On Sun, Jul 15, 2012 at 9:18 AM, Benjamin Peterson <benja...@python.org> wrote: >> Open questions >> ============== >> >> There are two open questions for this PEP: >> >> * Should ``list`` expose a kwarg in it's constructor for supplying a length >> hint. >> * Should a function be added either to ``builtins`` or some other module >> which >> calls ``__length_hint__``, like ``builtins.len`` calls ``__len__``. > > Let's try to keep this as limited as possible for a public API.
Length hints are very useful for *any* container implementation, whether those containers are in the standard library or not. Just as we exposed operator.index when __index__ was added, we should expose an "operator.length_hint" function with the following semantics: def length_hint(obj): """Return an estimate of the number of items in obj. This is useful for presizing containers when building from an iterable. If the object supports len(), the result will be exact. Otherwise, it may over or underestimate by an arbitrary amount. The result will be an integer >= 0. """ try: return len(obj) except TypeError: try: get_hint = obj.__length_hint__ except AttributeError: return 0 hint = get_hint() if not isinstance(hint, int): raise TypeError("Length hint must be an integer, not %r" % type(hint)) if hint < 0: raise ValueError("Length hint (%r) must be >= 0" % hint) return hint There's no reason to make pure Python container implementations reimplement all that for themselves. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia _______________________________________________ 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