What are decorated functions?

[Richard Jones]

Gabriel Genellina wrote:
> At Tuesday 22/8/2006 17:19, Wolfgang Draxinger wrote:
>> I'm just reading the python language reference and came around
>> the term "decorated functions", but I have no idea, for what
>> they could be used.
> 
> A decorator takes a function/method/callable, "decorates" (modifies)
> it in a certain way, and returns it.

It's important to note here that decorators, using the @decorate syntax[1],
*can only decorate methods and functions*. That is, they must be followed
by either another @decorator or a "def" statement. Anything else will
result in a SyntaxError. For example:

>>> def test(callable): return callable
...
>>> @test
... def foo(): pass
...
>>> @test
... @test
... @test
... def foo(): pass
...
>>> @test
... class bar:
  File "<stdin>", line 2
    class bar:
        ^
SyntaxError: invalid syntax
>>> @test
... file
  File "<stdin>", line 2
    file
       ^
SyntaxError: invalid syntax
>>>

So you can't decorate classes (the original PEP proposed it, but it was
dropped - see the discussion about this on python-dev[2]). You *can*
decorate a class' __init__ method, but that's not quite the same as eg.
implementing @singleton (mind you, we already have a bazillion ways to
implement the singleton pattern, so I don't think we're poorer for this
limitation <wink>)


    Richard

[1] of course, you can "anything = decorate(anything)"
[2] http://mail.python.org/pipermail/python-dev/2005-March/052369.html