134 lines
3.3 KiB
ReStructuredText
134 lines
3.3 KiB
ReStructuredText
PEP: 3129
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Title: Class Decorators
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Version: $Revision$
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Last-Modified: $Date$
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Author: Collin Winter <collinwinter@google.com>
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 01-May-2007
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Python-Version: 3.0
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Post-History: 07-May-2007
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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
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:pep:`obscure and unnecessary <318#motivation>`
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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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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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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 (:pep:`318#current-syntax`, :pep:`318#design-goals`);
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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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Adapting Python's grammar to support class decorators requires
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modifying two rules and adding a new rule::
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funcdef: [decorators] 'def' NAME parameters ['->' test] ':' suite
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compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt |
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with_stmt | funcdef | classdef
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need to be changed to ::
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decorated: decorators (classdef | funcdef)
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funcdef: 'def' NAME parameters ['->' test] ':' suite
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compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt |
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with_stmt | funcdef | classdef | decorated
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Adding ``decorated`` is necessary to avoid an ambiguity in the
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grammar.
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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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Acceptance
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==========
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There was virtually no discussion following the posting of this PEP,
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meaning that everyone agreed it should be accepted.
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The patch was committed to Subversion as revision 55430.
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References
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==========
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.. [#approval]
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https://mail.python.org/pipermail/python-dev/2006-March/062942.html
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.. [#motivation]
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https://mail.python.org/pipermail/python-dev/2006-March/062888.html
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.. [#implementation]
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https://bugs.python.org/issue1671208
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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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coding: utf-8
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End:
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