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.
class C(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 _testcapi import HeapCTypeWithDict class I3(HeapCTypeWithDict, list): pass i = 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__.
from _testcapi import HeapCTypeWithDict
class I3(HeapCTypeWithDict, list): pass
print("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
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