On Sat, Jul 14, 2012 at 4:21 PM, Alex Gaynor
On Sat, Jul 14, 2012 at 4:18 PM, Benjamin Peterson
wrote: 2012/7/14 Alex Gaynor
: 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.
-- Regards, Benjamin
Alex
-- "I disapprove of what you say, but I will defend to the death your right to say it." -- Evelyn Beatrice Hall (summarizing Voltaire) "The people's good is the highest law." -- Cicero
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