`Enum` types cannot be inherited

[sobolevn]

Feature

Subtyping with Enum exact values

This core currently passes mypy:

from enum import Enum

class A(Enum):
   x = 1

class B(A):
    ...

But, in runtime this code fails with:

Traceback (most recent call last):
  File "ex.py", line 6, in <module>
    class B(A):
  File "/Users/sobolev/.pyenv/versions/3.8.9/lib/python3.8/enum.py", line 146, in __prepare__
    metacls._check_for_existing_members(cls, bases)
  File "/Users/sobolev/.pyenv/versions/3.8.9/lib/python3.8/enum.py", line 527, in _check_for_existing_members
    raise TypeError(
TypeError: B: cannot extend enumeration 'A'

This is also the case for Flag, IntEnum, and IntFlag.

Subtyping without exact values

However, subtyping without Enum values is fine:

from enum import Enum

class A(Enum):
   ...

class B(A):
    ...

This is also the case for Flag, IntEnum, and IntFlag.

So, the rule is: types inherited from Enum with at least one enumeration value should be marked as implicitly final type.

Related #10852
Related #5599

[sobolevn]

I would love to work on it after #10852

[RJPercival]

@sobolevn, there appears to be a bug in the implementation of this feature. Mypy complains that error: Cannot inherit from final class LabeledEnum in the following example, even though it works at runtime:

class LabeledEnum(enum.Enum):
    __labels__: Dict[str, str]

class Foo(LabeledEnum):
    A = 1
    B = 2

    __labels__ = {
        "A": "first enum",
        "B": "second enum",
    }

I'm guessing mypy is unaware that members prefixed with __ are not considered enum values?

[RJPercival]

I see that this bug has been raised in #11850.