Think Python - Denison University

Chapter 14Files14.1 PersistenceMost of the programs we have seen so far are transient in the sense that they run for a short timeandproduce someoutput, butwhen theyend, theirdatadisappears. Ifyouruntheprogram again, itstartswithaclean slate.Other programs are persistent: they run for a long time (or all the time); they keep at least some oftheir data in permanent storage (a hard drive, for example); and if they shut down and restart, theypick up where they leftoff.Examplesofpersistentprogramsareoperatingsystems,whichrunprettymuchwheneveracomputerison, and web servers,which run allthe time, waitingforrequests tocome inon thenetwork.Oneofthesimplestwaysforprogramstomaintaintheirdataisbyreadingandwritingtextfiles. Wehavealreadyseenprogramsthatreadtextfiles;inthischapterwewillseeprogramsthatwritethem.Analternativeistostorethestateoftheprograminadatabase. InthischapterIwillpresentasimpledatabase and amodule,pickle,that makes iteasytostoreprogram data.14.2 Readingand writingAtextfileisasequenceofcharactersstoredonapermanentmediumlikeaharddrive,flashmemory,or CD-ROM. Wesaw how toopen and read afileinSection 9.1.To writeafile, you have toopen itwithmode'w' as asecond parameter:>>> fout = open('output.txt', 'w')>>> print foutIf the file already exists, opening it in write mode clears out the old data and starts fresh, so becareful! Ifthefiledoesn’t exist, anew one iscreated.Thewritemethod puts data intothe file.