pytut27

APython2.7programmingtutorialVrsion2.0JohnW.Shipman2013070712:18AbstractAtutorialforthPythonprogramminglanguag,rlas2.7.ThispublicationisaailablinWbform1andalsoasaPDFdocumnt2.Plasforwardanycommntstotccdoc@nmt.du.TablofContnts1.Introduction.21.1.StartingPythoninconrsationalmod..22.Python'snumrictyps32.1.Basicnumricoprations..32.2.Thassignmntstatmnt.52.3.Mormathmaticaloprations83.Charactrstringbasics103.1.Stringlitrals103.2.Indxingstrings..123.3.Stringmthods133.4.Thstringformatmthod184.Squnctyps..204.1.Functionsandopratorsforsquncs..214.2.Indxingthpositionsinasqunc..224.3.Slicingsquncs234.4.Squncmthods.254.5.Listmthods254.6.Thrang()function:cratingarithmticprogrssions264.7.Onalucanhamultiplnams275.Dictionaris295.1.Oprationsondictionaris..295.2.Dictionarymthods..315.3.Anamspacislikadictionary..336.Branching346.1.Conditionsandthbooltyp..346.2.Thifstatmnt356.3.Awordaboutindntingyourcod.386.4.Thforstatmnt:Looping..386.5.Thwhilstatmnt401http:www.nmt.dutcchlppubslangpytut272http:www.nmt.dutcchlppubslangpytut27pytut27.pdf1APython2.7programmingtutorialNwMxicoTchComputrCntrAboutthisdocumntThisdocumnthasbngnratdwithRndrXXEP. Visithttp:www.rndrx.comtolarnmorabout RndrXfamilyofsoftwarsolutionsfordigital typography.6.6.Spcialbranchstatmnts:brakandcontinu407.HowtowritaslfxcutingPythonscript..418.df:Dfiningfunctions.428.1.rturn:Rturningalusfromafunction..438.2.Functionargumntlistfaturs448.3.Kywordargumnts.458.4.Extrapositionalargumnts.468.5.Extrakywordargumnts..468.6.Documntingfunctionintrfacs.479.UsingPythonmoduls..479.1.Importingitmsfrommoduls.489.2.Importntirmoduls.499.3.Amodulisanamspac519.4.Buildyourownmoduls.5110.Inputandoutput5210.1.Radingfils..5210.2.Filpositioningforrandomaccssdics5410.3.Writingfils5411.Introductiontoobjctorintdprogramming.5511.1.Abrifhistoryofsnailracingtchnology5611.2.Scalarariabls.5611.3.Snailorintddatastructurs:Lists..5711.4.Snailorintddatastructurs:Alistoftupls..5811.5.Abstractdatatyps.6011.6.AbstractdatatypsinPython.6211.7.classSnailRun:Arysmallxamplclass..6211.8.Lifcyclofaninstanc.6411.9.Spcialmthods:Sortingsnailracdata..661.IntroductionThisdocumntcontainssomtutorialsforthPythonprogramminglanguag,asofPythonrsion2.7.ThstutorialsaccompanythfrPythonclassstaughtbythNwMxicoTchComputrCntr.AnothrgoodtutorialisatthPythonwbsit3.1.1.StartingPythoninconrsationalmodThistutorialmakshayusofPython'sconrsationalmod.WhnyoustartPythoninthisway,youwillsaninitialgrtingmssag,followdbythprompt“>>>”.OnaTCCworkstationinWindows,fromthStartmnu,slctAllProgramsPython2.7IDLE(PythonGUI).Youwillssomthinglikthis:3http:docs.python.orgtuttut.htmlNwMxicoTchComputrCntrAPython2.7programmingtutorial2ForLinuxorMacOS,fromashllprompt(suchas“$”forthbashshll),typ:pythonYouwillssomthinglikthis:$pythonPython2.7.1(r271:86832,Apr122011,16:15:16)[GCC4.6.020110331(RdHat4.6.02)]onlinux2Typhlp,copyright,crditsorlicnsformorinformation.>>>Whnyousth“>>>”prompt,youcantypaPythonxprssion,andPythonwillshowyouthrsultofthatxprssion.ThismaksPythonusfulasadskcalculator.