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 [1] 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 [2].

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 [3]. 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.

Semantics

The semantics and design goals of class decorators are the same as for function decorators ([4], [5]); the only difference is that you're decorating a class instead of a function. The following two snippets are semantically identical:

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


@foo
@bar
class A:
  pass

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

Implementation

Adapting 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 grammar.

The Python AST and bytecode must be modified accordingly.

A reference implementation [6] 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.

References

Copyright

This document has been placed in the public domain.