[Python-3000] PEP 3129: Class Decorators

Collin Winter collinw at gmail.com
Mon May 7 20:29:25 CEST 2007

Guido pointed out that this PEP hadn't been sent to the list yet.


This PEP proposes class decorators, an extension to the function
and method decorators introduced in PEP 318.


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.


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: ::

  class A:
  A = foo(bar(A))

  class A:

For a detailed examination of decorators, please refer to PEP 318.


Adapating Python's grammar to support class decorators requires
modifying two rules and adding a new rule ::

 funcdef: [decorators] 'def' NAME parameters ['->' test] ':' suite

 compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt |
                with_stmt | funcdef | classdef

need to be changed to ::

 decorated: decorators (classdef | funcdef)

 funcdef: 'def' NAME parameters ['->' test] ':' suite

 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

The Python AST and bytecode must be modified accordingly.

A reference implementation [#implementation]_ has been provided by
Jack Diederich.


.. [#obscure]

.. [#approval]

.. [#motivation]

.. [#semantics]

.. [#goals]

.. [#implementation]

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