[Python-Dev] PEP 0424: A method for exposing a length hint

Sun Jul 15 01:18:38 CEST 2012

2012年7月14日 Alex Gaynor <alex.gaynor at 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