3.12.0b4 enum.IntFlag `NoneType` object is not iterable

[cdce8p]

Bug report

import enum

class bitmap(enum.IntFlag):
    pass

print(bitmap.from_bytes(b'\x00', 'little'))

3.11

bitmap(0)

3.12.0b4

Traceback (most recent call last):
  File "/.../cpython/test.py", line 6, in <module>
    print(bitmap.from_bytes(b'\x00', 'little'))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.../cpython/Lib/enum.py", line 742, in __call__
    return cls._create_(
           ^^^^^^^^^^^^^
  File "/.../cpython/Lib/enum.py", line 870, in _create_
    for item in names:
TypeError: 'NoneType' object is not iterable

I bisected the issue to #99500.
/CC: @ethanfurman

Your environment

• CPython versions tested on: 3.12.0b4
• Operating system and architecture: macOS ARM64

[ethanfurman]

Thank you for the report!

What is your use-case for this behavior? Typically, an enum with no members is meant to be subclassed.

[cdce8p]

What is your use-case for this behavior? Typically, an enum with no members is meant to be subclassed.

Tbh I stumbled upon this while testing a third party library: https://github.com/zigpy/zigpy
As best as I can tell, they do also subclass it, but bitmap.from_bytes in that case is used in a deserialization routine from a binary struct.

I could post a longer example, just not sure it's all that useful, that's why I condensed it initially.

[ethanfurman]

Please open an issue with zigpy and reference this one. They should be able to confirm if this is a problem in practice.

[puddly]

While this new 3.12 behavior is backwards-incompatible with Python 3.11, we've resolved this problem by avoiding the functional API by replacing the enum metaclass (zigpy/zigpy#1236):

import enum


class AlwaysCreateEnumType(enum.EnumMeta):
    """Enum metaclass that skips the functional creation API."""

    def __call__(cls, value, names=None, *values) -> type[enum.Enum]:  # type: ignore
        return cls.__new__(cls, value)  # type: ignore


class bitmap8(
    int,
    enum.ReprEnum,
    enum.Flag,
    boundary=enum.KEEP,
    metaclass=AlwaysCreateEnumType,
):
    pass

bitmap8(0)  # works with 3.11 and 3.12

You can consider this issue resolved if no other projects rely on using the enum classes this way.