MCVE of how finalizers cannot depend on weak reference state in the presence of circular references.

test_finalizer_weak_dict.py

from weakref import WeakValueDictionary
import gc

objs = WeakValueDictionary()

class Registered:
    def __init__(self):
        objs[id(self)] = self
    def __del__(self):
        del objs[id(self)]

class Simple(Registered):
    pass

class Circular(Registered):
    def __init__(self):
        super().__init__()
        # create a circular reference
        def test():
            print(self)
        self._test_fn = test

def test_simple():
    gc.disable()

    local_objs = [Simple() for a in range(10)]
    assert len(objs) == 10

    local_objs = []
    assert len(objs) == 0


def test_circular():
    """
    This raises because the object is not longer in "objs" by the time the object's
    __del__ is called - but this is only the case when there are circular references.
    """
    gc.disable()

    local_objs = [Circular() for a in range(10)]
    assert len(objs) == 10

    local_objs = []
    assert len(objs) == 10

    gc.collect()

    assert len(objs) == 0

if __name__ == "__main__":
    test_simple()
    test_circular()