Python 3.11 changes behavior how enums are formatted as string

[OrangeTux]

@proelke made me aware about a changes to Python 3.11 that introduce subtle changes to all enums that subclass from both str and enum.Enum. Practically all enums defined in this library are subclassed from these 2 classes.

Consider the following code:

import enum

class ResetType(str, enum.Enum):
    hard = "hard"
    soft = "soft"

# Passes in both Python 3.11 and earlier.
assert ResetType.hard == "hard"

# Passes in Python 3.10 and below, but fails in Python 3.11.
assert f'{ResetType.hard}' == "hard"

$  python3.11 /enum_test.py
Traceback (most recent call last):
  File "/tmp/enum_test.py", line 15, in <module>
    assert f'{ResetType.hard}' == "hard"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

The underlying cause is that Python 3.11 changes the behavior of enum.Enum.__format__() slightly. f-strings (and a few other string formatting methods) rely on dunder method __format___().

So far, I considered two solutions.

Solution 1: StrEnum

Python 3.11 introduces enum.StrEnum. One could implement a solution as described at https://tomwojcik.com/posts/2023-01-02/python-311-str-enum-breaking-change

try:
    # breaking change introduced in python 3.11
    from enum import StrEnum
except ImportError:  # pragma: no cover
    from enum import Enum  # pragma: no cover

    class StrEnum(str, Enum):  # pragma: no cover
        pass  # pragma: no cover

class ResetType(StrEnum):
    hard = "hard"
    soft = "soft"

# Passes in both Python 3.11 and earlier.
assert ResetType.hard == "hard"

# Passes now in Python 3.10 and below and Python 3.11!
assert f'{ResetType.hard}' == "hard"

Manually implement __format__()

The other solution is to implement __format__() manually.

import enum

class _Enum(str, enum.Enum):
    def __format__(self, spec) -> str:
        return str.__format__(str(self.name), spec)

class ResetType(_Enum):
    hard = "hard"
    soft = "soft"

# Passes in both Python 3.11 and earlier.
assert ResetType.hard == "hard"

# Passes now in Python 3.10 and below and Python 3.11!
assert f"{ResetType.hard}" == "hard"

See also

• https://blog.pecar.me/python-enum
• https://www.reddit.com/r/Python/comments/zt4ot4/enum_with_str_or_int_mixin_breaking_change_in/

[larrykluger]

A regression error introduced in a .01 update! Good grief.

[Jared-Newell-Mobility]

@OrangeTux - I would suggest we go with Solution 1 - in looks cleaner and more logical moving forward with new Python versions; that at some stage, long into the future, the ImportError handling could be removed without refactoring the rest of the code?

[Jared-Newell-Mobility]

@OrangeTux - Another option may be this, I think this could be non-breaking too (introduced in 3.6)?

import enum

def test_this():

    class ResetType(str, enum.auto):
        hard = "hard"
        soft = "soft"

    print(ResetType.hard == "hard")
    print(f'{type(ResetType.hard)}')

    # Passes in Python 3.10 and Python 3.11.
    assert f'{ResetType.hard}' == "hard"

===================================================================================================== test session starts ======================================================================================================
platform linux -- Python 3.10.12, pytest-7.4.3, pluggy-1.3.0
rootdir: /home/jared/PycharmProjects/test_project/tests
collected 1 item

test_enum.py . [100%]

====================================================================================================== 1 passed in 0.00s =======================================================================================================

===================================================================================================== test session starts ======================================================================================================
platform linux -- Python 3.11.5, pytest-7.4.3, pluggy-1.3.0
rootdir: /home/jared/PycharmProjects/test_project
collected 1 item

tests/test_enum.py . [100%]

====================================================================================================== 1 passed in 0.00s =======================================================================================================

[Jared-Newell-Mobility]

Related ticket has this same behaviour, see comments in #433

[OrangeTux]

That's a clever solution. I didn't knew of this enum.auto and it's behavior. But I'm afraid that doesn't work for 2 reason:

• my demo code contains a bug. The members string should be capitalized, so "Hard" instead of "hard"
• a lot of enums in this project consists of multiple terms, e.g. FirmwareStatus.download_failed which should match the string "DownloadFailed". enum.auto does't work in this case.

[OrangeTux]

I suggest that we pick Solution #1.