Alex Gaynor, 15.07.2012 00:11: > CPython currently defines an ``__length_hint__`` method on several types, such > as various iterators. This method is then used by various other functions > (such > as ``map``) to presize lists based on the estimated returned by > ``__length_hint__``. Types can then define ``__length_hint__`` which are not > sized, and thus should not define ``__len__``, but can estimate or compute a > size (such as many iterators). > > Proposal > ======== > > This PEP proposes formally documenting ``__length_hint__`` for other > interpreter and non-standard library Python to implement. > > ``__length_hint__`` must return an integer, and is not required to be > accurate. > It may return a value that is either larger or smaller than the actual size of > the container. It may raise a ``TypeError`` if a specific instance cannot have > its length estimated. It may not return a negative value.
I'd like to more visibly repeat my suggestion to make this a slot method "tp_length_hint()" in extension types that returns a Py_ssize_t. That suggests that a negative return value would have a special meaning instead of relying on return values like NotImplemented. The Python wrapper of that slot method could still implement a mapping for this. Return values could be -1 for "don't know" and -2 for "infinite" at the C level, and NotImplemented for "don't know" at the Python level. Not sure about a good Python value for "infinite". Maybe return -1 for "infinite" at both levels and -2/NotImplemented for "don't know" in C/Python? That would suggest -3 to propagate exceptions at the C level. Stefan _______________________________________________ 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
