I've been doing more thinking, and I think the problem is more deeply rooted than this.
From the original doc: > The operators is and is not compare whether two objects are really the same > object; > this only matters for mutable objects like lists. This is actually wrong. If x is y, then you _aren't_ comparing two objects, you are comparing one object with itself, and that is the whole point. You are comparing two names, and seeing if they are bound to the same object. What I really want to tell the perplexed to do is read: http://effbot.org/zone/python-objects.htm However: Given that this is in the section on comparing sequences and other types: The operators ``is`` and ``is not`` test for object identity. If two different names are bound to the same object, then they compare the same: ``x is y`` . >>> v = [1,2,3] >>> w = v >>> v is w True >>> v.append(4) >>> v [1, 2, 3, 4] >>> w [1, 2, 3, 4] Confusion arises because implementors are free to cache immutable objects with the same value and type to the same object for reasons of efficiency. [PyPy 1.4.1] Type "help", "copyright", "credits" or "license" for more information. >>>> w = 1 >>>> y = 1 >>>> w is y False Python 2.6.6 #(I don't have a 3.x around , I assume it works the same way) Type "help", "copyright", "credits" or "license" for more information. >>> w = 1 >>> y = 1 >>> w is y # this might surprise you True Testing the object identity of unmutable objects with the ``is`` or ``is not`` operators is rarely something you want to do, because the result is implementation dependent. Singleton objects such as ``True``, ``False``, and ``None`` are always the same object. The canonical way to test whether an object is a singleton is to test for object identity, not equality. So >>> if x is None: ... do something ... not >>> if x == None: ... do something ... -------------- I think this will fix the confusion, but maybe it belongs someplace other than the comparing sequence section. http://docs.python.org/py3k/tutorial/datastructures.html#comparing-sequences-and-other-types _______________________________________________ Doc-SIG maillist - Doc-SIG@python.org http://mail.python.org/mailman/listinfo/doc-sig
