Alex Gaynor wrote:
Hi all,
I've just submitted a PEP proposing making __length_hint__ a public API for users to define and other VMs to implement:
These seems back-to-front. __length_hint__ is *used* by the VM, not provided by it. It should be part of the object model, rather than the API.
PEP: 424 Title: A method for exposing a length hint Version: $Revision$ Last-Modified: $Date Author: Alex Gaynor
Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 14-July-2012 Python-Version: 3.4 Abstract ========
CPython currently defines an ``__length_hint__`` method on several types, such as various iterators. This method is then used by various other functions (such as ``map``) to presize lists based on the estimated returned by
Don't use "map" as an example. map returns an iterator so it doesn't need __length_hint__
``__length_hint__``. Types can then define ``__length_hint__`` which are not sized, and thus should not define ``__len__``, but can estimate or compute a size (such as many iterators).
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.
Rather than raising a TypeError, why not return NotImplemented?
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__``.
Copyright =========
This document has been placed into the public domain.
.. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8
Alex
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