On Monday March 28, 2011, Laura Creighton wrote:
> In a message of Tue, 29 Mar 2011 09:59:40 +1300, Carl Cerecke writes: > >Well, not really. Although I see what you are trying to say. > > >numbers (like strings) are immutable. There can ever be only one number 1 > >You can't change a number to something else. If add 5 to 3, the number 3 > >doesn't change, I get a new number, 8. > > This is merely an implementation detail. Note well that the issue has changed since the original post. The question at the time concerned the following code >>> a = 1 >>> b = a # b and a are two different 1s Carl correctly points out that b and a are not two different 1s in this case. The assignment statment of a = b has the same semantics for integers as it does for any other data type. a and b become aliases for the same underlying instance. On any platform, it will be the case that >>> a is b True folowing that assignment. The original misunderstanding involved the semantics of b += 2; that command does not mutate the underlying instance, but re-associates the identifier b to a new value (while a remains associated with the value 1). In your reply, the code fragment (below) shows a different issue regarding the re-use of integer literals in source code (as you assigned b=1 rather than b=a). In that case, it is system-dependent whether the interpretter uses different instances or the same instance, and you are right that there is no guarantee that equivalent instances will be identical instances. With regard, Michael > > Python 2.6.6 > Type "help", "copyright", "credits" or "license" for more information. > >>> w = 1 > >>> y = 1 > >>> w is y # this might surprise you > True > > [PyPy 1.4.1] > Type "help", "copyright", "credits" or "license" for more information. > > >>>> w = 1 > >>>> y = 1 > >>>> w is y # or are you so used to CPython that this surprises you? > False > > You can have as many number ones as you like. In CPython they are > cached to be the same object, but you shouldn't rely on that behaviour. > It's actually pretty dodgy to test the object identity on immutable > objects -- unless you want to surprise people like I tried to. > > Incidentally > > Python 2.6.6 > Type "help", "copyright", "credits" or "license" for more information. > >>> w = 5234 > >>> y = 5324 > >>> w is y > False > > so after a certain point, the CPython developers have decided that the > performance gains of cacheing all number xs as the same x isn't worth > it. > > Laura +----------------------------------------------- | Michael H. Goldwasser, Ph.D. | Associate Professor | Director of Computer Science | Dept. Mathematics and Computer Science | Saint Louis University | 220 North Grand Blvd. | St. Louis, MO 63103-2007 | | Office: Ritter Hall 108 | Email: goldw...@slu.edu | URL: http://cs.slu.edu/~goldwasser | Phone: (314) 977-7039 | Fax: (314) 977-1452 _______________________________________________ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
