An IntFlag member before 3.8.6 was converted to an integer in an f-string. After 3.8.6, the textual IntFlag class and member name are placed in the interpolated f-string instead of the integer.

3.8.3: f"... {X.F} ..." where X.F = 1 << 4 will give "... 16 ..."
3.8.5: Same
3.8.6: f"... {X.F} ..." where X.F = 1 << 4 will give "... X.F ..."

I have reproduced this on Linux using the version compiled and installed by pyenv, and on Windows using the 32-bit versions from the downloadable standalone installer.

Now I have to locate and go through all my SQL statements and verify that they explicitly convert IntFlag values to integer to avoid the errors introduced by this regression.

I believe the regression is due to GH-14545 which "fixed" bpo-37479. The news entry states:

"When `Enum.__str__` is overridden in a derived class, the override will be used by `Enum.__format__` regardless of whether mixin classes are present."

.

The change is in the __format__ method of Enum. It now checks if type(self).__str__ != Enum.__str__ and if True calls str(self). Because Flag defines its own __str__ method, the expression evaluates to True. I do not think this is the indented behaviour.

In 3.8.5 str(self) was only called if _member_type_ is object.

Thank you for the bug report.

Another solution would be to subclass IntFlag and specify the `__format__` method:


    from enum import IntFlag as _IntFlag
    class IntFlag(_IntFlag):
        def __format__(self, spec):
            return format(self.value, spec)

New changeset 37440eef7f9a0c27e13fc9ce0850574bb00688b0 by Ethan Furman in branch 'master':
bpo-41907: [Enum] fix format() behavior for IntFlag (GH-22497)
https://github.com/python/cpython/commit/37440eef7f9a0c27e13fc9ce0850574bb00688b0

New changeset 14eaa7d75282d8458455c41e9e871c56db8b9a10 by Miss Islington (bot) in branch '3.9':
bpo-41907: [Enum] fix format() behavior for IntFlag (GH-22497) (GH-23703)
https://github.com/python/cpython/commit/14eaa7d75282d8458455c41e9e871c56db8b9a10

New changeset cbfcc67170d459bcf3e9d63d2f44eadec740bf69 by Miss Islington (bot) in branch '3.8':
bpo-41907: [Enum] fix format() behavior for IntFlag (GH-22497) (GH-23704)
https://github.com/python/cpython/commit/cbfcc67170d459bcf3e9d63d2f44eadec740bf69