextstatement.

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52

TheelseblockisexecutedwhenthewhileloopconditionbecomesFalse-thismayevenbethefirsttimethattheconditionischecked.Ifthereisanelseclauseforawhileloop,itisalwaysexecutedunlessyoubreakoutoftheloopwithabreakstatement.

TheTrueandFalsearecalledBooleantypesandyoucanconsiderthemtobeequivalenttothevalue1and0respectively.

NoteforC/C++Programmers

Rememberthatyoucanhaveanelseclauseforthewhileloop.

TheforloopThefor..instatementisanotherloopingstatementwhichiteratesoverasequenceofobjectsi.e.gothrougheachiteminasequence.Wewillseemoreaboutsequencesindetailinlaterchapters.Whatyouneedtoknowrightnowisthatasequenceisjustanorderedcollectionofitems.

Example(saveasfor.py):

foriinrange(1,5):

print(i)

else:

print('Theforloopisover')

Output:

$pythonfor.py

1

2

3

4

Theforloopisover

HowItWorks

Inthisprogram,weareprintingasequenceofnumbers.Wegeneratethissequenceofnumbersusingthebuilt-inrangefunction.

Whatwedohereissupplyittwonumbersandrangereturnsasequenceofnumbersstartingfromthefirstnumberanduptothesecondnumber.Forexample,range(1,5)givesthesequence[1,2,3,4].Bydefault,rangetakesastepcountof1.Ifwesupplyathird

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53

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

Notethatrange()generatesonlyonenumberatatime,ifyouwantthefulllistofnumbers,calllist()ontherange(),forexample,list(range(5))willresultin[0,1,2,3,4].Listsareexplainedinthedatastructureschapter.

Theforlooptheniteratesoverthisrange-foriinrange(1,5)isequivalenttoforiin[1,2,3,4]whichislikeassigningeachnumber(orobject)inthesequencetoi,oneatatime,andthenexecutingtheblockofstatementsforeachvalueofi.Inthiscase,wejustprintthevalueintheblockofstatements.

Rememberthattheelsepartisoptional.Whenincluded,itisalwaysexecutedonceaftertheforloopisoverunlessabreakstatementisencountered.

Rememberthatthefor..inloopworksforanysequence.Here,wehavealistofnumbersgeneratedbythebuilt-inrangefunction,butingeneralwecanuseanykindofsequenceofanykindofobjects!Wewillexplorethisideaindetailinlaterchapters.

NoteforC/C++/Java/C#Programmers

ThePythonforloopisradicallydifferentfromtheC/C++forloop.C#programmerswillnotethattheforloopinPythonissimilartotheforeachloopinC#.Javaprogrammerswillnotethatthesameissimilartofor(inti:IntArray)inJava1.5.

InC/C++,ifyouwanttowritefor(inti=0;i<5;i++),theninPythonyouwritejustforiinrange(0,5).Asyoucansee,theforloopissimpler,moreexpressiveandlesserrorproneinPython.

ThebreakStatementThebreakstatementisusedtobreakoutofaloopstatementi.e.stoptheexecutionofaloopingstatement,eveniftheloopconditionhasnotbecomeFalseorthesequenceofitemshasnotbeencompletelyiteratedover.

Animportantnoteisthatifyoubreakoutofafororwhileloop,anycorrespondingloopelseblockisnotexecuted.

Example(saveasbreak.py):

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54

whileTrue:

s=input('Entersomething:')

ifs=='quit':

break

print('Lengthofthestringis',len(s))

print('Done')

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-inlenfunction.

Rememberthatthebreakstatementcanbeusedwiththeforloopaswell.

Swaroop'sPoeticPython

TheinputIhaveusedhereisaminipoemIhavewritten:

Programmingisfun

Whentheworkisdone

ifyouwannamakeyourworkalsofun:

usePython!

ThecontinueStatementThecontinuestatementisusedtotellPythontoskiptherestofthestatementsinthecurrentloopblockandtocontinuetothenextiterationoftheloop.

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55

Example(saveascontinue.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

HowItWorks

Inthisprogram,weacceptinputfromtheuser,butweprocesstheinputstringonlyifitisatleast3characterslong.So,weusethebuilt-inlenfunctiontogetthelengthandifthelengthislessthan3,weskiptherestofthestatementsintheblockbyusingthecontinuestatement.Otherwise,therestofthestatementsintheloopareexecuted,doinganykindofprocessingwewanttodohere.

Notethatthecontinuestatementworkswiththeforloopaswell.

SummaryWehaveseenhowtousethethreecontrolflowstatements-if,whileandforalongwiththeirassociatedbreakandcontinuestatements.ThesearesomeofthemostcommonlyusedpartsofPythonandhence,becomingcomfortablewiththemisessential.

Next,wewillseehowtocreateandusefunctions.

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56

FunctionsFunctionsarereusablepiecesofprograms.Theyallowyoutogiveanametoablockofstatements,allowingyoutorunthatblockusingthespecifiednameanywhereinyourprogramandanynumberoftimes.Thisisknownascallingthefunction.Wehavealreadyusedmanybuilt-infunctionssuchaslenandrange.

Thefunctionconceptisprobablythemostimportantbuildingblockofanynon-trivialsoftware(inanyprogramminglanguage),sowewillexplorevariousaspectsoffunctionsinthischapter.

Functionsaredefinedusingthedefkeyword.Afterthiskeywordcomesanidentifiernameforthefunction,followedbyapairofparentheseswhichmayenclosesomenamesofvariables,andbythefinalcolonthatendstheline.Nextfollowstheblockofstatementsthatarepartofthisfunction.Anexamplewillshowthatthisisactuallyverysimple:

Example(saveasfunction1.py):

defsay_hello():

#blockbelongingtothefunction

print('helloworld')

#Endoffunction

say_hello()#callthefunction

say_hello()#callthefunctionagain

Output:

$pythonfunction1.py

helloworld

helloworld

HowItWorks

Wedefineafunctioncalledsay_hellousingthesyntaxasexplainedabove.Thisfunctiontakesnoparametersandhencetherearenovariablesdeclaredintheparentheses.Parameterstofunctionsarejustinputtothefunctionsothatwecanpassindifferentvaluestoitandgetbackcorrespondingresults.

Noticethatwecancallthesamefunctiontwicewhichmeanswedonothavetowritethesamecodeagain.

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57

FunctionParametersAfunctioncantakeparameters,whicharevaluesyousupplytothefunctionsothatthefunctioncandosomethingutilisingthosevalues.Theseparametersarejustlikevariablesexceptthatthevaluesofthesevariablesaredefinedwhenwecallthefunctionandarealreadyassignedvalueswhenthefunctionruns.

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

Example(saveasfunction_param.py):

defprint_max(a,b):

ifa>b:

print(a,'ismaximum')

elifa==b:

print(a,'isequalto',b)

else:

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,wedefineafunctioncalledprint_maxthatusestwoparameterscalledaandb.Wefindoutthegreaternumberusingasimpleif..elsestatementandthenprintthebiggernumber.

Thefirsttimewecallthefunctionprint_max,wedirectlysupplythenumbersasarguments.Inthesecondcase,wecallthefunctionwithvariablesasarguments.print_max(x,y)causesthevalueofargumentxtobeassignedtoparameteraandthevalueof

Functions

58

argumentytobeassignedtoparameterb.Theprint_maxfunctionworksthesamewayinbothcases.

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

Example(saveasfunction_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.

Next,weassignthevalue2tox.Thenamexislocaltoourfunction.So,whenwechangethevalueofxinthefunction,thexdefinedinthemainblockremainsunaffected.

Withthelastprintstatement,wedisplaythevalueofxasdefinedinthemainblock,therebyconfirmingthatitisactuallyunaffectedbythelocalassignmentwithinthepreviouslycalledfunction.

Functions

59

TheglobalstatementIfyouwanttoassignavaluetoanamedefinedatthetopleveloftheprogram(i.e.notinsideanykindofscopesuchasfunctionsorclasses),thenyouhavetotellPythonthatthenameisnotlocal,butitisglobal.Wedothisusingtheglobalstatement.Itisimpossibletoassignavaluetoavariabledefinedoutsideafunctionwithouttheglobalstatement.

Youcanusethevaluesofsuchvariablesdefinedoutsidethefunction(assumingthereisnovariablewiththesamenamewithinthefunction).However,thisisnotencouragedandshouldbeavoidedsinceitbecomesuncleartothereaderoftheprogramastowherethatvariable'sdefinitionis.Usingtheglobalstatementmakesitamplyclearthatthevariableisdefinedinanoutermostblock.

Example(saveasfunction_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

Theglobalstatementisusedtodeclarethatxisaglobalvariable-hence,whenweassignavaluetoxinsidethefunction,thatchangeisreflectedwhenweusethevalueofxinthemainblock.

Youcanspecifymorethanoneglobalvariableusingthesameglobalstatemente.g.globalx,y,z.

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60

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

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

Example(saveasfunction_default.py):

defsay(message,times=1):

print(message*times)

say('Hello')

say('World',5)

Output:

$pythonfunction_default.py

Hello

WorldWorldWorldWorldWorld

HowItWorks

Thefunctionnamedsayisusedtoprintastringasmanytimesasspecified.Ifwedon'tsupplyavalue,thenbydefault,thestringisprintedjustonce.Weachievethisbyspecifyingadefaultargumentvalueof1totheparametertimes.

Inthefirstusageofsay,wesupplyonlythestringanditprintsthestringonce.Inthesecondusageofsay,wesupplyboththestringandanargument5statingthatwewanttosaythestringmessage5times.

CAUTION

Onlythoseparameterswhichareattheendoftheparameterlistcanbegivendefaultargumentvaluesi.e.youcannothaveaparameterwithadefaultargumentvalueprecedingaparameterwithoutadefaultargumentvalueinthefunction'sparameterlist.

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

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61

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

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

Example(saveasfunction_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

HowItWorks

Thefunctionnamedfunchasoneparameterwithoutadefaultargumentvalue,followedbytwoparameterswithdefaultargumentvalues.

Inthefirstusage,func(3,7),theparameteragetsthevalue3,theparameterbgetsthevalue7andcgetsthedefaultvalueof10.

Inthesecondusagefunc(25,c=24),thevariableagetsthevalueof25duetothepositionoftheargument.Then,theparametercgetsthevalueof24duetonamingi.e.keywordarguments.Thevariablebgetsthedefaultvalueof5.

Inthethirdusagefunc(c=50,a=100),weusekeywordargumentsforallspecifiedvalues.Noticethatwearespecifyingthevalueforparametercbeforethatforaeventhoughaisdefinedbeforecinthefunctiondefinition.

VarArgsparameters

Functions

62

Sometimesyoumightwanttodefineafunctionthatcantakeanynumberofparameters,i.e.variablenumberofarguments,thiscanbeachievedbyusingthestars(saveasfunction_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)

print(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

None

HowItWorks

Whenwedeclareastarredparametersuchas*param,thenallthepositionalargumentsfromthatpointtilltheendarecollectedasatuplecalled'param'.

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

Wewillexploretuplesanddictionariesinalaterchapter.

ThereturnstatementThereturnstatementisusedtoreturnfromafunctioni.e.breakoutofthefunction.Wecanoptionallyreturnavaluefromthefunctionaswell.

Example(saveasfunction_return.py):

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63

defmaximum(x,y):

ifx>y:

returnx

elifx==y:

return'Thenumbersareequal'

else:

returny

print(maximum(2,3))

Output:

$pythonfunction_return.py

3

HowItWorks

Themaximumfunctionreturnsthemaximumoftheparameters,inthiscasethenumberssuppliedtothefunction.Itusesasimpleif..elsestatementtofindthegreatervalueandthenreturnsthatvalue.

NotethatareturnstatementwithoutavalueisequivalenttoreturnNone.NoneisaspecialtypeinPythonthatrepresentsnothingness.Forexample,itisusedtoindicatethatavariablehasnovalueifithasavalueofNone.

EveryfunctionimplicitlycontainsareturnNonestatementattheendunlessyouhavewrittenyourownreturnstatement.Youcanseethisbyrunningprint(some_function())wherethefunctionsome_functiondoesnotusethereturnstatementsuchas:

defsome_function():

pass

ThepassstatementisusedinPythontoindicateanemptyblockofstatements.

TIP:Thereisabuilt-infunctioncalledmaxthatalreadyimplementsthe'findmaximum'functionality,sousethisbuilt-infunctionwheneverpossible.

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

Functions

64

Example(saveasfunction_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')

print_max(3,5)

print(print_max.__doc__)

Output:

$pythonfunction_docstring.py

5ismaximum

Printsthemaximumoftwonumbers.

Thetwovaluesmustbeintegers.

HowItWorks

Astringonthefirstlogicallineofafunctionisthedocstringforthatfunction.NotethatDocStringsalsoapplytomodulesandclasseswhichwewilllearnaboutintherespectivechapters.

Theconventionfollowedforadocstringisamulti-linestringwherethefirstlinestartswithacapitalletterandendswithadot.Thenthesecondlineisblankfollowedbyanydetailedexplanationstartingfromthethirdline.Youarestronglyadvisedtofollowthisconventionforallyourdocstringsforallyournon-trivialfunctions.

Wecanaccessthedocstringoftheprint_maxfunctionusingthe__doc__(noticethedoubleunderscores)attribute(namebelongingto)ofthefunction.JustrememberthatPythontreatseverythingasanobjectandthisincludesfunctions.We'lllearnmoreaboutobjectsinthechapteronclasses.

Ifyouhaveusedhelp()inPython,thenyouhavealreadyseentheusageofdocstrings!Whatitdoesisjustfetchthe__doc__attributeofthatfunctionanddisplaysitinaneatmannerforyou.Youcantryitoutonthefunctionabove-justincludehelp(print_max)inyourprogram.Remembertopresstheqkeytoexithelp.

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Automatedtoolscanretrievethedocumentationfromyourprograminthismanner.Therefore,Istronglyrecommendthatyouusedocstringsforanynon-trivialfunctionthatyouwrite.ThepydoccommandthatcomeswithyourPythondistributionworkssimilarlytohelp()usingdocstrings.

SummaryWehaveseensomanyaspectsoffunctionsbutnotethatwestillhaven'tcoveredallaspectsofthem.However,wehavealreadycoveredmostofwhatyou'lluseregardingPythonfunctionsonaneverydaybasis.

Next,wewillseehowtouseaswellascreatePythonmodules.

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ModulesYouhaveseenhowyoucanreusecodeinyourprogrambydefiningfunctionsonce.Whatifyouwantedtoreuseanumberoffunctionsinotherprogramsthatyouwrite?Asyoumighthaveguessed,theanswerismodules.

Therearevariousmethodsofwritingmodules,butthesimplestwayistocreateafilewitha.pyextensionthatcontainsfunctionsandvariables.

AnothermethodistowritethemodulesinthenativelanguageinwhichthePythoninterpreteritselfwaswritten.Forexample,youcanwritemodulesintheCprogramminglanguageandwhencompiled,theycanbeusedfromyourPythoncodewhenusingthestandardPythoninterpreter.

Amodulecanbeimportedbyanotherprogramtomakeuseofitsfunctionality.ThisishowwecanusethePythonstandardlibraryaswell.First,wewillseehowtousethestandardlibrarymodules.

Example(saveasmodule_using_sys.py):

importsys

print('Thecommandlineargumentsare:')

foriinsys.argv:

print(i)

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

Output:

$pythonmodule_using_sys.pywearearguments

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

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First,weimportthesysmoduleusingtheimportstatement.Basically,thistranslatestoustellingPythonthatwewanttousethismodule.ThesysmodulecontainsfunctionalityrelatedtothePythoninterpreteranditsenvironmenti.e.thesystem.

WhenPythonexecutestheimportsysstatement,itlooksforthesysmodule.Inthiscase,itisoneofthebuilt-inmodules,andhencePythonknowswheretofindit.

Ifitwasnotacompiledmodulei.e.amodulewritteninPython,thenthePythoninterpreterwillsearchforitinthedirectorieslistedinitssys.pathvariable.Ifthemoduleisfound,thenthestatementsinthebodyofthatmodulearerunandthemoduleismadeavailableforyoutouse.Notethattheinitializationisdoneonlythefirsttimethatweimportamodule.

Theargvvariableinthesysmoduleisaccessedusingthedottednotationi.e.sys.argv.Itclearlyindicatesthatthisnameispartofthesysmodule.Anotheradvantageofthisapproachisthatthenamedoesnotclashwithanyargvvariableusedinyourprogram.

Thesys.argvvariableisalistofstrings(listsareexplainedindetailinalaterchapter.Specifically,thesys.argvcontainsthelistofcommandlineargumentsi.e.theargumentspassedtoyourprogramusingthecommandline.

IfyouareusinganIDEtowriteandruntheseprograms,lookforawaytospecifycommandlineargumentstotheprograminthemenus.

Here,whenweexecutepythonmodule_using_sys.pywearearguments,werunthemodulemodule_using_sys.pywiththepythoncommandandtheotherthingsthatfollowareargumentspassedtotheprogram.Pythonstoresthecommandlineargumentsinthesys.argvvariableforustouse.

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

Thesys.pathcontainsthelistofdirectorynameswheremodulesareimportedfrom.Observethatthefirststringinsys.pathisempty-thisemptystringindicatesthatthecurrentdirectoryisalsopartofthesys.pathwhichissameasthePYTHONPATHenvironmentvariable.Thismeansthatyoucandirectlyimportmoduleslocatedinthecurrentdirectory.Otherwise,youwillhavetoplaceyourmoduleinoneofthedirectorieslistedinsys.path.

Notethatthecurrentdirectoryisthedirectoryfromwhichtheprogramislaunched.Runimportos;print(os.getcwd())tofindoutthecurrentdirectoryofyourprogram.

Byte-compiled.pycfiles

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

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

Thefrom..importstatementIfyouwanttodirectlyimporttheargvvariableintoyourprogram(toavoidtypingthesys.everytimeforit),thenyoucanusethefromsysimportargvstatement.

WARNING:Ingeneral,avoidusingthefrom..importstatement,usetheimportstatementinstead.Thisisbecauseyourprogramwillavoidnameclashesandwillbemorereadable.

Example:

frommathimportsqrt

print("Squarerootof16is",sqrt(16))

Amodule's__name__Everymodulehasanameandstatementsinamodulecanfindoutthenameoftheirmodule.Thisishandyfortheparticularpurposeoffiguringoutwhetherthemoduleisbeingrunstandaloneorbeingimported.Asmentionedpreviously,whenamoduleisimportedforthefirsttime,thecodeitcontainsgetsexecuted.Wecanusethistomakethemodulebehaveindifferentwaysdependingonwhetheritisbeingusedbyitselforbeingimportedfromanothermodule.Thiscanbeachievedusingthe__name__attributeofthemodule.

Example(saveasmodule_using_name.py):

if__name__=='__main__':

print('Thisprogramisbeingrunbyitself')

else:

print('Iambeingimportedfromanothermodule')

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Output:

$pythonmodule_using_name.py

Thisprogramisbeingrunbyitself

$python

>>>importmodule_using_name

Iambeingimportedfromanothermodule

>>>

HowItWorks

EveryPythonmodulehasits__name__defined.Ifthisis'__main__',thatimpliesthatthemoduleisbeingrunstandalonebytheuserandwecantakeappropriateactions.

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

Example(saveasmymodule.py):

defsay_hi():

print('Hi,thisismymodulespeaking.')

__version__='0.1'

Theabovewasasamplemodule.Asyoucansee,thereisnothingparticularlyspecialaboutitcomparedtoourusualPythonprogram.WewillnextseehowtousethismoduleinourotherPythonprograms.

Rememberthatthemoduleshouldbeplacedeitherinthesamedirectoryastheprogramfromwhichweimportit,orinoneofthedirectorieslistedinsys.path.

Anothermodule(saveasmymodule_demo.py):

importmymodule

mymodule.say_hi()

print('Version',mymodule.__version__)

Output:

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$pythonmymodule_demo.py

Hi,thisismymodulespeaking.

Version0.1

HowItWorks

Noticethatweusethesamedottednotationtoaccessmembersofthemodule.Pythonmakesgoodreuseofthesamenotationtogivethedistinctive'Pythonic'feeltoitsothatwedon'thavetokeeplearningnewwaystodothings.

Hereisaversionutilisingthefrom..importsyntax(saveasmymodule_demo2.py):

frommymoduleimportsay_hi,__version__

say_hi()

print('Version',__version__)

Theoutputofmymodule_demo2.pyissameastheoutputofmymodule_demo.py.

Noticethatiftherewasalreadya__version__namedeclaredinthemodulethatimportsmymodule,therewouldbeaclash.Thisisalsolikelybecauseitiscommonpracticeforeachmoduletodeclareit'sversionnumberusingthisname.Hence,itisalwaysrecommendedtoprefertheimportstatementeventhoughitmightmakeyourprogramalittlelonger.

