I am trying to define a type in C, that derives from PyTypeObject.

I want to override tp_clear. To do so properly, I should call base type's tp_clear and have it perform its cleanup steps. PyTypeObject has a tp_clear implementation: subtype_clear. Problem is, it assumes the instance it gets is of a type, that does not override PyTypeObject's tp_clear, and behaves incorrectly in 2 ways:

1) it does not perform the usual cleanup, because in this code
base = type;
while ((baseclear = base->tp_clear) == subtype_clear)

the loop condition is immediately false, as my types overrode tp_clear

2) later on it calls baseclear on the same object. But because of the loop above baseclear actually points to my type's custom tp_clear implementation, which leads to reentry to that function (basically a stack overflow, unless there's a guard against it).

To put it differently, if you think in terms of MRO, my custom type's MRO is

my_type_clear (from my type), subtype_clear (from PyTypeObject), etc

And subtype_clear incorrectly assumes that it is the first entry in the object's MRO list for tp_clear.

Victor, it's not clear to me whether this is a bug report or a "how do I do this" question, but in either case if you posted your code along with "I expected X but got Y", it would be easier to understand the issue.