gh-149828: make Protocol compatible with Enum instantiation

[randolf-scholz]

Fixes #149828

This was surprisingly easy, the issue is that with enums, __init_subclass__ is only called after EnumType.__new__ is finished.

However, when subclassing from typing.Protocol, _is_protocol is only set during __init_subclass__. The fix is to replace the lookup of _is_protocol inside _no_init_or_replace_init with the actual check.

A more robust solution would probably be for _is_protocol to be a lazy class property, but that would be a larger change and class-properties are not natively supported...

cc @sobolevn

• Issue: Cannot subclass from Protocol and Enum at the same time. #149828

[sobolevn]

Looks fine to me! Let's wait for @ethanfurman :)

[bedevere-bot]

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[ethanfurman]

I'm not sure the ()'s are needed -- can someone test it both ways and verify which version results in the expected enum member and value?

impl = Impl vs impl = Impl()

[randolf-scholz]

Without the () the enum member would be the class Impl, not an instance of the class.

>>> from enum import Enum, EnumType
... from typing import Protocol
... 
... class Example(Protocol):
...     def method(self) -> None: ...
... 
... class Impl(Example):
...     def method(self) -> None: ...
... 
... class CompatEnumType(type(Protocol), EnumType): ...
... 
... class ProtoEnumA(Example, Enum, metaclass=CompatEnumType):
...     impl = Impl
... 
... class ProtoEnumB(Example, Enum, metaclass=CompatEnumType):
...     impl = Impl()
... 
... print(repr(ProtoEnumA.impl))
... print(repr(ProtoEnumB.impl))
... 
<ProtoEnumA.impl: <class '__main__.Impl'>>
<ProtoEnumB.impl: <__main__.Impl object at 0x74e096376660>>

[ethanfurman]

Very good, thank you!

[randolf-scholz]

Had to update the PR title since I copy-pasted the wrong issue number. The Misc/NEWS.d already used the correct number though, so no code changes are needed.