Applying Metaclass to int-derived classes gives 'Inconsistent metaclass structure' error.

[isundaylee]

On both 0.510-dev and 0.520-dev-94e3f9cfc2510e651a0147de2626e084a7433f6d, I have been running into the following error:

error: Inconsistent metaclass structure for 'BitMask'

when trying to apply a metaclass to a class derived from int (or float). Repro:

class BitMaskMeta(type):
    pass

class BitMask(int, metaclass=BitMaskMeta):
    pass

When I change BitMask to derive from object the error goes away. Is this the expected behavior? Or a potential BUG?

Thank you! :)

[ilevkivskyi]

Thanks for reporting!

The problem that int has SupportsAbs, SupportsInt, etc. in bases in typeshed, this creates a spurious metaclass conflict (since the latter are instances of ABCMeta). This will be super-simple to fix after protocols land (also in typeshed) by just removing those bases.

In the meantime, maybe @elazarg knows some quick workaround?

[elazarg]

I think inheriting from ABCMeta is a working hack

from abc import ABCMeta
class BitMaskMeta(ABCMeta):
    pass

class BitMask(int, metaclass=BitMaskMeta):
    pass

Alternatively, you can use # type: ignore:

class BitMask(int, metaclass=BitMaskMeta): # type: ignore
    pass

[elazarg]

python/typeshed#1595

[viniciusd]

I think inheriting from ABCMeta is a working hack

from abc import ABCMeta
class BitMaskMeta(ABCMeta):
    pass

class BitMask(int, metaclass=BitMaskMeta):
    pass

Tried that in a similar case, but it didn't work. Going with type ignore for now.

[ilevkivskyi]

This is essentially a duplicate of the typeshed issue (or rather the other way around, but the typeshed one has more discussion). I think it doesn't make sense to keep both open.