Combining int and Enum, as with enum.IntEnum results in a class where __str__ cannot be effectively overridden. For example:


from enum import IntEnum

class myIntEnum(IntEnum):
    x = 1
    
    def __str__(self):
        return 'aaaaAAAa'
    
f'{myIntEnum.x}, {str(myIntEnum.x)}'


Expected output:
'aaaaAAAa, aaaaAAAa'

Actual output:
'1, aaaaAAAa'

Overriding __str__ in this way works as expected if the inherited classes are int or Enum individually. However, it does not work when inheriting (int, Enum) or when inheriting (intEnum).

Presumably this is a side effect of Enum's mixin behavior documented at https://docs.python.org/3/library/enum.html#others and it is possibly related to https://bugs.python.org/issue18264 .

I mistyped - __str__ can be overridden effectively, but __format__ has to be overridden instead or overridden also to get the desired f-string behavior.

related cPython code: https://github.com/python/cpython/blob/19a1e1eb86115db66c1faae5927f87e3a12692fc/Lib/enum.py#L626

If we're in a mixed-in class but the class has overridden __str__(), we should probably use str(self) instead of self._value_.

Note that this isn't really related to f-strings, except that they use the __format__ protocol, as does str.__format__.

>>> format(myIntEnum.x)
'1'

New changeset 2f19e82fbe98ce86bcd98a176328af2808b678e8 by Ethan Furman (thatneat) in branch 'master':
bpo-37479: on Enum subclasses with mixins, __format__ uses overridden __str__ (GH-14545)
https://github.com/python/cpython/commit/2f19e82fbe98ce86bcd98a176328af2808b678e8

New changeset 38c8d3930eb872258a82339bcba3bce1d0e3ac2c by Ethan Furman in branch '3.8':
[3.8] bpo-37479: Enum - use correct __format__ (GH-14545)
https://github.com/python/cpython/commit/38c8d3930eb872258a82339bcba3bce1d0e3ac2c

Thank you, Jason!