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Re: Callable or not callable, that is the question!

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Ulrich Eckhardt wrote:

> Hello!
> 
> I just stumbled over a case where Python (2.7 and 3.3 on MS Windows)
> fail to detect that an object is a function, using the callable()
> builtin function. Investigating, I found out that the object was indeed
> not callable, but in a way that was very unexpected to me:
> 
>      class X:
>          @staticmethod
>          def example():
>              pass
>          test1 = example
>          test2 = [example,]
> 
>      X.example() # OK
>      X.test1() # OK
>      X.test2[0]() # TypeError: 'staticmethod' object is not callable

Slightly modified example:

>>> @staticmethod
... def example(): return 42
... 
>>> class X:
...     example = example
... 
>>> X.example()
42
>>> example()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'staticmethod' object is not callable

When you access an attribute its __get__() method is implicitly called. This 
is part of the descriptor protocol:

>>> X.example
<function example at 0x7fe45c0ea3b0>
>>> example
<staticmethod object at 0x7fe45aafc090>
>>> example.__get__(X)
<function example at 0x7fe45c0ea3b0>

While it would be easy to make staticmethod callable

>>> class Staticmethod(staticmethod):
...     def __call__(self, *args, **kw):
...             return self.__func__(*args, **kw)
... 
>>> @Staticmethod
... def foo(): return "bar"
... 
>>> foo()
'bar'
>>> X.baz = foo
>>> X.baz()
'bar'

I see no clear advantage over the current situation.

> Bug or feature?

No bug. Missing feature if you come up with a convincing use-case.

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Callable or not callable, that is the question! Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2013-07-11 15:05 +0200
  Re: Callable or not callable, that is the question! Peter Otten <__peter__@web.de> - 2013-07-11 16:11 +0200
    Re: Callable or not callable, that is the question! Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2013-07-12 08:41 +0200
      Re: Callable or not callable, that is the question! Duncan Booth <duncan.booth@invalid.invalid> - 2013-07-12 07:36 +0000
        Re: Callable or not callable, that is the question! Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-12 09:49 +0000
          Re: Callable or not callable, that is the question! Peter Otten <__peter__@web.de> - 2013-07-12 12:36 +0200
  Re: Callable or not callable, that is the question! Jason Swails <jason.swails@gmail.com> - 2013-07-11 10:28 -0400
  Re: Callable or not callable, that is the question! Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-12 02:12 +0000
    Re: Callable or not callable, that is the question! Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-12 01:11 -0600

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