Youcouldalsouse:

frommymoduleimport*

Thiswillimportallpublicnamessuchassay_hibutwouldnotimport__version__becauseitstartswithdoubleunderscores.

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

ZenofPython

OneofPython'sguidingprinciplesisthat"ExplicitisbetterthanImplicit".RunimportthisinPythontolearnmore.

ThedirfunctionBuilt-indir()functionreturnslistofnamesdefinedbyanobject.Iftheobjectisamodule,thislistincludesfunctions,classesandvariables,definedinsidethatmodule.

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Thisfunctioncanacceptarguments.Iftheargumentisthenameofthemodule,functionreturnslistofnamesfromthatspecifiedmodule.Ifthereisnoargument,functionreturnslistofnamesfromthecurrentmodule.

Example:

$python

>>>importsys

#getnamesofattributesinsysmodule

>>>dir(sys)

['__displayhook__','__doc__',

'argv','builtin_module_names',

'version','version_info']

#onlyfewentriesshownhere

#getnamesofattributesforcurrentmodule

>>>dir()

['__builtins__','__doc__',

'__name__','__package__']

#createanewvariable'a'

>>>a=5

>>>dir()

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

#delete/removeaname

>>>dela

>>>dir()

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

HowItWorks

First,weseetheusageofdirontheimportedsysmodule.Wecanseethehugelistofattributesthatitcontains.

Next,weusethedirfunctionwithoutpassingparameterstoit.Bydefault,itreturnsthelistofattributesforthecurrentmodule.Noticethatthelistofimportedmodulesisalsopartofthislist.

Inordertoobservethedirinaction,wedefineanewvariableaandassignitavalueandthencheckdirandweobservethatthereisanadditionalvalueinthelistofthesamename.Weremovethevariable/attributeofthecurrentmoduleusingthedelstatementandthechangeisreflectedagainintheoutputofthedirfunction.

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Anoteondel-thisstatementisusedtodeleteavariable/nameandafterthestatementhasrun,inthiscasedela,youcannolongeraccessthevariablea-itisasifitneverexistedbeforeatall.

Notethatthedir()functionworksonanyobject.Forexample,rundir(str)fortheattributesofthestr(string)class.

Thereisalsoavars()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

Packagesarejustaconveniencetohierarchicallyorganizemodules.Youwillseemanyinstancesofthisinthestandardlibrary.

Summary

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73

Justlikefunctionsarereusablepartsofprograms,modulesarereusableprograms.Packagesareanotherhierarchytoorganizemodules.ThestandardlibrarythatcomeswithPythonisanexampleofsuchasetofpackagesandmodules.

Wehaveseenhowtousethesemodulesandcreateourownmodules.

Next,wewilllearnaboutsomeinterestingconceptscalleddatastructures.

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DataStructuresDatastructuresarebasicallyjustthat-theyarestructureswhichcanholdsomedatatogether.Inotherwords,theyareusedtostoreacollectionofrelateddata.

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

ListAlistisadatastructurethatholdsanorderedcollectionofitemsi.e.youcanstoreasequenceofitemsinalist.Thisiseasytoimagineifyoucanthinkofashoppinglistwhereyouhavealistofitemstobuy,exceptthatyouprobablyhaveeachitemonaseparatelineinyourshoppinglistwhereasinPythonyouputcommasinbetweenthem.

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

QuickIntroductionToObjectsAndClassesAlthoughI'vebeengenerallydelayingthediscussionofobjectsandclassestillnow,alittleexplanationisneededrightnowsothatyoucanunderstandlistsbetter.Wewillexplorethistopicindetailinalaterchapter.

Alistisanexampleofusageofobjectsandclasses.Whenweuseavariableiandassignavaluetoit,sayinteger5toit,youcanthinkofitascreatinganobject(i.e.instance)iofclass(i.e.type)int.Infact,youcanreadhelp(int)tounderstandthisbetter.

Aclasscanalsohavemethodsi.e.functionsdefinedforusewithrespecttothatclassonly.Youcanusethesepiecesoffunctionalityonlywhenyouhaveanobjectofthatclass.Forexample,Pythonprovidesanappendmethodforthelistclasswhichallowsyoutoaddanitemtotheendofthelist.Forexample,mylist.append('anitem')willaddthatstringtothelistmylist.Notetheuseofdottednotationforaccessingmethodsoftheobjects.

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

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Example(saveasds_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)

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

Thevariableshoplistisashoppinglistforsomeonewhoisgoingtothemarket.Inshoplist,weonlystorestringsofthenamesoftheitemstobuybutyoucanaddanykindofobjecttoalistincludingnumbersandevenotherlists.

Wehavealsousedthefor..inlooptoiteratethroughtheitemsofthelist.Bynow,youmusthaverealisedthatalistisalsoasequence.Thespecialityofsequenceswillbediscussedinalatersection.

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Noticetheuseoftheendparameterinthecalltoprintfunctiontoindicatethatwewanttoendtheoutputwithaspaceinsteadoftheusuallinebreak.

Next,weaddanitemtothelistusingtheappendmethodofthelistobject,asalreadydiscussedbefore.Then,wecheckthattheitemhasbeenindeedaddedtothelistbyprintingthecontentsofthelistbysimplypassingthelisttotheprintfunctionwhichprintsitneatly.

Then,wesortthelistbyusingthesortmethodofthelist.Itisimportanttounderstandthatthismethodaffectsthelistitselfanddoesnotreturnamodifiedlist-thisisdifferentfromthewaystringswork.Thisiswhatwemeanbysayingthatlistsaremutableandthatstringsareimmutable.

Next,whenwefinishbuyinganiteminthemarket,wewanttoremoveitfromthelist.Weachievethisbyusingthedelstatement.Here,wementionwhichitemofthelistwewanttoremoveandthedelstatementremovesitfromthelistforus.Wespecifythatwewanttoremovethefirstitemfromthelistandhenceweusedelshoplist[0](rememberthatPythonstartscountingfrom0).

Ifyouwanttoknowallthemethodsdefinedbythelistobject,seehelp(list)fordetails.

TupleTuplesareusedtoholdtogethermultipleobjects.Thinkofthemassimilartolists,butwithouttheextensivefunctionalitythatthelistclassgivesyou.Onemajorfeatureoftuplesisthattheyareimmutablelikestringsi.e.youcannotmodifytuples.

Tuplesaredefinedbyspecifyingitemsseparatedbycommaswithinanoptionalpairofparentheses.

Tuplesareusuallyusedincaseswhereastatementorauser-definedfunctioncansafelyassumethatthecollectionofvaluesi.e.thetupleofvaluesusedwillnotchange.

Example(saveasds_using_tuple.py):

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#Iwouldrecommendalwaysusingparentheses

#toindicatestartandendoftuple

#eventhoughparenthesesareoptional.

#Explicitisbetterthanimplicit.

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

print('Numberofanimalsinthezoois',len(zoo))

new_zoo='monkey','camel',zoo

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

Thevariablezooreferstoatupleofitems.Weseethatthelenfunctioncanbeusedtogetthelengthofthetuple.Thisalsoindicatesthatatupleisasequenceaswell.

Wearenowshiftingtheseanimalstoanewzoosincetheoldzooisbeingclosed.Therefore,thenew_zootuplecontainssomeanimalswhicharealreadytherealongwiththeanimalsbroughtoverfromtheoldzoo.Backtoreality,notethatatuplewithinatupledoesnotloseitsidentity.

Wecanaccesstheitemsinthetuplebyspecifyingtheitem'spositionwithinapairofsquarebracketsjustlikewedidforlists.Thisiscalledtheindexingoperator.Weaccessthethirditeminnew_zoobyspecifyingnew_zoo[2]andweaccessthethirditemwithinthethirditeminthenew_zootuplebyspecifyingnew_zoo[2][2].Thisisprettysimpleonceyou'veunderstoodtheidiom.

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78

Tuplewith0or1items

Anemptytupleisconstructedbyanemptypairofparenthesessuchasmyempty=().However,atuplewithasingleitemisnotsosimple.Youhavetospecifyitusingacommafollowingthefirst(andonly)itemsothatPythoncandifferentiatebetweenatupleandapairofparenthesessurroundingtheobjectinanexpressioni.e.youhavetospecifysingleton=(2,)ifyoumeanyouwantatuplecontainingtheitem2.

NoteforPerlprogrammers

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

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

Notethatyoucanuseonlyimmutableobjects(likestrings)forthekeysofadictionarybutyoucanuseeitherimmutableormutableobjectsforthevaluesofthedictionary.Thisbasicallytranslatestosaythatyoushoulduseonlysimpleobjectsforkeys.

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

Rememberthatkey-valuepairsinadictionaryarenotorderedinanymanner.Ifyouwantaparticularorder,thenyouwillhavetosortthemyourselfbeforeusingit.

Thedictionariesthatyouwillbeusingareinstances/objectsofthedictclass.

Example(saveasds_using_dict.py):

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79

#'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

Wecreatethedictionaryabusingthenotationalreadydiscussed.Wethenaccesskey-valuepairsbyspecifyingthekeyusingtheindexingoperatorasdiscussedinthecontextoflistsandtuples.Observethesimplesyntax.

Wecandeletekey-valuepairsusingouroldfriend-thedelstatement.Wesimplyspecifythedictionaryandtheindexingoperatorforthekeytoberemovedandpassittothedelstatement.Thereisnoneedtoknowthevaluecorrespondingtothekeyforthisoperation.

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Next,weaccesseachkey-valuepairofthedictionaryusingtheitemsmethodofthedictionarywhichreturnsalistoftupleswhereeachtuplecontainsapairofitems-thekeyfollowedbythevalue.Weretrievethispairandassignittothevariablesnameandaddresscorrespondinglyforeachpairusingthefor..inloopandthenprintthesevaluesinthefor-block.

Wecanaddnewkey-valuepairsbysimplyusingtheindexingoperatortoaccessakeyandassignthatvalue,aswehavedoneforGuidointheabovecase.

Wecancheckifakey-valuepairexistsusingtheinoperator.

Forthelistofmethodsofthedictclass,seehelp(dict).

KeywordArgumentsandDictionaries

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

SequenceLists,tuplesandstringsareexamplesofsequences,butwhataresequencesandwhatissospecialaboutthem?

Themajorfeaturesaremembershiptests,(i.e.theinandnotinexpressions)andindexingoperations,whichallowustofetchaparticulariteminthesequencedirectly.

