Prior to 3.8 it was possible to create "abstract" enums (without members) and mix them together. To motivate with an example, perhaps we're modeling an API and want to be robust in the face of inconsistent casing

class CaseInsensitiveStrEnum(str, Enum):
    @classmethod
    def _missing_(cls, value):
        for member in cls._member_map_.values():
            if member._value_.lower() == value.lower():
                return member
        return super()._missing_(value)

and perhaps we also want to be robust in response to extensibility

class LenientStrEnum(str, Enum):
    @classmethod
    def _missing_(cls, value):
        logger.warning(
            f"[{cls.__name__}] encountered an unknown value!\n"
            f"Luckily I'm a LenientStrEnum, so I won't crash just yet.\n"
            f"You might want to add a new case though.\n"
            f"Value was: '{value}'"
        )
        return UnexpectedStr(value)

but we also want to model some known good set of values, so mixing together the abstract enums we'd get something like

class JobStatus(CaseInsensitiveStrEnum, LenientStrEnum):
    ACTIVE = "active"
    PENDING = "pending"
    TERMINATED = "terminated"

However, due to the resolution of https://bugs.python.org/issue39587 this no longer works, instead producing:

TypeError: 'JobStatus': too many data types: [<class 'str'>, <class 'str'>]

The relevant change is

ethanfurman@bff01f3

I believe that if we made data_types a set rather than a list then the example would become valid once again.

Excellent bug report.

But what is an UnexpectedString() ?

But what is an UnexpectedString()

Sorry, that’s maybe less relevant to the example. It’s just a subclass of string with some marker to make it detectable later on, similar to schemes that taint user input to prevent sql injection or whatever

On Jun 8, 2021, at 23:20, Ethan Furman report@bugs.python.org wrote:


Ethan Furman ethan@stoneleaf.us added the comment:

Excellent bug report.

But what is an UnexpectedString() ?

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue44356\>

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Oh, on further investigation I see that the example wouldn't have worked on 3.8 anyway, due to bpo-34536 adding some checks to the type returned by _missing_, so maybe I'm pushing Enum too far in that case.

Since I like puzzles, here is a working LenientStrEnum:

    class LenientStrEnum(str, Enum):
        #
        def __init__(self, *args):
            self._valid = True
        #
        @classmethod
        def _missing_(cls, value):
            logger.warning(
                f"[{cls.__name__}] encountered an unknown value!\n"
                f"Luckily I'm a LenientStrEnum, so I won't crash just yet.\n"
                f"You might want to add a new case though.\n"
                f"Value was: '{value}'"
                )
            unknown = cls._member_type_.__new__(cls, value)
            unknown._valid = False
            unknown._name_ = value.upper()
            unknown._value_ = value
            cls._member_map_[value] = unknown
            return unknown
        #
        @property
        def valid(self):
            return self._valid

_member_map_ is not guaranteed, but is unlikely to change.

New changeset 8a4f085 by Ethan Furman in branch 'main':
bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649)
8a4f085

New changeset 0128601 by Miss Islington (bot) in branch '3.10':
bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649) (GH-26653)
0128601

New changeset 304ec53 by Miss Islington (bot) in branch '3.9':
bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649) (GH-26652)
304ec53

On 10.06.2021 15:33, Ethan Furman wrote:

Since I like puzzles, here is a working LenientStrEnum:
...

Oh indeed, that's really cool!

Glad you like it!

Please don't change the title back. :-)

Oops, still getting used to the Python mailing list format. Learned
something new! Thanks for all your awesome work on Python.

On 11.06.2021 08:19, Ethan Furman wrote:

Ethan Furman <ethan@stoneleaf.us> added the comment:

Glad you like it!

Please don't change the title back. :-)

----------
resolution: -> fixed
stage: patch review -> resolved
status: open -> closed
title: Abstract enum mixins not allowed -> Multiple enum mixins not allowed even when they have the same datatype
versions: -Python 3.8

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue44356\>

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