On Mon, Dec 29, 2008 at 11:46 AM, Feat <j...@ai.univ-paris8.fr> wrote: > At 11:30 -0500 29/12/08, David Hostetler wrote: >>The 'weird' results you were seeing when using 'is' were really just the >>python interpretor lifting up its skirts a bit and (inadvertantly perhaps) >>revealing when it had shared the memory storage for a string literal and when >>it hadn't. > > Yes: thank you. Is there any way to predict whether the values are going to > be shared or not? As a matter of fact, the ids are the same as long as I do > not use any for statement with the values in a sequence, and when I use any, > it will sometimes get them from the context and some other times it will make > up new values.
I think the interpreter only automatically interns extremely short strings, like one or two characters long, and the empty string of course. In any case, you shouldn't worry about it or really care about the mysterious rules the interpreter uses to decide when to share a string. As explained below, you almost always should use == and not `is`; thus, comparing strings for pointer equality almost never comes up in practice, so the 'sharing' rules don't matter much at all. > I had hoped I could tell my students how to discriminate between the two > cases. They are no C programmers yet, but they're fully aware of the > difference between the eq() and equal() Lisp predicates, eq() being totally > reliable as for pointers comparison. The difference between `is` and == is exactly the same as between eq() and equal(), pointer equality versus value equality. `is` is used extremely rarely, apart from comparisons with None, with `if foo is None:` being idiomatic in that case. Unless you're comparing with None or have very good reason, always use ==. It's similar to how one should always use .equals() rather than == with objects in Java. If you want to give your students a more straightforward example to demonstrate the difference, use a normal, non-builtin object instead of a string so that the interpreter doesn't do any unexpected magical sharing. For example, write a simple Point class and show how: a = Point(1, 2) b = Point(1, 2) c = a print a == b #---> True print a is b, b is a #---> False False print a is c, c is a #---> True True Cheers, Chris -- Follow the path of the Iguana... http://rebertia.com _______________________________________________ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig
