Think Python - Denison University

96 Chapter 10. Listsa = 'banana'b = 'banana'Weknowthataandbbothrefertoastring,butwedon’tknowwhethertheyrefertothesamestring.There aretwo possiblestates:ab’banana’’banana’ab’banana’In one case,aandbrefer to two different objects that have the same value. In the second case, theyrefer tothe sameobject.To check whether twovariables refertothe sameobject, you can use theisoperator.>>> a = 'banana'>>> b = 'banana'>>> a is bTrueInthisexample, Python only created one stringobject, and bothaandbrefertoit.But when you create twolists,you get two objects:>>> a = [1, 2, 3]>>> b = [1, 2, 3]>>> a is bFalseSo thestatediagram looks likethis:ab[ 1, 2, 3 ][ 1, 2, 3 ]In this case we would say that the two lists are equivalent, because they have the same elements,but not identical, because they are not the same object. If two objects are identical, they are alsoequivalent, but ifthey are equivalent, they arenot necessarily identical.Until now, we have been using “object” and “value” interchangeably, but it is more precise to saythat an object has a value. If you execute [1,2,3], you get a list object whose value is a sequenceofintegers. Ifanotherlisthasthesameelements,wesayithasthesamevalue,butitisnotthesameobject.10.11 AliasingIfareferstoan object and you assignb = a,then both variables refertothe sameobject:>>> a = [1, 2, 3]>>> b = a>>> b is aTrue