> Either list creation is somewhat > costly, or "if var is None" is really cheap.
"if x is y" is extremely cheap, I believe. Unlike most comparisons which are (relatively) expensive, that one is just comparing simple object address. You can't override "is" so there's a whole series of checks that don't have to get done. You don't have to go through the richcompare machinery, check if there's a __ne__, etc, etc. --S -- http://mail.python.org/mailman/listinfo/python-list
