On 9/30/05, Jim Jewett <[EMAIL PROTECTED]> wrote: > If I submit a documentation patch, should I say that > numbers, lists, strings, dictionaries, and tuples are > a special case, or should I just warn that some > container-like objects (including iterators) are always > True?
You seem to be going at this from the wrong direction. Boolean value are defined by calling __nonzero__ or __len__, whichever exists; if neither exists the answer is true (except for None which is special-cased only for historical reasons -- there's no reason why it couldn't have a __nonzero__ method. __nonzero__ is intended for object types that either want to have number-like behavior or have a special reason for wanting to act like a Boolean. __len__ is for sequences and mappings specifically -- everything that supports __getitem__ should have __len__ and everything that has __len__ should have __getitem__. (This is what broke for iterators in 2.4.) So if anything's an "exception", it's numbers -- strings, lists, tuples are sequences and dicts are mappings, and that's where they get their definition of Booleanness from. Always remember, user-defined classes can define __nonzero__ any way they wish, and they get what they deserve. Library designers however should try to follow established patterns. Testing for Queue emptiness via __nonzero__ seems unwarranted since a Queue doesn't have any other sequence behavior. "Containerish" behavior isn't enough to warrant empty <--> false; in some sense every object is a container (at least every object with a __dict__ attribute) and you sure don't want to map __len__ to self.__dict__.__len__... -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
