Think Python - Denison University

140 Chapter 14. FilesThepicklemodulecanhelp. Ittranslatesalmostanytypeofobjectintoastringsuitableforstorageinadatabase, and then translates stringsback intoobjects.pickle.dumpstakesanobjectasaparameterandreturnsastringrepresentation(dumpsisshortfor“dump string”):>>> import pickle>>> t = [1, 2, 3]>>> pickle.dumps(t)'(lp0\nI1\naI2\naI3\na.'The format isn’t obvious to human readers; it is meant to be easy for pickle to interpret.pickle.loads(“load string”) reconstitutes the object:>>> t1 = [1, 2, 3]>>> s = pickle.dumps(t1)>>> t2 = pickle.loads(s)>>> print t2[1, 2, 3]Although thenew object has thesame value asthe old, itisnot (ingeneral) the sameobject:>>> t1 == t2True>>> t1 is t2FalseInother words, pickling and then unpickling has the sameeffect ascopying theobject.You can use pickle to store non-strings in a database. In fact, this combination is so common thatithas been encapsulated inamodule calledshelve.Exercise14.3 IfyoudidExercise12.4,modifyyoursolutionsothatitcreatesadatabasethatmapsfrom each word inthelisttoalistof words that use thesame setof letters.Writeadifferentprogramthatopensthedatabaseandprintsthecontentsinahuman-readableformat.14.8 PipesMost operating systems provide a command-line interface, also known as a shell. Shells usuallyprovide commands to navigate the file system and launch applications. For example, in Unix, youcanchangedirectorieswithcd,displaythecontentsofadirectorywithls,andlaunchawebbrowserby typing (forexample)firefox.Any program that you can launch from the shell can also be launched from Python using a pipe. Apipe isan object that represents arunning process.For example, the Unix command ls -l normally displays the contents of the current directory (inlong format). You can launchlswithos.popen:>>> cmd = 'ls -l'>>> fp = os.popen(cmd)