PEP:
Title: Change to decorator syntax
Version:
Andy Dirnberger <andy@nycpython.com>
Status: Type: Content-Type: text/x-rst Created: Post-History: Resolution: http://bugs.python.org/issue19660
This PEP provides a relaxed syntax for decorators. Currently, a decorator statement is limited in what it can accept, and arbitrary expressions will not work. The @ sign may be followed only by a dotted name with an optional, single set of parentheses, itself optionally containing an argument list. This restriction was introduced deliberately, and this PEP argues for its relaxation.
Currently there is little documentation about the syntax that can be used for decorators. PEP-318 documents the syntax, but the Python documentation only specifies the syntax using Extended Backus-Naur Form (EBNF). There is also little explanation available for why the syntax was used.
Upon investigation of the EBNF, these restrictions seem to stand out
in the grammar. The only other part of the grammar that enforces the
use of a dotted name is the import statement.
Relaxing the restrictions imposed on decorators, new, arguably useful forms can be introduced. It would be possible to use decorators consisting of syntaxes available elsewhere in the grammar. Examples include chained callables and a decorator registry. While such techniques can be used today, they must be wrapped inside another callable or another similar approach.
- some of these restrictions may appear arbitrary a.b() but not a().b
- the wrapper approach makes all of these possible, but unsightly _ = lambda x: x @_( a().b ) def foo(): pass
- the restriction serves better as a style guide issue
The change described here requires only a lifting of a deliberate restriction, which requires very minimal changes to the Grammar and interpreter. Most work will be focused on modifying documentation and adding unit tests. Beyond the designs discussed in PEP-318, there were no alternative designs considered.
This relaxed syntax supports any callable expression for use as a decorator:
# indexed callables
@registry[name]
def foo():
return 42
# chained callables or other expressions
@spam().eggs
def foo():
return 42
@spam()()
def foo():
return 42
# callables determined by a conditional
@(spam if exp else eggs)
def foo():
return 42
# lambda expressions
@(lambda f: f)
def foo():
return 42
This document has been placed in the public domain.