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

Example(saveasds_seq.py):

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81

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']

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

characters1to3iswa

characters2toendisaroop

characters1to-1iswaroo

charactersstarttoendisswaroop

HowItWorks

First,weseehowtouseindexestogetindividualitemsofasequence.Thisisalsoreferredtoasthesubscriptionoperation.Wheneveryouspecifyanumbertoasequencewithinsquarebracketsasshownabove,Pythonwillfetchyoutheitemcorrespondingtothat

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82

positioninthesequence.RememberthatPythonstartscountingnumbersfrom0.Hence,shoplist[0]fetchesthefirstitemandshoplist[3]fetchesthefourthitemintheshoplistsequence.

Theindexcanalsobeanegativenumber,inwhichcase,thepositioniscalculatedfromtheendofthesequence.Therefore,shoplist[-1]referstothelastiteminthesequenceandshoplist[-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.

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TryvariouscombinationsofsuchslicespecificationsusingthePythoninterpreterinteractivelyi.e.thepromptsothatyoucanseetheresultsimmediately.Thegreatthingaboutsequencesisthatyoucanaccesstuples,listsandstringsallinthesameway!

SetSetsareunorderedcollectionsofsimpleobjects.Theseareusedwhentheexistenceofanobjectinacollectionismoreimportantthantheorderorhowmanytimesitoccurs.

Usingsets,youcantestformembership,whetheritisasubsetofanotherset,findtheintersectionbetweentwosets,andsoon.

>>>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

Theexampleisprettymuchself-explanatorybecauseitinvolvesbasicsettheorymathematicstaughtinschool.

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

Generally,youdon'tneedtobeworriedaboutthis,butthereisasubtleeffectduetoreferenceswhichyouneedtobeawareof:

Example(saveasds_reference.py):

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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']

HowItWorks

Mostoftheexplanationisavailableinthecomments.

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

NoteforPerlprogrammers

Rememberthatanassignmentstatementforlistsdoesnotcreateacopy.Youhavetouseslicingoperationtomakeacopyofthesequence.

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MoreAboutStringsWehavealreadydiscussedstringsindetailearlier.Whatmorecantherebetoknow?Well,didyouknowthatstringsarealsoobjectsandhavemethodswhichdoeverythingfromcheckingpartofastringtostrippingspaces!

Thestringsthatyouuseinprogramareallobjectsoftheclassstr.Someusefulmethodsofthisclassaredemonstratedinthenextexample.Foracompletelistofsuchmethods,seehelp(str).

Example(saveasds_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.Thestartswithmethodisusedtofindoutwhetherthestringstartswiththegivenstring.Theinoperatorisusedtocheckifagivenstringisapartofthestring.

Thefindmethodisusedtolocatethepositionofthegivensubstringwithinthestring;findreturns-1ifitisunsuccessfulinfindingthesubstring.Thestrclassalsohasaneatmethodtojointheitemsofasequencewiththestringactingasadelimiterbetweeneachitemofthesequenceandreturnsabiggerstringgeneratedfromthis.

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SummaryWehaveexploredthevariousbuilt-indatastructuresofPythonindetail.Thesedatastructureswillbeessentialforwritingprogramsofreasonablesize.

NowthatwehavealotofthebasicsofPythoninplace,wewillnextseehowtodesignandwriteareal-worldPythonprogram.

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ProblemSolvingWehaveexploredvariouspartsofthePythonlanguageandnowwewilltakealookathowallthesepartsfittogether,bydesigningandwritingaprogramwhichdoessomethinguseful.TheideaistolearnhowtowriteaPythonscriptonyourown.

TheProblemTheproblemwewanttosolveis:

Iwantaprogramwhichcreatesabackupofallmyimportantfiles.

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

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

Thefilesanddirectoriestobebackeduparespecifiedinalist.Thebackupmustbestoredinamainbackupdirectory.Thefilesarebackedupintoazipfile.Thenameoftheziparchiveisthecurrentdateandtime.WeusethestandardzipcommandavailablebydefaultinanystandardGNU/LinuxorUnixdistribution.Notethatyoucanuseanyarchivingcommandyouwantaslongasithasacommandlineinterface.

ForWindowsusers

WindowsuserscaninstallthezipcommandfromtheGnuWin32projectpageandaddC:\ProgramFiles\GnuWin32\bintoyoursystemPATHenvironmentvariable,similartowhatwedidforrecognizingthepythoncommanditself.

TheSolutionAsthedesignofourprogramisnowreasonablystable,wecanwritethecodewhichisanimplementationofoursolution.

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Saveasbackup_ver1.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

#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:

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$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,copythelineprintedaftertheZipcommandislineintheoutput,pasteitintheshell(onGNU/LinuxandMacOSX)/cmd(onWindows),seewhattheerrorisandtrytofixit.Alsocheckthezipcommandmanualonwhatcouldbewrong.Ifthiscommandsucceeds,thentheproblemmightbeinthePythonprogramitself,socheckifitexactlymatchestheprogramwrittenabove.

HowItWorks

Youwillnoticehowwehaveconvertedourdesignintocodeinastep-by-stepmanner.

Wemakeuseoftheosandtimemodulesbyfirstimportingthem.Then,wespecifythefilesanddirectoriestobebackedupinthesourcelist.Thetargetdirectoryiswherewestoreallthebackupfilesandthisisspecifiedinthetarget_dirvariable.Thenameoftheziparchivethatwearegoingtocreateisthecurrentdateandtimewhichwegenerateusingthetime.strftime()function.Itwillalsohavethe.zipextensionandwillbestoredinthetarget_dirdirectory.

Noticetheuseoftheos.sepvariable-thisgivesthedirectoryseparatoraccordingtoyouroperatingsystemi.e.itwillbe'/'inGNU/LinuxandUnix,itwillbe'\\'inWindowsand':'inMacOS.Usingos.sepinsteadofthesecharactersdirectlywillmakeourprogramportableandworkacrossallofthesesystems.

Thetime.strftime()functiontakesaspecificationsuchastheonewehaveusedintheaboveprogram.The%Yspecificationwillbereplacedbytheyearwiththecentury.The%mspecificationwillbereplacedbythemonthasadecimalnumberbetween01and12andsoon.ThecompletelistofsuchspecificationscanbefoundinthePythonReferenceManual.

Wecreatethenameofthetargetzipfileusingtheadditionoperatorwhichconcatenatesthestringsi.e.itjoinsthetwostringstogetherandreturnsanewone.Then,wecreateastringzip_commandwhichcontainsthecommandthatwearegoingtoexecute.Youcancheckif

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thiscommandworksbyrunningitintheshell(GNU/LinuxterminalorDOSprompt).

Thezipcommandthatweareusinghassomeoptionsandparameterspassed.The-roptionspecifiesthatthezipcommandshouldworkrecursivelyfordirectoriesi.e.itshouldincludeallthesubdirectoriesandfiles.Thetwooptionsarecombinedandspecifiedinashortcutas-qr.Theoptionsarefollowedbythenameoftheziparchivetocreatefollowedbythelistoffilesanddirectoriestobackup.Weconvertthesourcelistintoastringusingthejoinmethodofstringswhichwehavealreadyseenhowtouse.

Then,wefinallyrunthecommandusingtheos.systemfunctionwhichrunsthecommandasifitwasrunfromthesystemi.e.intheshell-itreturns0ifthecommandwassuccessfully,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.

SecondVersionThefirstversionofourscriptworks.However,wecanmakesomerefinementstoitsothatitcanworkbetteronadailybasis.Thisiscalledthemaintenancephaseofthesoftware.

OneoftherefinementsIfeltwasusefulisabetterfile-namingmechanism-usingthetimeasthenameofthefilewithinadirectorywiththecurrentdateasadirectorywithinthemainbackupdirectory.Thefirstadvantageisthatyourbackupsarestoredinahierarchicalmannerandthereforeitismucheasiertomanage.Thesecondadvantageisthatthe

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filenamesaremuchshorter.Thethirdadvantageisthatseparatedirectorieswillhelpyoucheckifyouhavemadeabackupforeachdaysincethedirectorywouldbecreatedonlyifyouhavemadeabackupforthatday.

Saveasbackup_ver2.py:

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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')

#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')

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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.Thechangesarethatwecheckifthereisadirectorywiththecurrentdayasitsnameinsidethemainbackupdirectoryusingtheos.path.existsfunction.Ifitdoesn'texist,wecreateitusingtheos.mkdirfunction.

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

WARNING:Thefollowingprogramdoesnotwork,sodonotbealarmed,pleasefollowalongbecausethere'salessoninhere.

Saveasbackup_ver3.py:

importos

importtime

#1.Thefilesanddirectoriestobebackedupare

#specifiedinalist.

#ExampleonWindows:

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

#ExampleonMacOSXandLinux:

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

#Noticewehadtousedoublequotesinsidethestring

#fornameswithspacesinit.

#2.Thebackupmustbestoredina

#mainbackupdirectory

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#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:

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$pythonbackup_ver3.py

File"backup_ver3.py",line39

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

^

SyntaxError:invalidsyntax

HowThis(doesnot)Work

Thisprogramdoesnotwork!PythonsaysthereisasyntaxerrorwhichmeansthatthescriptdoesnotsatisfythestructurethatPythonexpectstosee.WhenweobservetheerrorgivenbyPython,italsotellsustheplacewhereitdetectedtheerroraswell.Sowestartdebuggingourprogramfromthatline.

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

FourthVersionSaveasbackup_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

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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_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

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HowItWorks

Thisprogramnowworks!Letusgothroughtheactualenhancementsthatwehadmadeinversion3.Wetakeintheuser'scommentsusingtheinputfunctionandthencheckiftheuseractuallyenteredsomethingbyfindingoutthelengthoftheinputusingthelenfunction.Iftheuserhasjustpressedenterwithoutenteringanything(maybeitwasjustaroutinebackupornospecialchangesweremade),thenweproceedaswehavedonebefore.

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

MoreRefinementsThefourthversionisasatisfactorilyworkingscriptformostusers,butthereisalwaysroomforimprovement.Forexample,youcanincludeaverbositylevelfortheprogramwhereyoucanspecifya-voptiontomakeyourprogrambecomemoretalkativeora-qtomakeitquiet.

Anotherpossibleenhancementwouldbetoallowextrafilesanddirectoriestobepassedtothescriptatthecommandline.Wecangetthesenamesfromthesys.argvlistandwecanaddthemtooursourcelistusingtheextendmethodprovidedbythelistclass.

Themostimportantrefinementwouldbetonotusetheos.systemwayofcreatingarchivesandinsteadusingthezipfileortarfilebuilt-inmodulestocreatethesearchives.Theyarepartofthestandardlibraryandavailablealreadyforyoutousewithoutexternaldependenciesonthezipprogramtobeavailableonyourcomputer.

