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

Alex Gaynor alex.gaynor at gmail.com
Sun Jul 15 00:11:46 CEST 2012

Hi all,

I've just submitted a PEP proposing making __length_hint__ a public API for 
users to define and other VMs to implement:

PEP: 424
Title: A method for exposing a length hint
Version: $Revision$
Last-Modified: $Date
Author: Alex Gaynor <alex.gaynor at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 14-July-2012
Python-Version: 3.4


CPython currently defines an ``__length_hint__`` method on several types, such
as various iterators. This method is then used by various other functions (such 
``map``) to presize lists based on the estimated returned by
``__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).


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.


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
* Should a function be added either to ``builtins`` or some other module which
  calls ``__length_hint__``, like ``builtins.len`` calls ``__len__``.


This document has been placed into the public domain.

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