In Python 3.6 the following works: class HexInt(int): def __repr__(self): return hex(self) class MyEnum(HexInt, enum.Enum): A = 1 B = 2 C = 3 >>> MyEnum.A <MyEnum.A: 0x1> However in Python 3.7/8 it instead prints >>> MyEnum.A 0x1 It uses HexInt's repr instead of Enum's. Looking at the enum.py module it seems that this occurs for mixin classes that don't define __new__ due to a change in the _get_mixins_ method. If I define a __new__ method on the HexInt class then the expected behavior occurs.

Bisecting points to issue29577 that introduced this change.

Yes, the change only considered types with their own copy of `__new__` to be actual data types, so in 3.6 `HexInt` was the recognized data type, but in 3.7+ it was `int` -- which also meant that HexEnum was considered a simple mix-in and its `__repr__` was used instead of `Enum.__repr__`.

 New changeset bff01f3a3aac0c15fe8fbe8b2f561f7927d117a1 by Ethan Furman in branch 'master': bpo-39587: Enum - use correct mixed-in data type (GH-22263) https://github.com/python/cpython/commit/bff01f3a3aac0c15fe8fbe8b2f561f7927d117a1 

 New changeset 8f8ebcca95d3b6ed0a522a9736ab53d6d4f0208c by Miss Islington (bot) in branch '3.8': bpo-39587: Enum - use correct mixed-in data type (GH-22263) https://github.com/python/cpython/commit/8f8ebcca95d3b6ed0a522a9736ab53d6d4f0208c 

 New changeset 95b81e2f8c955823dbf5632a817902b8a4916eaa by Miss Islington (bot) in branch '3.9': bpo-39587: Enum - use correct mixed-in data type (GH-22263) (GH-22266) https://github.com/python/cpython/commit/95b81e2f8c955823dbf5632a817902b8a4916eaa