However,Ihavebeenusingtheos.systemwayofcreatingabackupintheaboveexamplespurelyforpedagogicalpurposes,sothattheexampleissimpleenoughtobeunderstoodbyeverybodybutrealenoughtobeuseful.

Canyoutrywritingthefifthversionthatusesthezipfilemoduleinsteadoftheos.systemcall?

TheSoftwareDevelopmentProcessWehavenowgonethroughthevariousphasesintheprocessofwritingasoftware.Thesephasescanbesummarisedasfollows:

1. What(Analysis)

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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

SummaryWehaveseenhowtocreateourownPythonprograms/scriptsandthevariousstagesinvolvedinwritingsuchprograms.YoumayfinditusefultocreateyourownprogramjustlikewedidinthischaptersothatyoubecomecomfortablewithPythonaswellasproblem-solving.

Next,wewilldiscussobject-orientedprogramming.

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

Classesandobjectsarethetwomainaspectsofobjectorientedprogramming.Aclasscreatesanewtypewhereobjectsareinstancesoftheclass.Ananalogyisthatyoucanhavevariablesoftypeintwhichtranslatestosayingthatvariablesthatstoreintegersarevariableswhichareinstances(objects)oftheintclass.

NoteforStaticLanguageProgrammers

Notethatevenintegersaretreatedasobjects(oftheintclass).ThisisunlikeC++andJava(beforeversion1.5)whereintegersareprimitivenativetypes.

Seehelp(int)formoredetailsontheclass.

C#andJava1.5programmerswillfindthissimilartotheboxingandunboxingconcept.

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

Fieldsareoftwotypes-theycanbelongtoeachinstance/objectoftheclassortheycanbelongtotheclassitself.Theyarecalledinstancevariablesandclassvariablesrespectively.

Aclassiscreatedusingtheclasskeyword.Thefieldsandmethodsoftheclassarelistedinanindentedblock.

Theself

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Classmethodshaveonlyonespecificdifferencefromordinaryfunctions-theymusthaveanextrafirstnamethathastobeaddedtothebeginningoftheparameterlist,butyoudonotgiveavalueforthisparameterwhenyoucallthemethod,Pythonwillprovideit.Thisparticularvariablereferstotheobjectitself,andbyconvention,itisgiventhenameself.

Although,youcangiveanynameforthisparameter,itisstronglyrecommendedthatyouusethenameself-anyothernameisdefinitelyfrownedupon.Therearemanyadvantagestousingastandardname-anyreaderofyourprogramwillimmediatelyrecognizeitandevenspecializedIDEs(IntegratedDevelopmentEnvironments)canhelpyouifyouuseself.

NoteforC++/Java/C#Programmers

TheselfinPythonisequivalenttothethispointerinC++andthethisreferenceinJavaandC#.

YoumustbewonderinghowPythongivesthevalueforselfandwhyyoudon'tneedtogiveavalueforit.Anexamplewillmakethisclear.SayyouhaveaclasscalledMyClassandaninstanceofthisclasscalledmyobject.Whenyoucallamethodofthisobjectasmyobject.method(arg1,arg2),thisisautomaticallyconvertedbyPythonintoMyClass.method(myobject,arg1,arg2)-thisisallthespecialselfisabout.

Thisalsomeansthatifyouhaveamethodwhichtakesnoarguments,thenyoustillhavetohaveoneargument-theself.

ClassesThesimplestclasspossibleisshowninthefollowingexample(saveasoop_simplestclass.py).

classPerson:

pass#Anemptyblock

p=Person()

print(p)

Output:

$pythonoop_simplestclass.py

<__main__.Personinstanceat0x10171f518>

HowItWorks

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101

Wecreateanewclassusingtheclassstatementandthenameoftheclass.Thisisfollowedbyanindentedblockofstatementswhichformthebodyoftheclass.Inthiscase,wehaveanemptyblockwhichisindicatedusingthepassstatement.

Next,wecreateanobject/instanceofthisclassusingthenameoftheclassfollowedbyapairofparentheses.(Wewilllearnmoreaboutinstantiationinthenextsection).Forourverification,weconfirmthetypeofthevariablebysimplyprintingit.IttellsusthatwehaveaninstanceofthePersonclassinthe__main__module.

Noticethattheaddressofthecomputermemorywhereyourobjectisstoredisalsoprinted.TheaddresswillhaveadifferentvalueonyourcomputersincePythoncanstoretheobjectwhereveritfindsspace.

MethodsWehavealreadydiscussedthatclasses/objectscanhavemethodsjustlikefunctionsexceptthatwehaveanextraselfvariable.Wewillnowseeanexample(saveasoop_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

Hereweseetheselfinaction.Noticethatthesay_himethodtakesnoparametersbutstillhastheselfinthefunctiondefinition.

The__init__methodTherearemanymethodnameswhichhavespecialsignificanceinPythonclasses.Wewillseethesignificanceofthe__init__methodnow.

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The__init__methodisrunassoonasanobjectofaclassisinstantiated.Themethodisusefultodoanyinitializationyouwanttodowithyourobject.Noticethedoubleunderscoresbothatthebeginningandattheendofthename.

Example(saveasoop_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__methodastakingaparametername(alongwiththeusualself).Here,wejustcreateanewfieldalsocalledname.Noticethesearetwodifferentvariableseventhoughtheyarebothcalled'name'.Thereisnoproblembecausethedottednotationself.namemeansthatthereissomethingcalled"name"thatispartoftheobjectcalled"self"andtheothernameisalocalvariable.Sinceweexplicitlyindicatewhichnamewearereferringto,thereisnoconfusion.

Whencreatingnewinstancep,oftheclassPerson,wedosobyusingtheclassname,followedbytheargumentsintheparentheses:p=Person('Swaroop').

Wedonotexplicitlycallthe__init__method.Thisisthespecialsignificanceofthismethod.

Now,weareabletousetheself.namefieldinourmethodswhichisdemonstratedinthesay_himethod.

ClassAndObjectVariables

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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(saveasoop_objvar.py):

classRobot:

"""Representsarobot,withaname."""

#Aclassvariable,countingthenumberofrobots

population=0

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."""

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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.

Wehave0robots.

HowItWorks

Thisisalongexamplebuthelpsdemonstratethenatureofclassandobjectvariables.Here,populationbelongstotheRobotclassandhenceisaclassvariable.Thenamevariablebelongstotheobject(itisassignedusingself)andhenceisanobjectvariable.

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Thus,werefertothepopulationclassvariableasRobot.populationandnotasself.population.Werefertotheobjectvariablenameusingself.namenotationinthemethodsofthatobject.Rememberthissimpledifferencebetweenclassandobjectvariables.Alsonotethatanobjectvariablewiththesamenameasaclassvariablewillhidetheclassvariable!

InsteadofRobot.population,wecouldhavealsousedself.__class__.populationbecauseeveryobjectreferstoitsclassviatheself.__class__attribute.

Thehow_manyisactuallyamethodthatbelongstotheclassandnottotheobject.Thismeanswecandefineitaseitheraclassmethodorastaticmethoddependingonwhetherweneedtoknowwhichclasswearepartof.Sincewerefertoaclassvariable,let'suseclassmethod.

Wehavemarkedthehow_manymethodasaclassmethodusingadecorator.

Decoratorscanbeimaginedtobeashortcuttocallingawrapperfunction,soapplyingthe@classmethoddecoratorissameascalling:

how_many=classmethod(how_many)

Observethatthe__init__methodisusedtoinitializetheRobotinstancewithaname.Inthismethod,weincreasethepopulationcountby1sincewehaveonemorerobotbeingadded.Alsoobservethatthevaluesofself.nameisspecifictoeachobjectwhichindicatesthenatureofobjectvariables.

Remember,thatyoumustrefertothevariablesandmethodsofthesameobjectusingtheselfonly.Thisiscalledanattributereference.

Inthisprogram,wealsoseetheuseofdocstringsforclassesaswellasmethods.WecanaccesstheclassdocstringatruntimeusingRobot.__doc__andthemethoddocstringasRobot.say_hi.__doc__

Inthediemethod,wesimplydecreasetheRobot.populationcountby1.

Allclassmembersarepublic.Oneexception:Ifyouusedatamemberswithnamesusingthedoubleunderscoreprefixsuchas__privatevar,Pythonusesname-manglingtoeffectivelymakeitaprivatevariable.

Thus,theconventionfollowedisthatanyvariablethatistobeusedonlywithintheclassorobjectshouldbeginwithanunderscoreandallothernamesarepublicandcanbeusedbyotherclasses/objects.RememberthatthisisonlyaconventionandisnotenforcedbyPython(exceptforthedoubleunderscoreprefix).

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NoteforC++/Java/C#Programmers

Allclassmembers(includingthedatamembers)arepublicandallthemethodsarevirtualinPython.

InheritanceOneofthemajorbenefitsofobjectorientedprogrammingisreuseofcodeandoneofthewaysthisisachievedisthroughtheinheritancemechanism.Inheritancecanbebestimaginedasimplementingatypeandsubtyperelationshipbetweenclasses.

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

Youcancreatetwoindependentclassesforeachtypeandprocessthembutaddinganewcommoncharacteristicwouldmeanaddingtobothoftheseindependentclasses.Thisquicklybecomesunwieldy.

AbetterwaywouldbetocreateacommonclasscalledSchoolMemberandthenhavetheteacherandstudentclassesinheritfromthisclassi.e.theywillbecomesub-typesofthistype(class)andthenwecanaddspecificcharacteristicstothesesub-types.

Therearemanyadvantagestothisapproach.Ifweadd/changeanyfunctionalityinSchoolMember,thisisautomaticallyreflectedinthesubtypesaswell.Forexample,youcanaddanewIDcardfieldforbothteachersandstudentsbysimplyaddingittotheSchoolMemberclass.However,changesinthesubtypesdonotaffectothersubtypes.AnotheradvantageisthatifyoucanrefertoateacherorstudentobjectasaSchoolMemberobjectwhichcouldbeusefulinsomesituationssuchascountingofthenumberofschoolmembers.Thisiscalledpolymorphismwhereasub-typecanbesubstitutedinanysituationwhereaparenttypeisexpectedi.e.theobjectcanbetreatedasaninstanceoftheparentclass.

Alsoobservethatwereusethecodeoftheparentclassandwedonotneedtorepeatitinthedifferentclassesaswewouldhavehadtoincasewehadusedindependentclasses.

TheSchoolMemberclassinthissituationisknownasthebaseclassorthesuperclass.TheTeacherandStudentclassesarecalledthederivedclassesorsubclasses.

