[Python-Dev] PEP 0424: A method for exposing a length hint
alexandre.zani at gmail.com
Sun Jul 15 01:28:07 CEST 2012
On Sat, Jul 14, 2012 at 4:21 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
> On Sat, Jul 14, 2012 at 4:18 PM, Benjamin Peterson <benjamin at python.org>
>> 2012/7/14 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?
> ValueError, the same as with len.
>> > 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.
> Sounds reasonable to me! Should we just go ahead and strip those out now?
I'm +1 on not having a public API for this. Ultimately the contract
for a length hint will depend heavily upon what you need it for. Some
applications would require a length hint to be an "at least" others an
"at most" and others something else entirely. Given that the contract
here appears to be >=0, I don't think the length hint is particularly
useful to the public at large.
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