Issue 13264: Monkeypatching using metaclass (original) (raw)

Issue13264

Created on 2011-10-25 12:45 by Artem.Tomilov, last changed 2022-04-11 14:57 by admin. This issue is now closed.

Messages (5)
msg146366 - (view)	Author: Artem Tomilov (Artem.Tomilov)	Date: 2011-10-25 12:45
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
msg146395 - (view)	Author: Daniel Urban (daniel.urban) *	Date: 2011-10-25 19:40
> 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).
msg147483 - (view)	Author: Éric Araujo (eric.araujo) *	Date: 2011-11-12 13:47
It seems to me this is not a bug. Closing?
msg147528 - (view)	Author: Daniel Urban (daniel.urban) *	Date: 2011-11-12 19:40
> It seems to me this is not a bug. +1
msg147593 - (view)	Author: Éric Araujo (eric.araujo) *	Date: 2011-11-14 13:24
Artem, if you have further information to make us reconsider this issue, please add a new message.