Metaclass of a metaclass

[Peter Otten]

Steven D'Aprano wrote:

> I was playing around with metaclasses and I wondered what would happen if
> the metaclass itself had a metaclass. Sort of a metametaclass.
> 
> Apparently it gives an error. Can anyone explain why this does not work?
> 
> # Python 3.2
> 
> 
> 
>>>> class MyType(type):  # A metaclass...
> ...     def __repr__(self):
> ...             s = super().__repr__()
> ...             return s.replace('class', 'metaclass')
> ...
>>>> class Meta(metaclass=MyType):  # ... of a metaclass.
> ...     pass
> ...
>>>> Meta
> <metaclass '__main__.Meta'>
>>>> 
>>>> isinstance(Meta, type)
> True

I think you want isinstance(Meta(), type), and this returns False.

>>>> class MyClass(metaclass=Meta):  # And now try to use it.
> ...     pass
> ...
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: object.__new__() takes no parameters
> 
> 
> 
> What am I doing wrong?

class A(metaclass=M):
    pass

is equivalent to

A = M(name, bases, classdict)

and as the error message suggests B needs a compatible signature.