(That'swhywsomtimsrfrtoconrsationalmodas“calculatormod”.)Forxampl:>>>1+12>>>EachsctionofthistutorialintroducsagroupofrlatdPythonfaturs.2.Python'snumrictypsPrttymuchallprogramsndtodonumriccalculations.Pythonhassralwaysofrprsntingnumbrs,andanassortmntofopratorstoopratonnumbrs.2.1.BasicnumricoprationsTodonumriccalculationsinPython,youcanwritxprssionsthatlookmororlsslikalgbraicxprssionsinmanyothrcommonlanguags.Th“+”opratorisaddition;“”issubtraction;us“”tomultiply;andus“”todiid.Hrarsomxampls:>>>99+1100>>>19998>>>7535>>>81993APython2.7programmingtutorialNwMxicoTchComputrCntrThxamplsinthisdocumntwilloftnusalotofxtraspacbtwnthpartsofthxprssion,justtomakthingsasirtorad.Howr,thsspacsarnotrquird:>>>99+1100>>>19998Whnanxprssioncontainsmorthanonopration,Pythondfinsthusualordrofoprations,sothathighrprcdncoprationslikmultiplicationanddiisionardonbforadditionandsubtraction.Inthisxampl,nthoughthmultiplicationcomsaftrthaddition,itisdonfirst.>>>2+3414IfyouwanttoorridthusualprcdncofPythonoprators,usparnthss:>>>(2+3)420Hr'sarsultyoumaynotxpct:>>>150Youmightxpctarsultof0.2,notzro.Howr,Pythonhasdiffrntkindsofnumbrs.Anynumbrwithoutadcimalpointisconsidrdanintgr,awholnumbr.Ifanyofthnumbrsinoldcontainadcimalpoint,thcomputationisdonusingfloatingpointtyp:>>>1.04.00.25>>>1.03.00.33333333333333331Thatscondxamplabomayalsosurprisyou.Whyisthlastdigitaon?InPython(andinprttymuchallothrcontmporaryprogramminglanguags),manyralnumbrscannotbrprsntdxactly.Thrprsntationof1.03.0hasaslightrrorinthsntnthdcimalplac.Thisbhaiormaybslightlyannoying,butinconrsationalmod,Pythondosn'tknowhowmuchprcisionyouwant,soyougtaridiculousamountofprcision,andthisshowsupthfactthatsomalusarapproximations.YoucanusPython'sprintstatmnttodisplayaluswithoutquitsomuchprcision:>>>print1.03.00.333333333333It'sokaytomixintgrandfloatingpointnumbrsinthsamxprssion.Anyintgralusarcorcdtothirfloatingpointquialnts.>>>print1.050.2>>>print15.00.2LatrwwilllarnaboutPython'sstringformatmthod,whichallowsyoutospcifyxactlyhowmuchprcisiontouswhndisplayingnumbrs.Fornow,lt'smoontosommorofthopratorsonnumbrs.NwMxicoTchComputrCntrAPython2.7programmingtutorial4Th“%”opratorbtwntwonumbrsgisyouthmodulo.Thatis,thxprssion“x%y”rturnsthrmaindrwhnxisdiiddbyy.>>>13%53>>>5.72%0.50.21999999999999975>>>print5.72%0.50.22Exponntiationisxprssdas“xy”,maningxtothypowr.>>>28256>>>2301073741824>>>2.00.51.4142135623730951>>>10.05.2158489.31924611141>>>2.01001.2676506002282294+30Thatlastnumbr,1.2676506002282294+30,isanxamplofxponntialorscintificnotation.Thisnumbrisradas“1.26765timstntoth30thpowr”.Similarly,anumbrlik1.6624isradas“1.66timstntothminus24thpowr”.Sofarwhasnxamplsofthintgrtyp,whichiscalldintinPython,andthfloatingpointtyp,calldthfloattypinPython.Pythonguarantsthatinttypsupportsalusbtwn2,147,483,648and2,147,483,647(inclusi).Thrisanothrtypcalldlong,thatcanrprsntmuchlargrintgralus.Pythonautomaticallyswitchstothistypwhnranxprssionhasalusoutsidthrangofintalus.Youwillslttr“L”apparatthndofsuchalus,butthyactjustlikrgularintgrs.