[Python-ideas] Fix that broken callable builtin

[Guido van Rossum]

You won't have any more luck defining __add__ as a property -- just don't
do that.

On how to implement a proxy, I'll let other explain. But this is not it.

On Fri, Apr 17, 2015 at 2:04 PM, Ionel Cristian Mărieș <contact at ionelmc.ro>
wrote:

> 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
>>>
>>> _______________________________________________
>>> Python-ideas mailing list
>>> Python-ideas at python.org
>>> https://mail.python.org/mailman/listinfo/python-ideas
>>> Code of Conduct: http://python.org/psf/codeofconduct/
>>>
>>
>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
>>
>
>


-- 
--Guido van Rossum (python.org/~guido)
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