Bug report

For example the following program fails with either assert WeirdClass.value == 2 or assert WeirdClass.value == 3 in recent Python versions:

import sys

class Base:
    value = 1

class Meta(type):
    def mro(cls):
        return (cls, Base, object)

class WeirdClass(metaclass=Meta):
    pass

assert Base.value == 1
assert WeirdClass.value == 1

Base.value = 2
assert Base.value == 2
assert WeirdClass.value == 2

Base.value = 3
assert Base.value == 3
assert WeirdClass.value == 3

Adding intervening calls to sys _clear_internal_caches() makes the test pass.

We have code that checks for this case, but it hasn't worked properly in Python 3.10+:

We also have a test that partly covers this case, but doesn't detect the bug:

Linked PRs

• gh-127773: Disable attribute cache on incompatible MRO entries #127924