>>>2501125899906842624L>>>21001267650600228229401496703205376L>>>2100010715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376L2.2.ThassignmntstatmntSofarwhaworkdonlywithnumricconstantsandoprators.Youcanattachanamtoaalu,andthataluwillstayaroundforthrstofyourconrsationalPythonsssion.Pythonnamsmuststartwithalttrorthundrbar()charactr;thrstofthnammayconsistoflttrs,undrbars,ordigits.Namsarcassnsiti:thnamCountisadiffrntnamthancount.5APython2.7programmingtutorialNwMxicoTchComputrCntrForxampl,supposyouwantdtoanswrthqustion,“howmanydaysisamillionsconds?”Wcanstartbyattachingthnamsctoaaluofamillion:>>>sc=16>>>sc1000000.0Astatmntofthistypiscalldanassignmntstatmnt.Tocomputthnumbrofminutsinamillionsconds,wdiidby60.Toconrtminutstohours,wdiidby60again.Toconrthourstodays,diidby24,andthatisthfinalanswr.>>>minuts=sc60.0>>>minuts16666.666666666668>>>hours=minuts60>>>hours277.77777777777777>>>days=hours24.>>>days11.574074074074074>>>printdays,hours,minuts,sc11.5740740741277.77777777816666.66666671000000.0Youcanattachmorthanonnamtoaalu.Usasrisofnams,sparatdbyqualsigns,likthis.>>>total=rmaining=50>>>printtotal,rmaining5050Thgnralformofanassignmntstatmntlookslikthis:nam1=nam2==xprssionHrarthrulsforaluatinganassignmntstatmnt:EachnamiissomPythonariablnam.Variablnamsmuststartwithithralttrorthundrbar()charactr,andthrmainingcharactrsmustblttrs,digits,orundrbarcharactrs.Exampls:skatKy;x47;sumofallfars.ThxprssionisanyPythonxprssion.Whnthstatmntisaluatd,firstthxprssionisaluatdsothatitisasinglalu.Forxampl,ifthxprssionis“(2+3)4”,thrsultingsinglaluisthintgr20.Thnallthnamsnamiarboundtothatalu.Whatdositmanforanamtobboundtoaalu?WhnyouarusingPythoninconrsationalmod,thnamsandaluyoudfinarstordinanaracalldthglobalnamspac.Thisaraislikatwocolumntabl,withnamsonthlftandalusonthright.Hrisanxampl.SupposyoustartwithabrandnwPythonsssion,andtypthislin:>>>i=5100Hriswhatthglobalnamspaclookslikaftrthxcutionofthisassignmntstatmnt.NwMxicoTchComputrCntrAPython2.7programmingtutorial6GlobalnamspacNamValuinti5100Inthisdiagram,thaluapparingonthrightshowsitstyp,int(intgr),andthalu,5100.InPython,alushatyps,butnamsarnotassociatdwithanytyp.Anamcanbboundtoaaluofanytypatanytim.So,aPythonnamislikaluggagtag:itidntifisaalu,andltsyourtriitlatr.Hrisanothrassignmntstatmnt,andadiagramshowinghowthglobalnamspacapparsaftrthstatmntisxcutd.>>>j=foo=i+1NamValuint5101int5100ifoojThxprssion“i+1”isquialntto“5100+1”,sincariabliisboundtothintgr5100.Thisxprssionrducstothintgralu5101,andthnthnamsjandfooarbothboundtothatalu.Youmightthinkofthissituationasbinglikonpicofbaggagwithtwotagstidtoit.Lt'sxaminthglobalnamspacaftrthxcutionofthisassignmntstatmnt:>>>foo=foo+1NamValuint5101int5100i5102intfoojBcausfoostartsoutboundtothintgralu5101,thxprssion“foo+1”simplifistothalu5102.Obiously,foo=foo+1dosn'tmaksnsinalgbra!Howr,itisacommonwayforprogrammrstoaddontoaalu.Notthatnamjisstillboundtoitsoldalu,5101.7APython2.7programmingtutorialNwMxicoTchComputrCntr2.3.MormathmaticaloprationsPythonhasanumbrofbuiltinfunctions.TocallafunctioninPython,usthisgnralform:f(arg1,arg2,)Thatis,usthfunctionnam,followdbyanopnparnthsis“(”,followdbyzroormorargumntssparatdbycommas,followdbyaclosingparnthsis“)”.Forxampl,throundfunctiontaksonnumricargumnt,andrturnsthnarstwholnumbr(asafloatnumbr).Exampls:>>>round(4.1)4.0>>>round(4.9)5.0>>>round(4.5)5.0Thrsultofthatlastcasissomwhatarbitrary,sinc4.5isquidistantfrom4.0and5.0.Howr,asinmostothrmodrnprogramminglanguags,thaluchosnisthonfurthrfromzro.Morxampls:>>>round(4.1)4.0>>>round(4.9)5.0>>>round(4.5)5.0Forhistoricalrasons,trigonomtricandtranscndntalfunctionsarnotbuiltintoPython.Ifyouwanttodocalculationsofthoskinds,youwillndtotllPythonthatyouwanttousthmathpackag.Typthislin:>>>frommathimportOncyouhadonthis,youwillbabltousanumbrofmathmaticalfunctions.Forxampl,sqrt(x)computsthsquarrootofx:>>>sqrt(4.0)2.0>>>sqrt(81)9.0>>>sqrt(100000)316.22776601683796Importingthmathmodulalsoaddstwoprdfindariabls,pi(asinπ)and,thbasofnaturallogarithms:>>>printpi,3.141592653592.71828182846Hr'sanxamplofafunctionthattaksmorthanargumnt.Thfunctionatan2(dy,dx)rturnstharctangntofalinwhosslopisdydx.>>>atan2(1.0,0.0)1.5707963267948966NwMxicoTchComputrCntrAPython2.7programmingtutorial8>>>atan2(0.0,1.0)0.0>>>atan2(1.0,1.0)0.78539816339744828>>>printpi40.785398163397Foracompltlistofallthfacilitisinthmathmodul,sthPythonquickrfrnc4.Hrarsommorxampls;logisthnaturallogarithm,andlog10isthcommonlogarithm:>>>log()1.0>>>log10()0.43429448190325182>>>xp(1.0)2.7182818284590451>>>sin(pi2)1.0>>>cos(pi2)6.123031769111886317Mathmatically,cos(π2)shouldbzro.Howr,likprttymuchallothrmodrnprogramminglanguags,transcndntalfunctionslikthisusapproximations.6.12×1017is,aftrall,prttyclostozro.Twomathfunctionsthatyoumayfindusfulincrtainsituations:floor(x)rturnsthlargstwholnumbrthatislssthanorqualtox.cil(x)rturnsthsmallstwholnumbrthatisgratrthanorqualtox.>>>floor(4.9)4.0>>>floor(4.1)4.0>>>floor(4.1)5.0>>>floor(4.9)5.0>>>cil(4.9)5.0>>>cil(4.1)5.0>>>cil(4.1)4.0>>>cil(4.9)4.0Notthatthfloorfunctionalwaysmostoward(minusinfinity),andcilalwaysmostoward+.4http:www.nmt.dutcchlppubspythonwbmathmodul.html9APython2.7programmingtutorialNwMxicoTchComputrCntr3.CharactrstringbasicsPythonhasxtnsifatursforhandlingstringsofcharactrs.Thrartwotyps:Astraluisastringofzroormor8bitcharactrs.ThcommoncharactrsyousonNorthAmricankyboardsallus8bitcharactrs.ThofficialnamforthischaractrstisASCII5,forAmricanStandardCodforInformationIntrchang.Thischaractrsthasonsurprisingproprty:allcapitallttrsarconsidrdlssthanalllowrcaslttrs,sothstringZsortsbforstringa.Aunicodaluisastringofzroormor32bitUnicodcharactrs.ThUnicodcharactrstcorsjustaboutrywrittnlanguagandalmostryspcialcharactrrinntd.W'llmainlytalkaboutworkingwithstralus,butmostunicodoprationsarsimilaroridntical,xcptthatUnicodlitralsarprcddwiththlttru:forxampl,abcistypstr,butuabcistypunicod.3.1.StringlitralsInPython,youcannclosstringconstantsinithrsinglquot('')ordoublquot()charactrs.