Enum with str or int Mixin Breaking Change in Python 3.11

[anze3db]

Bug report

Looks like there was a breaking change with the way str and int mixins work with Enums in Python 3.11:

from enum import Enum


class Foo(str, Enum):
    BAR = "bar"

# Python 3.10
f"{Foo.BAR}"  # > bar

# Python 3.11
f"{Foo.BAR}"  # > Foo.BAR

The same goes for Enum classes with the int mixin.

In my project we were relying on Foo.BAR to return the enum value, so this change broke our code. We fixed it by replacing str Enum mixin with the newly added StrEnum class (thanks for that, it's exactly what we needed!).

I think reverting the breaking change would only introduce another breaking change so that's probably not the way to go. But maybe updating the whatsnew page and call out the change there could help people stumbling into this when doing the upgrade. I've found the existing point about this change in the release notes a little bit confusing and I have already opened a PR to try and clear it up a bit: #100387

I've also written a longer blog post about it here, and there has been some lively discussion in r/python.

Your environment

• CPython versions tested on: 3.11.0, 3.11.1
• Operating system and architecture: MacOS, Ubuntu 22.04

Linked PRs

• gh-100458: Clarify Enum.__format__() change of mixed-in types in the whatsnew/3.11.rst #100387
• [3.11] gh-100458: Clarify Enum.__format__() change of mixed-in types in the whatsnew/3.11.rst (GH-100387) #104060

[JosephSBoyle]

@anze3db StrEnum was added in 3.11, does that work as a 'slot in' replacement for your use case?

docs: https://docs.python.org/3.11/library/enum.html#enum.StrEnum

[anze3db]

@JosephSBoyle yes, we replaced the str mixin with StrEnum to fix this on our end. If we could let people know about this breaking change in the what's new docs it would save me a lot of head scratching, especially since the current note about this change seems wrong (see my attempt of at least making it a bit more accurate: #100387)

[JosephSBoyle]

Coincidentally I've also used the mixin approach you describe in professional projects, which suggests that this use is at the very least not uncommon.

As such it makes sense that the docs also suggest a way to achieve the old behaviour, like you say @anze3db. I've added a small suggestion to your PR; hopefully that will help resolve things for others in the same position!

[anze3db]

Thank you! The text that you added would have helped me a lot when I was figuring out how to fix this 👍

[jkbrzt]

Also, str(HTTPStatus.OK) now returns '200' instead of 'HTTPStatus.OK'. Workaround:

from enum import Enum
from http import HTTPStatus

HTTPStatus.__str__ = Enum.__str__

[hauntsaninja]

Improving the What's New documentation was a good call, thank you! While I also found this needlessly breaking when updating my work codebase, I don't think there's anything left to be done here.