Enum: using _missing_ on Enums without any elements doesn't work since 3.11.6

Bug report

Bug description:

It seems the pull request (#108704) that tried to handle erroneous calls to Enum.__new__ introduced some other behavior as well. In a few projects that I'm involved with we use this pattern:

from enum import Enum

class Result(Enum):
    @classmethod
    def _missing_(cls, value):
        v = None
        for subclass in cls.__subclasses__():
            try:
                v = subclass(value)
            except:
                pass

        return v


class CommonResult(Result):
    OK = 0
    ERR_FOO = -0x1
    ERR_BAR = -0x2

where calling Result(0) would return CommonResult.OK. Now in 3.11.6 and onwards this throws a TypeError. Was this intentional? It's a nice pattern to have and there are to me no obvious ways to replace it with something as elegant, so I hope that it can be solved some other way (maybe by checking if the class actually overrides __new__ and if not, then allowing the call to _missing_?)

CPython versions tested on:

3.11, 3.12

Operating systems tested on:

Linux, Windows

Found a workaround which is not horrible. I still think that it would be nice for a few other use cases if the _missing_ pattern worked, but if anyone else needs it, here's a solution that replicates what at least I did:

from enum import Enum, EnumMeta

class Fudgeulator(EnumMeta):
    def __call__(cls, value, *args, **kwargs):
        for subclass in cls.__subclasses__():
            try:
                return subclass(value)
            except Exception:
                pass
        
        return super().__call__(value, *args, **kwargs)

class Result(Enum, metaclass=Fudgeulator):
    pass

class Foo(Result):
    pass

class CommonResult(Foo):
    OK = 0
    ERR_FOO = -0x1
    ERR_BAR = -0x2

class AnotherResult(Result):
    ERR_QUUX = -0x4
    ERR_BAAA = -0x5


print(Result(-0x4))
print(CommonResult.OK)

@mikrodust-henrikp

@mikrodust-henrikp: Definitely an intriguing use-case. Your subclass of EnumType is the correct solution (and hopefully not onerous).

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Enum: using _missing_ on Enums without any elements doesn't work since 3.11.6 #111103

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Enum: using _missing_ on Enums without any elements doesn't work since 3.11.6 #111103

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mikrodust-henrikp commented on Oct 20, 2023

ethanfurman commented on Oct 25, 2023

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