Currently, an enum with a mixed-in data type, such as IntEnum, will use that data type's `__format__` -- unless the user provides their own `__str__`, in which case the `str()` of the enum member will be used in the `format()` call.

This behavior will be deprecated in 3.10, and in 3.12 the default `__format__` will use the default `__str__`, which is the standard behavior for Python objects

For those that were relying on, for example,

    class Color(IntEnum):
        RED = 1

    f'{Color.RED}' # -> '2'

They will need to add ":d" to ensure the integer output:

    f'{Color.RED:d}'

This change does work now.

New changeset 5987b8c463892e0ab7a63cdae92f34b5eb79732d by Ethan Furman in branch 'master':
bpo-43945: [Enum] Deprecate non-standard mixin format() behavior (GH-25649)
https://github.com/python/cpython/commit/5987b8c463892e0ab7a63cdae92f34b5eb79732d

Thinking about this some more I am partially reversing course.  The idea behind `IntEnum` and `IntFlag` is to be, as close as possible, drop-in replacements for existing integer-based constants.  If the format() of those two change then they become much less attractive.

So it won't.  Instead, any user-mixed enumerations will get the new behavior:

    class Grades(int, Enum):
        A = 5
        F = 0

will emit a DeprecationWarning now, and in 3.12 `format(Grades.A)` will product 'A' instead of '5'.

New changeset f60b07ab6c943fce084772c3c7731ab3bbd213ff by Ethan Furman in branch 'main':
bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752)
https://github.com/python/cpython/commit/f60b07ab6c943fce084772c3c7731ab3bbd213ff

New changeset 1b4addf3cbd0ef424939681ee67c802328d567ba by Ethan Furman in branch '3.10':
[3.10] bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752) 
https://github.com/python/cpython/commit/1b4addf3cbd0ef424939681ee67c802328d567ba