[Python-Dev] Dinamically set __call__ method

[Gregory Ewing]

Ethan Furman wrote:
>>
>>> On 11/06/2014 10:59 PM, dieter wrote:
>>>
>>>> A possibility to get the original approach implemented looks like:
>>>>
>>>>     make "__call__" a descriptor on the class which looks up the real
>>>>     method on the instance.
>>>
>>> This still wouldn't get the signatrue correct, though.
>>
>> Why not? Once the descriptor is resolved, you get the final
>> instance method - with the correct signature.
> 
> Hmmm... well, it wouldn't be correct on a class lookup, but, yeah, it 
> would be correct for an instance lookup -- and maybe that's good enough 
> for the OP.

Seems to depend on how you get hold of the object you're
inspecting the signature of. I did an experiment:

   class C(object):

   @property
     def __call__(self):
       return self.call

   def f(x, y):
     print("Called f with %s, %s" % (x, y))

   c = C()
   c.call = f
   c(17, 42)

prints:
   Called f with 17, 42

This works too:

   import inspect
   print(inspect.getargspec(c.__call__))

prints:
   ArgSpec(args=['x', 'y'], varargs=None, keywords=None, defaults=None)

But

   print(inspect.getargspec(c))

doesn't work:

   TypeError: <__main__.C object at 0x479250> is not a Python function

(BTW, I'm actually surprised that this technique makes c callable.
There must be more going on that just "look up __call__ in the class
object", because evaluating C.__call__ just returns the descriptor
and doesn't invoking the descriptor mechanism.)

-- 
Greg