Stale entries in MRO cache if MRO contains non-base classes

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

@encukou

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.

@colesbury

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

Skip test that triggers pythongh-127773 in refleak tests

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

@encukou

I think this was fixed by @encukou

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

@encukou

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

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

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

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

I've merged the backport to 3.13.

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Stale entries in MRO cache if MRO contains non-base classes #127773

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	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.

	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)

Stale entries in MRO cache if MRO contains non-base classes #127773

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colesbury commented on Dec 9, 2024

cfbolz commented on Dec 10, 2024

colesbury commented on Feb 6, 2025

encukou commented on Feb 7, 2025

kumaraditya303 commented on Jan 9, 2026

encukou commented on Jan 12, 2026

encukou commented on Jan 26, 2026

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