In Python, the__dict__ and__weakref__ slots are treated specially (slots meaning__slots__, nottp_slots)
They are automatically insert by the VM when creating a class.

classC(list):pass>>>C().__dict__{}

In order to support inheritance, specifically multiple inheritance, the VM can lay out subclasses in ways that differ from the superclass.
This is OK, provided__dict__ and__weakref__ are only accessed though thetp_dictoffset andtp_weaklistoffset offsets.
But, if either field is accessed directly, then we access invalid memory and 💥

test.py:

from_testcapiimportHeapCTypeWithDictclassI3(HeapCTypeWithDict,list):passi=I3()i.append(0)print(i.dictobj)

$ python3.10~/test/test.py Segmentation fault (core dumped)

We have (accidentally) fixed this for__dict__ in 3.11, although at the expense breaking backwards compatibility for some C extensions. However, the problem still remains for__weakref__.

Backwards incompatibility

from_testcapiimportHeapCTypeWithDictclassI3(HeapCTypeWithDict,list):passprint("OK")

$ python3.10 test.py OK$ python3.12 test.py Traceback (most recent call last):  File"test.py", line 3,in<module>    class I3(HeapCTypeWithDict, list): passTypeError: multiple bases have instance lay-out conflict