Wewillnowseethisexampleasaprogram(saveasoop_subclass.py):

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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

member.tell()

Output:

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108

$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.Next,weobservethatthe__init__methodofthebaseclassisexplicitlycalledusingtheselfvariablesothatwecaninitializethebaseclasspartoftheobject.Thisisveryimportanttoremember-Sincewearedefininga__init__methodinTeacherandStudentsubclasses,PythondoesnotautomaticallycalltheconstructorofthebaseclassSchoolMember,youhavetoexplicitlycallityourself.

Incontrast,ifwehavenotdefinedan__init__methodinasubclass,Pythonwillcalltheconstructorofthebaseclassautomatically.

Wealsoobservethatwecancallmethodsofthebaseclassbyprefixingtheclassnametothemethodcallandthenpassintheselfvariablealongwithanyarguments.

NoticethatwecantreatinstancesofTeacherorStudentasjustinstancesoftheSchoolMemberwhenweusethetellmethodoftheSchoolMemberclass.

Also,observethatthetellmethodofthesubtypeiscalledandnotthetellmethodoftheSchoolMemberclass.OnewaytounderstandthisisthatPythonalwaysstartslookingformethodsintheactualtype,whichinthiscaseitdoes.Ifitcouldnotfindthemethod,itstartslookingatthemethodsbelongingtoitsbaseclassesonebyoneintheordertheyarespecifiedinthetupleintheclassdefinition.

Anoteonterminology-ifmorethanoneclassislistedintheinheritancetuple,thenitiscalledmultipleinheritance.

Theendparameterisusedintheprintfunctioninthesuperclass'stell()methodtoprintalineandallowthenextprinttocontinueonthesameline.Thisisatricktomakeprintnotprinta\n(newline)symbolattheendoftheprinting.

Summary

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Wehavenowexploredthevariousaspectsofclassesandobjectsaswellasthevariousterminologiesassociatedwithit.Wehavealsoseenthebenefitsandpitfallsofobject-orientedprogramming.Pythonishighlyobject-orientedandunderstandingtheseconceptscarefullywillhelpyoualotinthelongrun.

Next,wewilllearnhowtodealwithinput/outputandhowtoaccessfilesinPython.

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110

InputandOutputTherewillbesituationswhereyourprogramhastointeractwiththeuser.Forexample,youwouldwanttotakeinputfromtheuserandthenprintsomeresultsback.Wecanachievethisusingtheinput()functionandprintfunctionrespectively.

Foroutput,wecanalsousethevariousmethodsofthestr(string)class.Forexample,youcanusetherjustmethodtogetastringwhichisrightjustifiedtoaspecifiedwidth.Seehelp(str)formoredetails.

Anothercommontypeofinput/outputisdealingwithfiles.Theabilitytocreate,readandwritefilesisessentialtomanyprogramsandwewillexplorethisaspectinthischapter.

InputfromuserSavethisprogramasio_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:

InputandOutput

111

$python3io_input.py

Entertext:sir

No,itisnotapalindrome

$python3io_input.py

Entertext:madam

Yes,itisapalindrome

$python3io_input.py

Entertext:racecar

Yes,itisapalindrome

HowItWorks

Weusetheslicingfeaturetoreversethetext.We'vealreadyseenhowwecanmakeslicesfromsequencesusingtheseq[a:b]codestartingfrompositionatopositionb.Wecanalsoprovideathirdargumentthatdeterminesthestepbywhichtheslicingisdone.Thedefaultstepis1becauseofwhichitreturnsacontinuouspartofthetext.Givinganegativestep,i.e.,-1willreturnthetextinreverse.

Theinput()functiontakesastringasargumentanddisplaysittotheuser.Thenitwaitsfortheusertotypesomethingandpressthereturnkey.Oncetheuserhasenteredandpressedthereturnkey,theinput()functionwillthenreturnthattexttheuserhasentered.

Wetakethattextandreverseit.Iftheoriginaltextandreversedtextareequal,thenthetextisapalindrome.

Homeworkexercise

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

Ifyouneedahint,theideaisthat...

FilesYoucanopenandusefilesforreadingorwritingbycreatinganobjectofthefileclassandusingitsread,readlineorwritemethodsappropriatelytoreadfromorwritetothefile.Theabilitytoreadorwritetothefiledependsonthemodeyouhavespecifiedforthefileopening.Thenfinally,whenyouarefinishedwiththefile,youcalltheclosemethodtotellPythonthatwearedoneusingthefile.

Example(saveasio_using_file.py):

1

InputandOutput

112

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

First,openafilebyusingthebuilt-inopenfunctionandspecifyingthenameofthefileandthemodeinwhichwewanttoopenthefile.Themodecanbeareadmode('r'),writemode('w')orappendmode('a').Wecanalsospecifywhetherwearereading,writing,orappendingintextmode('t')orbinarymode('b').Thereareactuallymanymoremodesavailableandhelp(open)willgiveyoumoredetailsaboutthem.Bydefault,open()considersthefiletobea't'extfileandopensitin'r'eadmode.

Inourexample,wefirstopenthefileinwritetextmodeandusethewritemethodofthefileobjecttowritetothefileandthenwefinallyclosethefile.

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113

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

Intheend,wefinallyclosethefile.

Now,checkthecontentsofthepoem.txtfiletoconfirmthattheprogramhasindeedwrittentoandreadfromthatfile.

PicklePythonprovidesastandardmodulecalledpickleusingwhichyoucanstoreanyplainPythonobjectinafileandthengetitbacklater.Thisiscalledstoringtheobjectpersistently.

Example(saveasio_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)

Output:

$pythonio_pickle.py

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

HowItWorks

InputandOutput

114

Tostoreanobjectinafile,wehavetofirstopenthefileinwritebinarymodeandthencallthedumpfunctionofthepicklemodule.Thisprocessiscalledpickling.

Next,weretrievetheobjectusingtheloadfunctionofthepicklemodulewhichreturnstheobject.Thisprocessiscalledunpickling.

UnicodeSofar,whenwehavebeenwritingandusingstrings,orreadingandwritingtoafile,wehaveusedsimpleEnglishcharactersonly.

NOTE:IfyouareusingPython2,andwewanttobeabletoreadandwriteothernon-Englishlanguages,weneedtousetheunicodetype,anditallstartswiththecharacteru,e.g.u"helloworld"

>>>"helloworld"

'helloworld'

>>>type("helloworld")

<class'str'>

>>>u"helloworld"

'helloworld'

>>>type(u"helloworld")

<class'str'>

WhenwereadorwritetoafileorwhenwetalktoothercomputersontheInternet,weneedtoconvertourunicodestringsintoaformatthatcanbesentandreceived,andthatformatiscalled"UTF-8".Wecanreadandwriteinthatformat,usingasimplekeywordargumenttoourstandardopenfunction:

#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

Youcanignoretheimportstatementfornow,we'llexplorethatindetailinthemoduleschapter.

InputandOutput

115

WheneverwewriteaprogramthatusesUnicodeliteralslikewehaveusedabove,wehavetomakesurethatPythonitselfistoldthatourprogramusesUTF-8,andwehavetoput#encoding=utf-8commentatthetopofourprogram.

Weuseio.openandprovidethe"encoding"and"decoding"argumenttotellPythonthatweareusingunicode.

Youshouldlearnmoreaboutthistopicbyreading:

"TheAbsoluteMinimumEverySoftwareDeveloperAbsolutely,PositivelyMustKnowAboutUnicodeandCharacterSets"PythonUnicodeHowtoPragmaticUnicodetalkbyNatBatchelder

SummaryWehavediscussedvarioustypesofinput/output,aboutfilehandling,aboutthepicklemoduleandaboutUnicode.

Next,wewillexploretheconceptofexceptions.

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

1

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116

ExceptionsExceptionsoccurwhenexceptionalsituationsoccurinyourprogram.Forexample,whatifyouaregoingtoreadafileandthefiledoesnotexist?Orwhatifyouaccidentallydeleteditwhentheprogramwasrunning?Suchsituationsarehandledusingexceptions.

Similarly,whatifyourprogramhadsomeinvalidstatements?ThisishandledbyPythonwhichraisesitshandsandtellsyouthereisanerror.

ErrorsConsiderasimpleprintfunctioncall.WhatifwemisspeltprintasPrint?Notethecapitalization.Inthiscase,Pythonraisesasyntaxerror.

>>>Print("HelloWorld")

Traceback(mostrecentcalllast):

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

NameError:name'Print'isnotdefined

>>>print("HelloWorld")

HelloWorld

ObservethataNameErrorisraisedandalsothelocationwheretheerrorwasdetectedisprinted.Thisiswhatanerrorhandlerforthiserrordoes.

ExceptionsWewilltrytoreadinputfromtheuser.Press[ctrl-d],(or[ctrl-z]ifyouareusingWindows),andseewhathappens.

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

Entersomething-->Traceback(mostrecentcalllast):

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

EOFError

PythonraisesanerrorcalledEOFErrorwhichbasicallymeansitfoundanendoffilesymbol(whichisrepresentedbyctrl-d)whenitdidnotexpecttoseeit.

HandlingExceptions

Exceptions

117

Wecanhandleexceptionsusingthetry..exceptstatement.Webasicallyputourusualstatementswithinthetry-blockandputallourerrorhandlersintheexcept-block.

Example(saveasexceptions_handle.py):

try:

text=input('Entersomething-->')

exceptEOFError:

print('WhydidyoudoanEOFonme?')

exceptKeyboardInterrupt:

print('Youcancelledtheoperation.')

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/errorsinsidethetryblockandthenputhandlersfortheappropriateerrors/exceptionsintheexceptclause/block.Theexceptclausecanhandleasinglespecifiederrororexception,oraparenthesizedlistoferrors/exceptions.Ifnonamesoferrorsorexceptionsaresupplied,itwillhandleallerrorsandexceptions.

Notethattherehastobeatleastoneexceptclauseassociatedwitheverytryclause.Otherwise,what'sthepointofhavingatryblock?

Ifanyerrororexceptionisnothandled,thenthedefaultPythonhandleriscalledwhichjuststopstheexecutionoftheprogramandprintsanerrormessage.Wehavealreadyseenthisinactionabove.

Youcanalsohaveanelseclauseassociatedwithatry..exceptblock.Theelseclauseisexecutedifnoexceptionoccurs.

Exceptions

118

Inthenextexample,wewillalsoseehowtogettheexceptionobjectsothatwecanretrieveadditionalinformation.

RaisingExceptionsYoucanraiseexceptionsusingtheraisestatementbyprovidingthenameoftheerror/exceptionandtheexceptionobjectthatistobethrown.

