2007-05-01 14:31:51 -04:00
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PEP: 3129
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Title: Class Decorators
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Version: $Revision: 53815 $
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Last-Modified: $Date: 2007-04-27 16:42:06 -0700 (Fri, 27 Apr 2007) $
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Author: Collin Winter <collinw at gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 1-May-2007
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Python-Version: 3.0
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Post-History:
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Abstract
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========
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This PEP proposes class decorators, an extension to the function
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and method decorators introduced in PEP 318.
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Rationale
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=========
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When function decorators were originally debated for inclusion in
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Python 2.4, class decorators were seen as obscure and unnecessary
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[#obscure]_ thanks to metaclasses. After several years' experience
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with the Python 2.4.x series of releases and an increasing
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familiarity with function decorators and their uses, the BDFL and
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the community re-evaluated class decorators and recommended their
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inclusion in Python 3.0 [#approval]_.
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The motivating use-case was to make certain constructs more easily
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expressed and less reliant on implementation details of the CPython
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interpreter. While it is possible to express class decorator-like
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functionality using metaclasses, the results are generally
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2007-05-01 15:08:28 -04:00
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unpleasant and the implementation highly fragile [#motivation]_. In
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addition, metaclasses are inherited, whereas class decorators are not,
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making metaclasses unsuitable for some, single class-specific uses of
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class decorators. The fact that large-scale Python projects like Zope
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were going through these wild contortions to achieve something like
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class decorators won over the BDFL.
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2007-05-01 14:31:51 -04:00
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Semantics
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=========
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The semantics and design goals of class decorators are the same as
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for function decorators ([#semantics]_, [#goals]_); the only
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difference is that you're decorating a class instead of a function.
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The following two snippets are semantically identical: ::
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class A:
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pass
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A = foo(bar(A))
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@foo
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@bar
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class A:
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pass
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For a detailed examination of decorators, please refer to PEP 318.
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Implementation
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==============
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The grammar for class declarations changes from ::
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classdef: 'class' NAME ['(' [testlist] ')'] ':' suite
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to ::
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classdef: [decorators] 'class' NAME ['(' [testlist] ')'] ':' suite
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The Python AST and bytecode must be modified accordingly.
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A reference implementation [#implementation]_ has been provided by
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Jack Diederich.
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References
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==========
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.. [#obscure]
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http://www.python.org/dev/peps/pep-0318/#motivation
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.. [#approval]
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http://mail.python.org/pipermail/python-dev/2006-March/062942.html
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.. [#motivation]
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http://mail.python.org/pipermail/python-dev/2006-March/062888.html
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.. [#semantics]
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http://www.python.org/dev/peps/pep-0318/#current-syntax
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.. [#goals]
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http://www.python.org/dev/peps/pep-0318/#design-goals
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.. [#implementation]
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http://python.org/sf/1671208
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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