@property; @classmethod; def f()

[Ian Kelly]

On 1/1/2011 6:55 PM, K. Richard Pixley wrote:
> Can anyone explain to me why this doesn't work?
>
> class Foo(object):
> @property
> @classmethod
> def f(cls):
> return 4
>
> I mean, I think it seems to be syntactically clear what I'm trying to
> accomplish. What am I missing?

First, because classmethod returns a classmethod instance, not a 
function, so what gets passed to property is the classmethod descriptor, 
not an actual callable.

Second, because a property descriptor just returns itself when accessed 
on the class.  It only works on instances.

To do what you want, I think you would need to write your own descriptor 
class.  Something like this:

class classproperty(object):

     def __init__(self, getter):
         self._getter = getter

     def __get__(self, instance, owner):
         return self._getter(owner)

class Foo(object):
     @classproperty
     def f(cls):
         return 4

Modify as needed if you want to accomodate setters and deleters as well.

Cheers,
Ian