[Python-Dev] isinstance() on old-style classes in Py 2.7

[Mark Shannon]

Hi,
The problem is a side effect of the fact that old-style classes are 
implemented on top of new-style meta-classes.
Consequently although C is the "class" of C() it is not its "type".

 >>> type(C())
<type 'instance'>

 >>> type(C()).__mro__
(<type 'instance'>, <type 'object'>)

therefore
 >>> issubclass(type(C()), object)
True

which implies
 >>> isinstance(C(),object)
True

Cheers,
Mark.


On 21/10/14 17:43, Andreas Maier wrote:
>
> Hi. Today, I ran across this, in Python 2.7.6:
>
>>>> class C:
> ...   pass
> ...
>>>> issubclass(C,object)
> False
>>>> isinstance(C(),object)
> True   <-- ???
>
> The description of isinstance() in Python 2.7 does not reveal this result
> (to my reading).
>
>  From a duck-typing perspective, one would also not guess that an instance
> of C would be considered an instance of object:
>
>>>> dir(C())
> ['__doc__', '__module__']
>>>> dir(object())
> ['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__',
> '__hash__', '__init__', '__new__', '__reduce__
> ', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
> '__subclasshook__']
>
> -> What is the motivation for isinstance(C,object) to return True in Python
> 2.7?
>
> Andy
>
> Andreas Maier
> IBM Senior Technical Staff Member, Systems Management Architecture & Design
> IBM Research & Development Laboratory Boeblingen, Germany
> maiera at de.ibm.com, +49-7031-16-3654
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