property using a classmethod

Bruno Desthuilliers bruno.42.desthuilliers at websiteburo.invalid
Fri Jul 10 12:14:26 CEST 2009


Lie Ryan a écrit :
> Bruno Desthuilliers wrote:
>> Lie Ryan a écrit :
>>> Emanuele D'Arrigo wrote:
>> (snip)
>>>> Ultimately all I want is a non-callable class-level attribute
>>>> MyClass.myProperty that gives the result of MyClass.myClassMethod().
>>> This works like what you seem to want (it's ugly):
>> Ugly, indeed. And an extreme case of arbitrary overcomplexification too :-/
>>
>> (snip rube goldberg code)
>>
> 
> Can't think of anything simpler than that without meddling with
> descriptor.

Hmmm... Rereading the OP's spec, I guess you understood it better than I 
did - seems the OP wants to be able to call the "property" on the class 
object itself - which won't work with the builtin property type. So my 
own proposed solution won't do :-/

But still, "meddling with descriptor" is *way* simpler than your 
proposed solution. Here's a simple non-binding descriptor that do the job:

class ClsProperty(object):
     def __init__(self, fget):
         if not isinstance(fget, (classmethod, staticmethod)):
             raise ValueError(
               "fget must be a classmethod or staticmethod"
               )
         self.fget = fget

     def __get__(self, obj, cls=None):
         if cls is None:
             assert obj is not None
             cls = type(obj)
         return self.fget.__get__(obj, cls)()


# example use
class Foo(object):
     @classmethod
     def bar(cls):
         return "%s.bar" % cls.__name__

     quux = ClsProperty(bar)


> I'm not even sure descriptor can help here as it seems
> descriptor needs an instance?

wrt/ descriptors, no, they don't "need" an instance, at least for 
non-binding descriptors. Else, MyClass.MyMethod would return the 
MyClass.__dict__['MyMethod'] function, not an unbound method object !-)





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