In Python, the __dict__ and __weakref__ slots are treated specially (slots meaning __slots__, not tp_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 the tp_dictoffset and tp_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): pass TypeError: multiple bases have instance lay-out conflict