>>>clonNam='Clm'>>>clonNam'Clm'>>>printclonNamClm>>>fairNam=FuturFair>>>printfairNamFuturFair>>>fairNam'FuturFair'Whnyoudisplayastringaluinconrsationalmod,Pythonwillusuallyussinglquotcharactrs.Intrnally,thalusarthsamrgardlssofwhichkindofquotsyouus.Notalsothatthprintstatmntshowsonlythcontntofastring,withoutanyquotsaroundit.Toconrtanintgr(inttyp)aluitoitsstringquialnt,usthfunction“str(i)”:>>>str(497)'497'>>>str(000)'0'Thinrsopration,conrtingastringsbackintoanintgr,iswrittnas“int(s)”:>>>>>>int(497)497>>>int(0)0>>>int(012thisain'tnonumbr)Tracback(mostrcntcalllast):5http:n.wikipdia.orgwikiASCIINwMxicoTchComputrCntrAPython2.7programmingtutorial10Fil<stdin>,lin1,in?ValuError:inalidlitralforint():012thisain'tnonumbrThlastxamplaboshowswhathappnswhnyoutrytoconrtastringthatisn'taalidnumbr.ToconrtastringscontaininganumbrinbasB,usthform“int(s,B)”:>>>int('0F',16)15>>>int(10101,2)21>>>int(0177776,8)65534Toobtainth8bitintgrcodcontaindinaoncharactrstrings,usthfunction“ord(s)”.Thinrsfunction,toconrtanintgritothcharactrthathascodi,us“chr(i)”.ThnumricalusofachcharactrardfindbythASCII6charactrst.>>>chr(97)'a'>>>ord(a)97>>>chr(65)'A'>>>ord('A')65Inadditiontothprintablcharactrswithcodsinthrangfrom32to127inclusi,aPythonstringcancontainanyofthothrunprintabl,spcialcharactrsaswll.Forxampl,thnullcharactr,whosofficialnamisNUL,isthcharactrwhoscodiszro.Onwaytowritsuchacharactristousthisform:'\xNN'whrNNisthcharactr'scodinhxadcimal(bas16)notation.>>>chr(0)'\x00'>>>ord('\x00')0Anothrspcialcharactryoumayndtodalwithisthnwlincharactr,whosofficialnamisLF(for“linfd”).Usthspcialscapsqunc“\n”toproducdthischaractr.>>>s=Twolin\nstring.>>>s'Twolin\nstring.'>>>printsTwolinstring.Asyoucans,whnanwlincharactrisdisplaydinconrsationalmod,itapparsas“\n”,butwhnyouprintit,thcharactrthatfollowsitwillapparonthnxtlin.Thcodforthischaractris10:6http:n.wikipdia.orgwikiASCII11APython2.7programmingtutorialNwMxicoTchComputrCntr>>>ord('\n')10>>>chr(10)'\n'Pythonhassralothrofthsscapsquncs.Thtrm“scapsqunc”rfrstoaconntionwhraspcialcharactr,th“scapcharactr”,changsthmaningofthcharactrsaftrit.Python'sscapcharactrisbackslash(\).ManingNamCodInputbackspacBS8\btabHT9\tDoublquot34\Singlquot'39\'Backslash\92\\Thrisanothrhandywaytogtastringthatcontainsnwlincharactrs:nclosthstringwithinthrpairsofquots,ithrsinglordoublquots.>>>multi=Thisstringcontainsthrlins.>>>multi'Thisstring\ncontainsthr\nlins.'>>>printmultiThisstringcontainsthrlins.>>>s2='''xyz'''>>>s2'\nxyz\n'>>>prints2xyz>>>NoticthatinPython'sconrsationalmod,whnyouprssEntratthndofalin,andPythonknowsthatyourlinisnotfinishd,itdisplaysa“”promptinstadofthusual“>>>”prompt.3.2.IndxingstringsToxtractonormorcharactrsfromastringalu,youhatoknowhowpositionsinastringarnumbrd.Hr,forxampl,isadiagramshowingallthpositionsofthstring'ijklm'.NwMxicoTchComputrCntrAPython2.7programmingtutorial12ijklm01245−5−4−3−2−13Inthdiagramabo,thnumbrsshowthpositionsbtwncharactrs.Position