[Python-Dev] Why does Signature.from_function() have to check the type of its argument?

[Stefan Behnel]

PJ Eby, 08.02.2013 19:46:
> On Fri, Feb 8, 2013 at 10:54 AM, Stefan Behnel wrote:
>> Nick Coghlan, 08.02.2013 16:20:
>>> On Sat, Feb 9, 2013 at 1:06 AM, Benjamin Peterson wrote:
>>>> 2013/2/8 Stefan Behnel:
>>>>> I'm wondering about the purpose of this code in
>>>>> inspect.Signature.from_function():
>>>>>
>>>>> """
>>>>>     if not isinstance(func, types.FunctionType):
>>>>>         raise TypeError('{!r} is not a Python function'.format(func))
>>>>> """
>>>>>
>>>>> Is there any reason why this method would have to explicitly check the type
>>>>> of its argument? Why can't it just accept any object that quacks like a
>>>>> function?
>>>>
>>>> The signature() function checks for types.FunctionType in order to
>>>> call Signature.from_function(). How would you reimplement that?
>>
>> It should call isfunction() instead of running an explicit type check.
> 
> Isn't it possible now for an object to implement __instancecheck__ and
> claim to be an instance of FunctionType, anyway?  (For that matter,
> shouldn't there be some ABCs for this?)

Wow, good call. Providing an __instancecheck__() method that simply says
yes when it's asked for PyFunction_Type really works. Thanks!

Stefan