2012/7/14 Alex Gaynor <alex.gaynor@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