Revised PEP 318 - Function/Method Decorator Syntax

Michele Simionato mis6 at pitt.edu
Thu Jun 12 19:01:17 CEST 2003


Steven Taschuk <staschuk at telusplanet.net> wrote in message news:<mailman.1055379262.15114.python-list at python.org>...
> Quoth Michele Simionato:
>   [...]
> > IMHO, there is only one reasonable way of defining a "decorator": it
> > should be defined as a descriptor class, that takes a function and returns 
> > a descriptor object. classmethods and staticmethods work this way and 
> > "protected" should be the same. Now, if 'protected' is such a decorator, 
> > it would only accepts *functions* as input, not classmethod objects.
> 
> Functions are descriptors too, though (since they have __get__ for
> the unbound/bound method machinery). 

This reminds me that the biggest difference between custom descriptors 
and functions is that we cannot subclass the built-in FunctionType at
the present. 
I remember Tim Peters saying that this is possible in principle and was 
not done because of lack of serious interests for the moment being. 

> What if we thought of these
> things as descriptor decorators instead of function decorators?
> Then transitivity is no problem.  (Of course, they'd expect that
> the descriptor they wrap would have a __get__ which provides a
> callable.)
> 
>     def pre(prefunc):
>         class prewrapper(object):
>             def __init__(self, descriptor):
>                 self.descriptor = descriptor
>             def __get__(self, obj, type_=None):
>                 func = self.descriptor.__get__(obj, type_)    # <--
>                 def prewrapped(*args, **kwargs):
>                     prefunc(*args, **kwargs)
>                     return func(*args, **kwargs)
>                 prewrapped.__doc__ = func.__doc__
>                 return prewrapped
>         return prewrapper
> 
> For the specific intended use, it wouldn't matter that other
> callables (types, instances of classes with __call__ ...) are not
> necessarily also descriptors.
> 
> (The implementation above does have the wart that the prefunc
> doesn't see the self argument.)

I see your point. Still, multiple inheritance would seem more consistent
to me, and also would be more natural when extending the notation to classes
i.e.

class C[Traced,Protected]: pass

would have a metaclass inherited from Traced and Protected.

            M.




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