TheerrororexceptionthatyoucanraiseshouldbeaclasswhichdirectlyorindirectlymustbeaderivedclassoftheExceptionclass.

Example(saveasexceptions_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))

else:

print('Noexceptionwasraised.')

Output:

$pythonexceptions_raise.py

Entersomething-->a

ShortInputException:Theinputwas1long,expectedatleast3

$pythonexceptions_raise.py

Entersomething-->abc

Noexceptionwasraised.

HowItWorks

Exceptions

119

Here,wearecreatingourownexceptiontype.ThisnewexceptiontypeiscalledShortInputException.Ithastwofields-lengthwhichisthelengthofthegiveninput,andatleastwhichistheminimumlengththattheprogramwasexpecting.

Intheexceptclause,wementiontheclassoferrorwhichwillbestoredasthevariablenametoholdthecorrespondingerror/exceptionobject.Thisisanalogoustoparametersandargumentsinafunctioncall.Withinthisparticularexceptclause,weusethelengthandatleastfieldsoftheexceptionobjecttoprintanappropriatemessagetotheuser.

Try...FinallySupposeyouarereadingafileinyourprogram.Howdoyouensurethatthefileobjectisclosedproperlywhetherornotanexceptionwasraised?Thiscanbedoneusingthefinallyblock.

Savethisprogramasexceptions_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:

Exceptions

120

$pythonexceptions_finally.py

Programmingisfun

Pressctrl+cnow

^C!!Youcancelledthereadingfromthefile.

(Cleaningup:Closedthefile)

HowItWorks

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

ObservethattheKeyboardInterruptexceptionisthrownandtheprogramquits.However,beforetheprogramexits,thefinallyclauseisexecutedandthefileobjectisalwaysclosed.

Notethatweusesys.stdout.flush()afterprintsothatitprintstothescreenimmediately.

ThewithstatementAcquiringaresourceinthetryblockandsubsequentlyreleasingtheresourceinthefinallyblockisacommonpattern.Hence,thereisalsoawithstatementthatenablesthistobedoneinacleanmanner:

Saveasexceptions_using_with.py:

withopen("poem.txt")asf:

forlineinf:

print(line,end='')

HowItWorks

Theoutputshouldbesameasthepreviousexample.Thedifferencehereisthatweareusingtheopenfunctionwiththewithstatement-weleavetheclosingofthefiletobedoneautomaticallybywithopen.

Whathappensbehindthescenesisthatthereisaprotocolusedbythewithstatement.Itfetchestheobjectreturnedbytheopenstatement,let'scallit"thefile"inthiscase.

Italwayscallsthethefile.__enter__functionbeforestartingtheblockofcodeunderitandalwayscallsthefile.__exit__afterfinishingtheblockofcode.

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121

Sothecodethatwewouldhavewritteninafinallyblockshouldbetakencareofautomaticallybythe__exit__method.Thisiswhathelpsustoavoidhavingtouseexplicittry..finallystatementsrepeatedly.

Morediscussiononthistopicisbeyondscopeofthisbook,sopleasereferPEP343foracomprehensiveexplanation.

SummaryWehavediscussedtheusageofthetry..exceptandtry..finallystatements.Wehaveseenhowtocreateourownexceptiontypesandhowtoraiseexceptionsaswell.

Next,wewillexplorethePythonStandardLibrary.

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StandardLibraryThePythonStandardLibrarycontainsahugenumberofusefulmodulesandispartofeverystandardPythoninstallation.ItisimportanttobecomefamiliarwiththePythonStandardLibrarysincemanyproblemscanbesolvedquicklyifyouarefamiliarwiththerangeofthingsthattheselibrariescando.

Wewillexploresomeofthecommonlyusedmodulesinthislibrary.YoucanfindcompletedetailsforallofthemodulesinthePythonStandardLibraryinthe'LibraryReference'sectionofthedocumentationthatcomeswithyourPythoninstallation.

Letusexploreafewusefulmodules.

CAUTION:Ifyoufindthetopicsinthischaptertooadvanced,youmayskipthischapter.However,IhighlyrecommendcomingbacktothischapterwhenyouaremorecomfortablewithprogrammingusingPython.

sysmoduleThesysmodulecontainssystem-specificfunctionality.Wehavealreadyseenthatthesys.argvlistcontainsthecommand-linearguments.

SupposewewanttochecktheversionofthePythonsoftwarebeingused,thesysmodulegivesusthatinformation.

>>>importsys

>>>sys.version_info

sys.version_info(major=3,minor=5,micro=1,releaselevel='final',serial=0)

>>>sys.version_info.major==3

True

HowItWorks

Thesysmodulehasaversion_infotuplethatgivesustheversioninformation.Thefirstentryisthemajorversion.Wecanpulloutthisinformationtouseit.

loggingmodule

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123

Whatifyouwantedtohavesomedebuggingmessagesorimportantmessagestobestoredsomewheresothatyoucancheckwhetheryourprogramhasbeenrunningasyouwouldexpectit?Howdoyou"storesomewhere"thesemessages?Thiscanbeachievedusingtheloggingmodule.

Saveasstdlib_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(

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

Ifyoudonothavethecatcommand,thenyoucanjustopenthetest.logfileinatexteditor.

HowItWorks

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124

Weusethreemodulesfromthestandardlibrary-theosmoduleforinteractingwiththeoperatingsystem,theplatformmoduleforinformationabouttheplatformi.e.theoperatingsystemandtheloggingmoduletologinformation.

First,wecheckwhichoperatingsystemweareusingbycheckingthestringreturnedbyplatform.platform()(formoreinformation,seeimportplatform;help(platform)).IfitisWindows,wefigureoutthehomedrive,thehomefolderandthefilenamewherewewanttostoretheinformation.Puttingthesethreepartstogether,wegetthefulllocationofthefile.Forotherplatforms,weneedtoknowjustthehomefolderoftheuserandwegetthefulllocationofthefile.

Weusetheos.path.join()functiontoputthesethreepartsofthelocationtogether.Thereasontouseaspecialfunctionratherthanjustaddingthestringstogetherisbecausethisfunctionwillensurethefulllocationmatchestheformatexpectedbytheoperatingsystem.

Weconfiguretheloggingmoduletowriteallthemessagesinaparticularformattothefilewehavespecified.

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

ModuleoftheWeekSeriesThereismuchmoretobeexploredinthestandardlibrarysuchasdebugging,handlingcommandlineoptions,regularexpressionsandsoon.

ThebestwaytofurtherexplorethestandardlibraryistoreadDougHellmann'sexcellentPythonModuleoftheWeekseries(alsoavailableasabook)andreadingthePythondocumentation.

SummaryWehaveexploredsomeofthefunctionalityofmanymodulesinthePythonStandardLibrary.ItishighlyrecommendedtobrowsethroughthePythonStandardLibrarydocumentationtogetanideaofallthemodulesthatareavailable.

Next,wewillcovervariousaspectsofPythonthatwillmakeourtourofPythonmorecomplete.

StandardLibrary

125

StandardLibrary

126

MoreSofarwehavecoveredamajorityofthevariousaspectsofPythonthatyouwilluse.Inthischapter,wewillcoversomemoreaspectsthatwillmakeourknowledgeofPythonmorewell-rounded.

PassingtuplesaroundEverwishedyoucouldreturntwodifferentvaluesfromafunction?Youcan.Allyouhavetodoisuseatuple.

>>>defget_error_details():

...return(2,'details')

...

>>>errnum,errstr=get_error_details()

>>>errnum

2

>>>errstr

'details'

Noticethattheusageofa,b=<someexpression>interpretstheresultoftheexpressionasatuplewithtwovalues.

ThisalsomeansthefastestwaytoswaptwovariablesinPythonis:

>>>a=5;b=8

>>>a,b

(5,8)

>>>a,b=b,a

>>>a,b

(8,5)

SpecialMethodsTherearecertainmethodssuchasthe__init__and__del__methodswhichhavespecialsignificanceinclasses.

Specialmethodsareusedtomimiccertainbehaviorsofbuilt-intypes.Forexample,ifyouwanttousethex[key]indexingoperationforyourclass(justlikeyouuseitforlistsandtuples),thenallyouhavetodoisimplementthe__getitem__()methodandyourjobis

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done.Ifyouthinkaboutit,thisiswhatPythondoesforthelistclassitself!

Someusefulspecialmethodsarelistedinthefollowingtable.Ifyouwanttoknowaboutallthespecialmethods,seethemanual.

__init__(self,...)

Thismethodiscalledjustbeforethenewlycreatedobjectisreturnedforusage.__del__(self)

Calledjustbeforetheobjectisdestroyed(whichhasunpredictabletiming,soavoidusingthis)

__str__(self)

Calledwhenweusetheprintfunctionorwhenstr()isused.__lt__(self,other)

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

__getitem__(self,key)

Calledwhenx[key]indexingoperationisused.__len__(self)

Calledwhenthebuilt-inlen()functionisusedforthesequenceobject.

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

>>>flag=True

>>>ifflag:print('Yes')

...

Yes

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

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LambdaFormsAlambdastatementisusedtocreatenewfunctionobjects.Essentially,thelambdatakesaparameterfollowedbyasingleexpression.Lambdabecomesthebodyofthefunction.Thevalueofthisexpressionisreturnedbythenewfunction.

Example(saveasmore_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

Noticethatthesortmethodofalistcantakeakeyparameterwhichdetermineshowthelistissorted(usuallyweknowonlyaboutascendingordescendingorder).Inourcase,wewanttodoacustomsort,andforthatweneedtowriteafunction.Insteadofwritingaseparatedefblockforafunctionthatwillgetusedinonlythisoneplace,weusealambdaexpressiontocreateanewfunction.

ListComprehensionListcomprehensionsareusedtoderiveanewlistfromanexistinglist.Supposeyouhavealistofnumbersandyouwanttogetacorrespondinglistwithallthenumbersmultipliedby2onlywhenthenumberitselfisgreaterthan2.Listcomprehensionsareidealforsuchsituations.

Example(saveasmore_list_comprehension.py):

listone=[2,3,4]

listtwo=[2*iforiinlistoneifi>2]

print(listtwo)

Output:

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$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*prefixontheargsvariable,allextraargumentspassedtothefunctionarestoredinargsasatuple.Ifa**prefixhadbeenusedinstead,theextraparameterswouldbeconsideredtobekey/valuepairsofadictionary.

