> If I understand Victor's test case correctly, the problem is caused if you have an extension type that implements tp_traverse but not tp_clear and that there is also a weakref involved in the trash cycle.

I'm not sure that "implements tp_traverse but not tp_clear" is enough to trigger the issue. It's unclear to me neither if it's a good practice or not to implement tp_clear... Since it seems like implementing tp_clear for functions (func_clear) caused this regression... Or maybe it's very specific to the magic combo: weak references with callback.