Python 3 Notes: Reading and Writing Methods

For such cases, the open() statement should include an encoding spcification, ... myfile = open('alice.txt', encoding='utf-8') # Reading a UTF-8 file; ... Goto:Na-RaeHan'shomepage   Python3Notes      [HOME|LING1330/2330] FileReadingandWritingMethods <> Onthispage:open(),file.read(),file.readlines(),file.write(),file.writelines(). Beforeproceeding,makesureyouunderstandtheconceptsoffilepathandCWD.Ifyourunintoproblems,visittheCommonPitfallssectionatthebottomofthispage. OpeningandClosinga"FileObject" AsseeninTutorials#12and#13,fileIO(input/output)operationsaredonethroughafiledataobject.Ittypicallyproceedsasfollows: Createafileobjectusingtheopen()function.Alongwiththefilename,specify: 'r'forreadinginanexistingfile(default;canbedropped), 'w'forcreatinganewfileforwriting, 'a'forappendingnewcontenttoanexistingfile. Dosomethingwiththefileobject(reading,writing). Closethefileobjectbycallingthe.close()methodonthefileobject. Below,myfileisthefiledataobjectwe'recreatingforreading.'alice.txt'isapre-existingtextfileinthesamedirectoryasthefoo.pyscript.Afterthefilecontentisreadin,.close()iscalledonmyfile,closingthefileobject. myfile=open('alice.txt','r')#Reading.'r'canbeomitted #...readfrommyfile... myfile.close()#Closingfile foo.py Below,myfileisopenedforwriting.Inthesecondinstance,the'a'switchmakessurethatthenewcontentistackedonattheendoftheexistingtextfile.Hadyouused'w'instead,theoriginalfilewouldhavebeenoverwritten. myfile=open('results.txt','w')#Thefileisnewlycreatedwherefoo.pyis #...writetomyfile... myfile.close()#Closingfile.VERYIMPORTANT! myfile=open('results.txt','a')#'a':appendinginsteadofoverwriting. #...addtexttothefile... myfile.close()#Closingfile.DON'TFORGET! foo.py Thereisonemorepieceofcrucialinformation:encoding.Somefilesmayhavetobereadasaparticularencodingtype,andsometimesyouneedtowriteoutafileinaspecificencodingsystem.Forsuchcases,theopen()statementshouldincludeanencodingspcification,withtheencoding='xxx'switch: myfile=open('alice.txt',encoding='utf-8')#ReadingaUTF-8file;'r'isomitted myfile=open('results.txt','w',encoding='utf-8')#FilewillbewritteninUTF-8 foo.py Mostly,youwillneed'utf-8'(8-bitUnicode),'utf-16'(16-bitUnicode),or'utf-32'(32-bit),butitmaybesomethingdifferent,especiallyifyouaredealingwithaforeignlanguagetext.Hereisafulllistofencodings. ReadingfromaFile OK,weknowhowtoopenandcloseafileobject.Butwhataretheactualcommandsforreading?Therearemultiplemethods. Firstoff,.read()readsintheentiretextcontentofthefileasasinglestring.Below,thefileisreadintoavariablenamedmarytxt,whichendsupbeingastring-typeobject.Downloadmary-short.txtandtryoutyourself.   >>>f=open('mary-short.txt') >>>marytxt=f.read()#Using.read() >>>f.close() >>>marytxt 'Maryhadalittlelamb,\nHisfleecewaswhiteassnow,\nAndeverywherethatMary went,\nThelambwassuretogo.\n' >>>type(marytxt)#marytxtisstringtype >>>len(marytxt)#marytxthas110characters 110 >>>print(marytxt[0]) M Next,.readlines()readsintheentiretextcontentofthefileasalistoflines,eachterminatingwithalinebreak.Below,youcanseemarylinesisalistofstrings,whereeachstringisalinefrommary-short.txt.   >>>f=open('mary-short.txt') >>>marylines=f.readlines()#Using.readlines() >>>f.close() >>>marylines ['Maryhadalittlelamb,\n','Hisfleecewaswhiteassnow,\n','Andeverywhere thatMarywent,\n','Thelambwassuretogo.\n'] >>>type(marylines)#marylinesislisttype >>>len(marylines)#marylineshas4lines 4 >>>print(marylines[0]) Maryhadalittlelamb, Lastly,ratherthanloadingtheentirefilecontentintomemory,youcaniteratethroughthefileobjectlinebylineusingthefor...inloop.Thismethodismorememory-efficientandthereforerecommendedwhendealingwithaverylargefile.Below,bible-kjv.txtisopened,andanylinecontainingsmiteisprintedout.Downloadbible-kjv.txtandtryoutyourself. f=open('bible-kjv.txt')#Thisisabigfile forlineinf:#Using'for...in'onfileobject if'smite'inline: print(line,)#','keepsprintfromaddingalinebreak f.close() foo.py WritingtoaFile Writingmethodsalsocomeinapair:.write()and.writelines().Likethecorrespondingreadingmethods,.write()handlesasinglestring,while.writelines()handlesalistofstrings. Below,.write()writesasinglestringeachtimetothedesignatedoutputfile:   >>>fout=open('hello.txt','w') >>>fout.write('Hello,world!