Description

colesbury

opened on Dec 9, 2024

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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.

Version	Result
3.7	OK
3.8	OK
3.9	OK
3.10	AssertionError: assert WeirdClass.value == 2
3.11	AssertionError: assert WeirdClass.value == 2
3.12	AssertionError: assert WeirdClass.value == 2
3.13	AssertionError: assert WeirdClass.value == 3
main	AssertionError: assert WeirdClass.value == 3

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

cpython/Objects/typeobject.c

Lines 1107 to 1112 in 5c89adf

	static void
	type_mro_modified(PyTypeObject *type, PyObject *bases) {
	/*
	Check that all base classes or elements of the MRO of type are
	able to be cached. This function is called after the base
	classes or mro of the type are altered.

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

cpython/Lib/test/test_capi/test_type.py

Lines 40 to 66 in 5c89adf

	def test_freeze_meta(self):
	"""test PyType_Freeze() with overridden MRO"""
	type_freeze = _testcapi.type_freeze
	class Base:
	value = 1
	class Meta(type):
	def mro(cls):
	return (cls, Base, object)
	class FreezeThis(metaclass=Meta):
	"""This has `Base` in the MRO, but not tp_bases"""
	self.assertEqual(FreezeThis.value, 1)
	with self.assertRaises(TypeError):
	type_freeze(FreezeThis)
	Base.value = 2
	self.assertEqual(FreezeThis.value, 2)
	type_freeze(Base)
	with self.assertRaises(TypeError):
	Base.value = 3
	type_freeze(FreezeThis)
	self.assertEqual(FreezeThis.value, 2)

Linked PRs

• gh-127773: Disable attribute cache on incompatible MRO entries #127924
• [3.13] gh-127773: Disable attribute cache on incompatible MRO entries (GH-127924) #143729

Reacted by Ken Jin and Petr Viktorin

Activity

1. colesbury

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   3.12only security fixesonly security fixes

   3.13only security fixesonly security fixes

   3.14bugs and security fixesbugs and security fixes

   on Dec 9, 2024
2. Eclips4

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Dec 9, 2024
3. colesbury commented on Dec 9, 2024

   

   colesbury

   on Dec 9, 2024

   ContributorAuthor

   More actions

   cc @encukou - this broke in a4760cc (GH-27260), which removed Py_TPFLAGS_HAVE_VERSION_TAG.

   The reason for removing Py_TPFLAGS_HAVE_VERSION_TAG was because "the field is always present in the type struct", but the flag indicates whether a type supports the method cache (i.e, supports having a valid version tag) and types with MRO entries that aren't superclasses don't support the method cache.
4. cfbolz commented on Dec 10, 2024

   

   cfbolz

   on Dec 10, 2024

   Contributor

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   Great (if very annoying) find, @colesbury! The bug even affects pypy, which has an entirely different architecture for its caches 😳
5. encukou

   self-assigned this

   on Dec 12, 2024
6. mpage

   added a commit that references this issue on Dec 20, 2024

   Skip test that triggers pythongh-127773 in refleak tests

   e5e2508
7. encukou

   added a commit that references this issue on Jan 13, 2025

   gh-127773: Disable attribute cache on incompatible MRO entries (GH-12…

   aa6579c
8. colesbury commented on Feb 6, 2025

   

   colesbury

   on Feb 6, 2025

   ContributorAuthor

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   I think this was fixed by @encukou
9. colesbury

   closed this as completedon Feb 6, 2025
10. encukou commented on Feb 7, 2025

    

    encukou

    on Feb 7, 2025

    Member

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    In the main branch. I don't know if I'll get to older branches in time.
11. encukou

    reopened this on Feb 7, 2025
12. kumaraditya303 commented on Jan 9, 2026

    

    kumaraditya303

    on Jan 9, 2026

    Contributor

    More actions

    @encukou do you want to backport it? Otherwise, this seems complete now.
13. encukou

    added a commit that references this issue on Jan 12, 2026

    pythongh-127773: Disable attribute cache on incompatible MRO entries (p…

    356fb83
14. encukou commented on Jan 12, 2026

    

    encukou

    on Jan 12, 2026

    Member

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    Oh, thanks for the ping!
    See #143729 for the backport.
15. encukou

    added a commit that references this issue on Jan 26, 2026

    [3.13] gh-127773: Disable attribute cache on incompatible MRO entries (…

    fdac87a
16. encukou commented on Jan 26, 2026

    

    encukou

    on Jan 26, 2026

    Member

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    I've merged the backport to 3.13.
17. encukou

    closed this as completedon Jan 26, 2026

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