4.5.FILEPROCESSING 53
Youcanthinkofa textfileasa (possiblylong)stringthathappenstobestoredondisk.Ofcourse,a
typicalfilegenerallycontainsmorethana singlelineoftext.A specialcharacterorsequenceofcharactersis
usedtomarktheendofeachline.Therearenumerousconventionsforend-of-linemarkers.Pythonusesa
singlecharactercallednewlineasa marker.
Youcanthinkofnewlineasthecharacterproducedwhenyoupressthe Enter key onyourkeyboard.
Althougha newlineis a singlecharacter, it is representedinPython(andmany othercomputerlanguages)
usingthespecialnotation’
n’. Otherspecialcharactersarerepresentedusinga similarnotation(i.e.,’t’
for Tab ).
Let’s take a lookat a concreteexample.Supposeyoutypethefollowinglinesintoa texteditorexactlyas
shownhere.
Hello
World
Goodbye 32
Whenstoredtoa file,yougetthissequenceofcharacters.
Hello\nWorld\n\nGoodbye 32\n
Noticethattheblanklinebecomesa barenewlineintheresultingfile/string.
Bytheway, byembeddingnewlinecharactersintooutputstrings,youcanproducemultiplelinesofoutput
witha singleprintstatement.Hereis theexamplefromabove printedinteractively.
print "Hello\nWorld\n\nGoodbye 32\n"
Hello
World
Goodbye 32
If yousimplyaskPythontoevaluatea stringcontainingnewlinecharacters,youwilljustgettheembedded
newlinerepresentationbackagain.
"Hello\nWorld\n\nGoodbye 32\n"
’Hello\nWorld\n\nGoodbye 32\n’
It’s onlywhena stringis printedthatthespecialcharactersaffecthow thestringis displayed.
4.5.2 FileProcessing
Theexactdetailsoffile-processingdiffersubstantiallyamongprogramminglanguages,butvirtuallyalllan-
guagessharecertainunderlyingfilemanipulationconcepts.First,weneedsomewaytoassociatea fileon
diskwitha variableina program.Thisprocessis calledopeninga file.Oncea filehasbeenopened,it is
manipulatedthroughthevariableweassigntoit.
Second,weneeda setofoperationsthatcanmanipulatethefilevariable.Attheveryleast,thisincludes
operationsthatallow ustoreadtheinformationfroma fileandwritenew informationtoa file.Typically, the
readingandwritingoperationsfortextfilesaresimilartotheoperationsfortext-based,interactive inputand
output.
Finally, whenwearefinishedwitha file,it isclosed. Closinga filemakessurethatany bookkeepingthat
wasnecessaryto maintainthecorrespondencebetweenthefileondiskandthefilevariableis finishedup.For
example,if youwriteinformationtoa filevariable,thechangesmightnotshow uponthediskversionuntil
thefilehasbeenclosed.
Thisideaofopeningandclosingfilesis closelyrelatedto how youmightworkwithfilesin anapplication
programlike a wordprocessor. However, theconceptsarenotexactlythesame.Whenyouopena fileina
programlike MicrosoftWord,thefileis actuallyreadfromthediskandstoredintoRAM.Inprogramming