[Python-Dev] Metatype conflict among bases?

[Guido van Rossum]

> Consider:
> 
>     class A(object):
>         class __metaclass__(type):
>             pass
> 
>     class B(A):  # TypeError: metatype conflict among bases
>         class __metaclass__(type):
>             pass
> 
> Now that's a weird error message at least!  There's only one base (A),
> and I'm telling Python explicitly to use the nested __metaclass__
> instead of A's __metaclass__!
> 
> Should I not be surprised that Python won't let me set the metatype
> explicitly?

The metaclass must be a subclass of the metaclass of all the bases.

--Guido van Rossum (home page: http://www.python.org/~guido/)