2012/7/14 Alex Gaynor <alex.gay...@gmail.com>: > > 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.
And what happens if you return a negative value? > > Rationale > ========= > > Being able to pre-allocate lists based on the expected size, as estimated by > ``__length_hint__``, can be a significant optimization. CPython has been > observed to run some code faster than PyPy, purely because of this > optimization > being present. > > 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. -- Regards, Benjamin _______________________________________________ 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
