Issue 41517: Able to subclass enum with members by using multiple inheritance

when inheriting an implemented enum, we get a runtime error
But when creating a multiple inheritance like the following, it works… so something feels broken in the enum mechanism

------------------------------------
from enum import IntEnum, Enum

class A(IntEnum):
   a = 1

class B(A, Enum):
   b= 1

print(B.b)
------------------------------------

Could you provide code which demonstrates the problem?

Eric V. Smith it's in the first comment

With Python 3.7.4, which is all I have handy, that code does not give a runtime error. It prints "B.b".

Your text says code "when creating a multiple inheritance like the following, it works". It sounds like you know the code sample works. So I don't understand what code gives a runtime error. What type of error are you seeing?

sorry, I'll explain. by "when inheriting an implemented enum, we get a runtime error" I refer to this https://docs.python.org/3/library/enum.html#restricted-enum-subclassing

You shouldn't be able to subclass an enum, yet I just showed a loophole. my example code shows you can subclass an enum by using a trick, multiple inheritance.

I hope it's more clear now :)

btw it also works with regular Enum inheritance (I just used IntEnum for simplicity)

The problem is that class B should raise an error as class A already has members.

Ethan Furman bullseye! thanks for clarifying

The documentation says:

"Allowing subclassing of enums that define members would lead to a violation of some important invariants of types and instances."

but it isn't clear what those invariants are, or why it is more of a problem for enums than any other subclassing situation. Could the docs be updated to explain why it is prohibited?

To answer the invariant question, see this post by Guido:

  https://mail.python.org/pipermail/python-dev/2013-April/125716.html

New changeset 3064dbf5df1021e85b507366a7ea448c8895efe7 by Ethan Furman in branch 'master':
bpo-41517: do not allow Enums to be extended (#22271)
https://github.com/python/cpython/commit/3064dbf5df1021e85b507366a7ea448c8895efe7

New changeset 3f4012117bf80aa7c005f8fa6fb8e1f8b1aef5d5 by Miss Islington (bot) in branch '3.8':
bpo-41517: do not allow Enums to be extended (GH-22271)
https://github.com/python/cpython/commit/3f4012117bf80aa7c005f8fa6fb8e1f8b1aef5d5

New changeset 48f99250ff319e36b15b555128cd62e408d8165f by Miss Islington (bot) in branch '3.9':
bpo-41517: do not allow Enums to be extended (GH-22271)
https://github.com/python/cpython/commit/48f99250ff319e36b15b555128cd62e408d8165f

messages: + msg375157
title: Enum multiple inheritance loophole -> Able to subclass enum with members by using multiple inheritance