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

Mark Shannon mark at hotpy.org
Sun Jul 15 16:14:59 CEST 2012

Alex Gaynor wrote:

Hi all,

I've just submitted a PEP proposing making lengthhint 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: 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 _lengthhint_ 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

_lengthhint_. Types can then define _lengthhint_ 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 _lengthhint_ for other interpreter and non-standard library Python to implement. _lengthhint_ 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 _lengthhint_, 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 _lengthhint_, 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

Python-Dev mailing list Python-Dev at python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/mark%40hotpy.org

More information about the Python-Dev mailing list