[Python-ideas] Fix that broken callable builtin

[Ionel Cristian Mărieș]

Well yes, from that example it look right, because the call operator uses
the __call__ attribute from the type of the object. However, when the call
operator gets the __call__ method it will actually use it as a descriptor.
>From that perspective it's inconsistent.

Also there's the issue about not being able to implement a true proxy (as
outlined before).

What actually prevents this being fixed?


Thanks,
-- Ionel Cristian Mărieș, http://blog.ionelmc.ro

On Fri, Apr 17, 2015 at 11:55 PM, Guido van Rossum <guido at python.org> wrote:

> I think you're fighting windmills. Like most special operations (e.g.
> __add__), a __call__ attribute on the object does not work, i.e. it does
> not make the object callable. E.g.
>
> $ python3
> Python 3.5.0a2 (v3.5.0a2:0337bd7ebcb6, Mar  8 2015, 01:12:06)
> [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
> >>> class C: pass
> ...
> >>> c = C()
> >>> c.__call__ = lambda *a: a
> >>> c()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: 'C' object is not callable
> >>> callable(c)
> False
> >>> hasattr(c, '__call__')
> True
> >>>
>
> On Fri, Apr 17, 2015 at 1:45 PM, Ionel Cristian Mărieș <contact at ionelmc.ro
> > wrote:
>
>> Hello,
>>
>> I had an issue today with the `callable` builtin because it doesn't
>> correctly check that the object has the __call__ attribute.
>>
>> Effectively what `callable(a)` does is `hasattr(type(a), '__call__')` but
>> that's not very straightforward. A more straightforward implementation
>> would do something like `hasattr(a, '__call__')`.
>>
>> For example:
>>
>> Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64
>>> bit (AMD64)] on win32
>>> Type "help", "copyright", "credits" or "license" for more information.
>>> >>> callable
>>> <built-in function callable>
>>> >>> class A:
>>> ...  @property
>>> ...  def __call__(self):
>>> ...   raise AttributeError('go away')
>>> ...
>>> >>> a = A()
>>> >>> a
>>> <__main__.A object at 0x000000000365B5C0>
>>> >>> a.__call__
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>>   File "<stdin>", line 4, in __call__
>>> AttributeError: go away
>>> >>> callable(a)
>>> True
>>> >>> # it should be False :(
>>>
>>
>> ​So it boils down to this:
>>
>>> >>> hasattr(a, "__call__")
>>> False
>>> >>> hasattr(type(a), "__call__")
>>> True
>>
>>
>> My issue is that I didn't call `callable(type(a))` but just
>> `callable(a)`. Clearly mismatching what happens when you do hasattr(a,
>> "__call__").
>>
>> To put in contrast, this is legal and clearly indicates the descriptors
>> are being used as expected:
>>
>> >>> class B:
>>> ...  @property
>>> ...  def __call__(self):
>>> ...   return lambda: 1
>>> ...
>>> >>> b = B()
>>> >>> b()
>>> 1
>>>
>>
>> ​There​
>> ​'s some more discussing in issue 23990
>> <http://bugs.python.org/issue23990>​
>> ​ where I get slightly angry, sorry.​
>>
>>
>> ​So were is this change actually useful? Proxies! Since new-style objects
>> in Python you cannot really proxy the callable aspect of objects, because
>> `callable` just checks that a field is set in a C struct.​
>> ​ This is fairly inconvenient because you have to know upfront if your
>> target is going to be callable or not.​
>>
>>
>>
>> Thanks,
>> -- Ionel Cristian Mărieș, http://blog.ionelmc.ro
>>
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>>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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