python-peps/pep-3129.txt

141 lines
3.5 KiB
Plaintext
Raw Normal View History

2007-05-01 14:31:51 -04:00
PEP: 3129
Title: Class Decorators
2007-05-07 15:05:28 -04:00
Version: $Revision$
Last-Modified: $Date$
Author: Collin Winter <collinwinter@google.com>
Status: Final
2007-05-01 14:31:51 -04:00
Type: Standards Track
Content-Type: text/x-rst
Created: 1-May-2007
Python-Version: 3.0
2007-05-07 15:05:28 -04:00
Post-History: 7-May-2007
2007-05-01 14:31:51 -04:00
Abstract
========
This PEP proposes class decorators, an extension to the function
and method decorators introduced in PEP 318.
Rationale
=========
When function decorators were originally debated for inclusion in
Python 2.4, class decorators were seen as obscure and unnecessary
[#obscure]_ thanks to metaclasses. After several years' experience
with the Python 2.4.x series of releases and an increasing
familiarity with function decorators and their uses, the BDFL and
the community re-evaluated class decorators and recommended their
inclusion in Python 3.0 [#approval]_.
The motivating use-case was to make certain constructs more easily
expressed and less reliant on implementation details of the CPython
interpreter. While it is possible to express class decorator-like
functionality using metaclasses, the results are generally
unpleasant and the implementation highly fragile [#motivation]_. In
addition, metaclasses are inherited, whereas class decorators are not,
making metaclasses unsuitable for some, single class-specific uses of
class decorators. The fact that large-scale Python projects like Zope
were going through these wild contortions to achieve something like
class decorators won over the BDFL.
2007-05-01 14:31:51 -04:00
Semantics
=========
The semantics and design goals of class decorators are the same as
for function decorators ([#semantics]_, [#goals]_); the only
difference is that you're decorating a class instead of a function.
The following two snippets are semantically identical::
2007-05-01 14:31:51 -04:00
class A:
pass
A = foo(bar(A))
2007-05-01 14:31:51 -04:00
@foo
@bar
class A:
pass
2007-05-01 14:31:51 -04:00
For a detailed examination of decorators, please refer to PEP 318.
Implementation
==============
2009-07-02 11:58:38 -04:00
Adapting Python's grammar to support class decorators requires
modifying two rules and adding a new rule::
2007-05-01 16:14:28 -04:00
funcdef: [decorators] 'def' NAME parameters ['->' test] ':' suite
2007-05-01 16:14:28 -04:00
compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt |
with_stmt | funcdef | classdef
2007-05-01 16:14:28 -04:00
need to be changed to ::
2007-05-01 16:14:28 -04:00
decorated: decorators (classdef | funcdef)
funcdef: 'def' NAME parameters ['->' test] ':' suite
2007-05-01 16:14:28 -04:00
compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt |
with_stmt | funcdef | classdef | decorated
Adding ``decorated`` is necessary to avoid an ambiguity in the
grammar.
2007-05-01 14:31:51 -04:00
The Python AST and bytecode must be modified accordingly.
A reference implementation [#implementation]_ has been provided by
Jack Diederich.
Acceptance
==========
There was virtually no discussion following the posting of this PEP,
meaning that everyone agreed it should be accepted.
The patch was committed to Subversion as revision 55430.
2007-05-01 14:31:51 -04:00
References
==========
.. [#obscure]
http://www.python.org/dev/peps/pep-0318/#motivation
2007-05-01 14:31:51 -04:00
.. [#approval]
2017-06-11 15:02:39 -04:00
https://mail.python.org/pipermail/python-dev/2006-March/062942.html
2007-05-01 14:31:51 -04:00
.. [#motivation]
2017-06-11 15:02:39 -04:00
https://mail.python.org/pipermail/python-dev/2006-March/062888.html
2007-05-01 14:31:51 -04:00
.. [#semantics]
http://www.python.org/dev/peps/pep-0318/#current-syntax
.. [#goals]
http://www.python.org/dev/peps/pep-0318/#design-goals
2007-05-01 14:31:51 -04:00
.. [#implementation]
http://python.org/sf/1671208
2007-05-01 14:31:51 -04:00
Copyright
=========
This document has been placed in the public domain.
..
Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 70
coding: utf-8
End: