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Impossible to specify a metaclass for a Generic class #449
Description
Activity
As a workaround in cases where possible, one can just do:
from typing import Generic, TypeVar, GenericMeta class FooMeta(type): pass class GenericFooMeta(GenericMeta, FooMeta): pass T = TypeVar('T') class Foo(Generic[T], metaclass=GenericFooMeta): pass
Reacted by Joar Wandborg and Sebastián RamírezReacted by Sebastián RamírezWe recently encountered this in a case where a superclass we don't control used
ABCMetaas its metaclass. The answer seems to be to use__extra__, but this doesn't appear to be documented anywhere. Should it be treated as an implementation detail not to be relied on?Should it be treated as an implementation detail not to be relied on?
Yes. Moreover
GenericMetais completely removed in Python 3.7.Genericwill no more have a non-trivial metaclass,type(Generic)is nowtype.This is fixed in Python 3.7+ and we can't really do anything for older versions, so I think this can be closed.
Below is a contrived example. In the real world I might not actually control the metaclass for
Fooor I may be inheriting from some other class that also has a metaclass.This passes MyPy just fine, but fails at runtime with:
Moved from python/mypy#3720