from abc import ABCMeta

class Meta(ABCMeta):
    def __instancecheck__(cls, instance):
        # monkeypatching class method
        cls.__subclasscheck__ = super(Meta, cls).__subclasscheck__
        return super(Meta, cls).__instancecheck__(instance)

    def __subclasscheck__(cls, sub):
        return cls in sub.mro()

class A(object):
    __metaclass__ = Meta

class B(object): pass

# registering class 'B' as a virtual subclass of 'A'
A.register(B)

>>> issubclass(B, A)
False
>>> isinstance(B(), A) # => method __subclasscheck__ is now monkeypatched
True
>>> issubclass(B, A) # => desire to get 'True' because 'B' is a virtual subclass
False

> class Meta(ABCMeta):
>     def __instancecheck__(cls, instance):
>         # monkeypatching class method
>         cls.__subclasscheck__ = super(Meta, cls).__subclasscheck__

This line is approximately the same as:
        cls.__dict__['__subclasscheck__'] = ...
So it will put the object in the namespace of cls. The function in the namespace of type(cls) will remain there. (Also, the object you put in there is a bound method, so this is probably not what you want.)


>         return super(Meta, cls).__instancecheck__(instance)
> 
>     def __subclasscheck__(cls, sub):
>         return cls in sub.mro()
> 
> class A(object):
>     __metaclass__ = Meta
> 
> class B(object): pass
> 
> # registering class 'B' as a virtual subclass of 'A'
> A.register(B)
> 
> >>> issubclass(B, A)
> False
> >>> isinstance(B(), A) # => method __subclasscheck__ is now
> monkeypatched

A.__dict__['__subclasscheck__'] is now indeed the other method you put in there, but issubclass will call  type(A).__dict__['__subclasscheck__'] which remain the same. (Because __special__ methods a looked up directly in the namespace of the type of an object, see http://docs.python.org/dev/py3k/reference/datamodel#special-lookup).

It seems to me this is not a bug.  Closing?

> It seems to me this is not a bug.

+1

Artem, if you have further information to make us reconsider this issue, please add a new message.