[Python-Dev] PEP 0424: A method for exposing a length hint (original) (raw)

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: RevisionRevisionRevision 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

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 __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.

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:

Copyright

This document has been placed into the public domain.

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Alex

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