0isthpositionbforthfirstcharactr;position1isthpositionbtwnthfirstandcharactrs;andsoon.Youmayalsorfrtopositionsrlatitothndofastring.Position1rfrstothpositionbforthlastcharactr;2isthpositionbforthnxttolastcharactr;andsoon.Toxtractfromastringsthcharactrthatoccursjustaftrpositionn,usanxprssionofthisform:s[n]Exampls:>>>stuff='ijklm'>>>stuff[0]'i'>>>stuff[1]'j'>>>stuff[4]'m'>>>stuff[1]'m'>>>stuff[3]'k'>>>stuff[5]Tracback(mostrcntcalllast):Fil<stdin>,lin1,in?IndxError:stringindxoutofrangThlastxamplshowswhathappnswhnyouspcifyapositionaftrallthcharactrsinthstring.Youcanalsoxtractmultiplcharactrsfromastring;sSction4.3,“Slicingsquncs”(p.23).3.3.StringmthodsManyofthoprationsonstringsarxprssdasmthods.Amthodissortoflikafunctionthatlisonlyinsidalusofacrtaintyp.Tocallamthod,usthissyntax:xpr.mthod(arg1,arg2,)whrachargiisanargumnttothmthod,justlikanargumnttoafunction.Forxampl,anystringaluhasamthodcalldcntrthatproducsanwstringwiththoldalucntrd,usingxtraspacstopadthaluouttoaginlngth.Thismthodtaksasanargumntthdsirdnwlngth.Hr'sanxampl:>>>star=>>>star.cntr(5)''Thfollowingsctionsdscribsomofthmorcommonandusfulstringmthods.13APython2.7programmingtutorialNwMxicoTchComputrCntr3.3.1..cntr():CntrsomtxtGinsomstringalus,toproducanwstringcontainingscntrdinastringoflngthn,usthismthodcall:s.cntr(n)Thismthodtaksonargumntn,thsizofthrsult.Exampls:>>>k=Ni>>>k.cntr(5)'Ni'>>><>.cntr(12)'<>'NotthatinthfirstxamplwaraskingPythontocntrthstringNiinafildoflngth5.Clarlywcan'tcntra2charactrstringin5charactrs,soPythonarbitrarilyaddsthlftorspaccharactrbfortholdalu.3.3.2..ljust()and.rjust():PadtolngthonthlftorrightAnothrusfulstringmthodlftjustifisaaluinafildofaginlngth.Thgnralform:s.ljust(n)Foranystringxprssions,thismthodrturnsanwstringcontainingthcharactrsfromswithnoughspacsadddaftrittomakanwstringoflngthn.>>>Ni.ljust(4)'Ni'>>>Toolongtofit.ljust(4)'Toolongtofit'Notthatth.ljust()mthodwillnrrturnashortrstring.Ifthlngthisn'tnough,itjustrturnsthoriginalalu.Thrisasimilarmthodthatrightjustifisastringalu:s.rjust(n)Thismthodrturnsastringwithnoughspacsadddbforthalutomakastringoflngthn.Aswithth.ljust()mthod,itwillnrrturnastringshortrthanthoriginal.>>>Ni.rjust(4)'Ni'>>>m=floccinaucinihilipilification>>>m.rjust(4)'floccinaucinihilipilification'3.3.3..strip(),.lstrip(),and.rstrip():RmoladingandortrailingwhitspacSomtimsyouwanttormowhitspac(spacs,tabs,andnwlins)fromastring.Forastrings,usthsmthodstormoladingandtrailingwhitspac:s.strip()rturnsswithanyladingortrailingwhitspaccharactrsrmod.s.lstrip()rmosonlyladingwhitspac.NwMxicoTchComputrCntrAPython2.7programmingtutorial14s.rstrip()rmosonlytrailingwhitspac.>>>spacCas='\n\tMoon\t\t'>>>spacCas'\n\tMoon\t\t'>>>spacCas.strip()'Moon'>>>spacCas.lstrip()'Moon\t\t'>>>spacCas.rstrip()'\n\tMoon'3.3.4..count():Howmanyoccurrncs?Thmthods.count(t)sarchsstringstoshowmanytimsstringtoccursinit.>>>chiq=banana>>>chiq'banana'>>>chiq.count(a)3>>>chiq.count(b)1>>>chiq.count(x)0>>>chiq.count(an)2>>>chiq.count(ana)1Notth