TheassertstatementTheassertstatementisusedtoassertthatsomethingistrue.Forexample,ifyouareverysurethatyouwillhaveatleastoneelementinalistyouareusingandwanttocheckthis,andraiseanerrorifitisnottrue,thenassertstatementisidealinthissituation.Whentheassertstatementfails,anAssertionErrorisraised.Thepop()methodremovesandreturnsthelastitemfromthelist.

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>>>mylist=['item']

>>>assertlen(mylist)>=1

>>>mylist.pop()

'item'

>>>assertlen(mylist)>=1

Traceback(mostrecentcalllast):

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

AssertionError

Theassertstatementshouldbeusedjudiciously.Mostofthetime,itisbettertocatchexceptions,eitherhandletheproblemordisplayanerrormessagetotheuserandthenquit.

DecoratorsDecoratorsareashortcuttoapplyingwrapperfunctions.Thisishelpfulto"wrap"functionalitywiththesamecodeoverandoveragain.Forexample,IcreatedaretrydecoratorformyselfthatIcanjustapplytoanyfunctionandifanyexceptionisthrownduringarun,itisretriedagain,tillamaximumof5timesandwithadelaybetweeneachretry.Thisisespeciallyusefulforsituationswhereyouaretryingtomakeanetworkcalltoaremotecomputer:

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fromtimeimportsleep

fromfunctoolsimportwraps

importlogging

logging.basicConfig()

log=logging.getLogger("retry")

defretry(f):

@wraps(f)

defwrapped_f(*args,**kwargs):

MAX_ATTEMPTS=5

forattemptinrange(1,MAX_ATTEMPTS+1):

try:

returnf(*args,**kwargs)

except:

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

attempt,

MAX_ATTEMPTS,

(args,kwargs))

sleep(10*attempt)

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

MAX_ATTEMPTS,

(args,kwargs))

returnwrapped_f

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:

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$pythonmore_decorator.py

Writetoadatabaseormakeanetworkcalloretc.

Thiswillbeautomaticallyretriedifexceptionisthrown.

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

Traceback(mostrecentcalllast):

File"more_decorator.py",line14,inwrapped_f

returnf(*args,**kwargs)

File"more_decorator.py",line39,insave_to_database

raiseValueError(arg)

ValueError:Somebadvalue

Writetoadatabaseormakeanetworkcalloretc.

Thiswillbeautomaticallyretriedifexceptionisthrown.

HowItWorks

See:

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

DifferencesbetweenPython2andPython3See:

"Six"libraryPortingtoPython3ReduxbyArminPython3experiencebyPyDannyOfficialDjangoGuidetoPortingtoPython3DiscussiononWhataretheadvantagestopython3.x?

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

Next,wewilldiscusshowtoexplorePythonfurther.

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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.

1

WhatNext

134

ExampleCodeThebestwaytolearnaprogramminglanguageistowritealotofcodeandreadalotofcode:

PythonCookbookisanextremelyvaluablecollectionofrecipesortipsonhowtosolvecertainkindsofproblemsusingPython.Thisisamust-readforeveryPythonuser.PythonModuleoftheWeekisanotherexcellentmust-readguidetotheStandardLibrary.

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

VideosFullStackWebDevelopmentwithFlaskPyVideo

QuestionsandAnswersOfficialPythonDosandDon'tsOfficialPythonFAQNorvig'slistofInfrequentlyAskedQuestionsPythonInterviewQ&AStackOverflowquestionstaggedwithpython

TutorialsHiddenfeaturesofPythonWhat'stheonecodesnippet/pythontrick/etcdidyouwishyouknewwhenyoulearnedpython?Awaretek'scomprehensivelistofPythontutorials

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135

DiscussionIfyouarestuckwithaPythonproblem,anddon'tknowwhomtoask,thenthepython-tutorlististhebestplacetoaskyourquestion.

Makesureyoudoyourhomeworkbytryingtosolvingtheproblemyourselffirstandasksmartquestions.

NewsIfyouwanttolearnwhatisthelatestintheworldofPython,thenfollowtheOfficialPythonPlanet.

InstallinglibrariesThereareahugenumberofopensourcelibrariesatthePythonPackageIndexwhichyoucanuseinyourownprograms.

Toinstallandusetheselibraries,youcanusepip.

CreatingaWebsiteLearnFlasktocreateyourownwebsite.Someresourcestogetstarted:

FlaskOfficialQuickstartTheFlaskMega-TutorialExampleFlaskProjects

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

TherearelotsofchoicesforGUIusingPython:

Kivy

http://kivy.org

WhatNext

136

PyGTK

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).

VariousImplementationsThereareusuallytwopartsaprogramminglanguage-thelanguageandthesoftware.Alanguageishowyouwritesomething.Thesoftwareiswhatactuallyrunsourprograms.

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137

WehavebeenusingtheCPythonsoftwaretorunourprograms.ItisreferredtoasCPythonbecauseitiswrittenintheClanguageandistheClassicalPythoninterpreter.

TherearealsoothersoftwarethatcanrunyourPythonprograms:

Jython

APythonimplementationthatrunsontheJavaplatform.ThismeansyoucanuseJavalibrariesandclassesfromwithinPythonlanguageandvice-versa.

IronPython

APythonimplementationthatrunsonthe.NETplatform.Thismeansyoucanuse.NETlibrariesandclassesfromwithinPythonlanguageandvice-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

Summary

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138

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

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

1

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139

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(including

Appendix:FLOSS

140

MicrosoftIIS)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

So,goaheadandexplorethevast,freeandopenworldofFLOSS!

Appendix:FLOSS

141

Appendix:ColophonAlmostallofthesoftwarethatIhaveusedinthecreationofthisbookareFLOSS.

BirthoftheBookInthefirstdraftofthisbook,IhadusedRedHat9.0Linuxasthefoundationofmysetupandinthesixthdraft,IusedFedoraCore3Linuxasthebasisofmysetup.

Initially,IwasusingKWordtowritethebook(asexplainedinthehistorylesson).

TeenageYearsLater,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

Appendix:About

142

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

Forthetenthdraft,IswitchedtowritinginMarkdown+GitBookformat,usingtheSpacemacseditor.

AbouttheAuthorSeehttp://www.swaroopch.com/about/

Appendix:About

143

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:RevisionHistory4.0

Appendix:RevisionHistory

144

19Jan2016SwitchedbacktoPython3SwitchedbacktoMarkdown,usingGitBookandSpacemacs

3.0

31Mar2014RewrittenforPython2usingAsciiDocandadoc-mode.

2.1

03Aug2013RewrittenusingMarkdownandJasonBlevins'MarkdownMode

2.0

20Oct2012RewritteninPandocformat,thankstomywifewhodidmostoftheconversionfromtheMediawikiformatSimplifyingtext,removingnon-essentialsectionssuchasnonlocalandmetaclasses

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

08Mar2004

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Aftertremendousfeedbackandsuggestionsfromreaders,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.

0.90

18Dec2003Added2morechapters.OpenOfficeformatwithrevisions.

0.60

21Nov2003Fullyrewrittenandexpanded.

0.20

20Nov2003Correctedsometyposanderrors.

0.15

20Nov2003

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ConvertedtoDocBookXMLwithXEmacs.0.10

14Nov2003InitialdraftusingKWord.

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147

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.

Catalan

Appendix:Translations

148

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

MoisèsGómez-Iamadeveloperandalsoateacherofprogramming(normallyforpeoplewithoutanypreviousexperience).

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

Afterthisexperience,Ithoughtsomeotherpeopleinmycountrycouldtakebenefitfromittoo.ButEnglishlanguagecanbeabarrier.

So,whynottrytotranslateit?AndIdidforapreviousversionofBoP.

Imycountrytherearetwoofficiallanguages.IselectedtheCatalanlanguageassumingthatotherswilltranslateittothemorewidespreadSpanish.

ChineseThefollowingURLsareunavailablenow.

Translationsareavailableathttp://woodpecker.org.cn/abyteofpython_cn/chinese/andhttp://zhgdg.gitcafe.com/static/doc/byte_of_python.html.

JuanShen(orion_val@163.com)hasvolunteeredtotranslatethebooktoChinese.

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.

ChineseTraditionalFredLin(gasolin@gmail.com)hasvolunteeredtotranslatethebooktoChineseTraditional.

Appendix:Translations

149

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

Anexcitingfeatureofthistranslationisthatitalsocontainstheexecutablechinesepythonsourcessidebysidewiththeoriginalpythonsources.

FredLin-I'mworkingasanetworkfirmwareengineeratDeltaNetwork,andI'malsoacontributorofTurboGearswebframework.

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

Thetranslationareoriginallybasedonsimplifiedchineseversion,andsoonalotofrewriteweremadetofitthecurrentwikiversionandthequalityofreading.

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

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

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

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

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

Theirtranslationislocatedathttp://ftp.jaist.ac.jp/pub//sourceforge/a/ab/abop-german.berlios/

LutzHornsays:

Appendix:Translations

150

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!

Greek

Appendix:Translations

151

TheGreekUbuntuCommunitytranslatedthebookinGreek,foruseinouron-lineasynchronousPythonlessonsthattakeplaceinourforums.Contact@savvasradevicformoreinformation.

IndonesianDaniel(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

Appendix:Translations

152

ShunroDozono(dozono@gmail.com)istranslatingthebooktoJapanese.

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

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).

Appendix:Translations

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

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

Update:ThetranslationiscompleteandreadyasofOct2,2009.ThankstoDominik,histwostudentsandtheirfriendfortheirtimeandeffort!

DominikKozaczko-I'maComputerScienceandInformationTechnologyteacher.

PortugueseFidelViegas(fidel.viegas@gmail.com)hasvolunteeredtotranslatethebooktoPortuguese.

RomanianPaul-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

Appendix:Translations

154

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

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

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

Thisdownloadlinkisnolongeraccessible.

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

SlovakAlbertioWard(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

155

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

156

TranslationHow-to1. Thefullsourceofthebookisavailablefromhttps://github.com/swaroopch/byte-of-

python.2. Pleaseforktherepository.3. Then,fetchtherepositorytoyourcomputer.YouneedtoknowhowtouseGittodothat.4. ReadtheGitBookdocumentation,esp.theMarkdownsection.5. Starteditingthe.mdfilestotranslatetoyourlocallanguage.6. SignuponGitBook.com,createabookandyoucanseeabeautifullyrenderedwebsite,

withlinkstodownloadPDF,EPUB,etc.

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FeedbackThebookneedsthehelpofitsreaderssuchasyourselvestopointoutanypartsofthebookwhicharenotgood,notcomprehensibleoraresimplywrong.Pleasewritetothemainauthorortherespectivetranslatorswithyourcommentsandsuggestions.

Feedback

158