\n')#.write(str) >>>fout.write('MynameisHomer.\n') >>>fout.write("Whatabeautifuldaywe'rehaving.\n") >>>fout.close() Thistime,wehavetobuy,alistofstrings,which.writelines()writesoutatonce:   >>>tobuy=['milk\n','butter\n','coffeebeans\n','arugula\n'] >>>fout=open('grocerylist.txt','w') >>>fout.writelines(tobuy)#.writelines(list) >>>fout.close() Notethatallstringsintheexampleshavethelinebreak'\n'attheend.Withoutit,allstringswillbeprintedoutonthesameline,whichiswhatwashappeninginTutorial13.Unliketheprintstatementwhichprintsoutastringonitsownnewline,writingmethodswillnottackonanewlinecharacter--youmustremembertosupply'\n'ifyouwishastringtooccupyitsownline. CommonPitfalls FileI/Oisnotoriouslyfraughtwithstumblingblocksforbeginningprogrammers.Belowarethemostcommonones. "Nosuchfileordirectory"error   >>>f=open('mary-short.txt') Traceback(mostrecentcalllast): File"",line1,in IOError:[Errno1]Nosuchfileordirectory:'mary-short.txt' YouaregettingthiserrorbecausePythonfailedtolocatethefileforreading.Makesureyouaresupplyingthecorrectfilepathandname.ReadfirstFilePathandCWD.Also,refertothis,thisandthisFAQ. Issueswithencoding   >>>f=open('mary-short.txt')#needencoding='utf-8' >>>marytxt=f.read() Traceback(mostrecentcalllast): File"",line1,in marytxt=f.read() File"C:\ProgramFiles(x86)\Python35-32\lib\encodings\cp1252.py",line23,indecode returncodecs.charmap_decode(input,self.errors,decoding_table)[0] UnicodeDecodeError:'charmap'codeccan'tdecodebyte0x81inposition36593:character mapsto "UnicodeDecodeError"meansyouhaveafileencodingissue.Eachcomputerhasitsownsystem-widedefaultencoding,andthefileyouaretryingtoopenisencodedinsomethingdifferent,mostlikelysomeversionofUnicode.Ifthishappens,youshouldspecifytheencodingusingtheencoding='xxx'switchwhileopeningthefile.Ifyouarenotsurewhichencodingtouse,try'utf-8','utf-16',and'utf-32'. EntirefilecontentcanbereadinonlyONCEperopening   >>>f=open('mary-short.txt') >>>marytxt=f.read()#Readsinentirefilecontent >>>marylines=f.readlines()#Nothinglefttoread,returnsnothing >>>f.close() >>>len(marytxt) 110 >>>len(marylines)#marylinesisempty! 0 Both.read()and.readlines()comewiththeconceptofacursor.Aftereithercommandisexecuted,thecursormovestotheendofthefile,leavingnothingmoretoreadin.Therefore,onceafilecontenthasbeenreadin,anotherattempttoreadfromthefileobjectwillproduceanemptydataobject.Ifforsomereasonyoumustreadthefilecontentagain,youmustcloseandre-openthefile. Onlythestringtypecanbewritten   >>>pi=3.141592 >>>fout=open('math.txt','w') >>>fout.write("Pi'svalueis") >>>fout.write(pi)#tryingtowritefloat,doesn'twork Traceback(mostrecentcalllast): File"",line1,in TypeError:expectedacharacterbufferobject >>>fout.write(str(pi))#turnnumberintostringusingstr() >>> Writingmethodsonlyworkswithstrings:.write()takesasinglestring,and.writelines()takesalistwhichcontainsstringsonly.Non-stringtypedatamustbefirstcoercedintothestringtypebyusingthestr()function. Youroutputfileisempty   output.txt Thishappenstoeveryone:youwritesomethingout,openupthefiletoview,onlytofinditempty.Inothertimes,thefilecontentmaybeincomplete.Curious,isn'tit?Well,thecauseissimple:YOUFORGOT.close().Writingouthappensinbuffers;flushingoutthelastwritingbufferdoesnothappenuntilyoucloseyourfileobject.ALWAYSREMEMBERTOCLOSEYOURFILEOBJECT. (Windows)Linebreaksdonotshowup IfyouopenupyourtextfileinNotepadappinWindowsandseeeverythinginoneline,don'tbealarmed.OpenthesametextfileinWordpador,evenbetter,Notepad++,andyouwillseethatthelinebreaksarethereafterall.SeethisFAQfordetails.