'Removes all registered finalizers.
:param clear_all: If `True`, all finalizers are deleted. Otherwise,
only the finalizers from an empty weak reference are deleted (i.e.,
weak references pointing to inexistent objects).'

@classmethod
def clear_finalizers(cls, clear_all=False):

    if clear_all:
        cls.finalizers.clear()
    else:
        for (id, ref) in items(cls.finalizers):
            if (ref() is None):
                cls.finalizers.pop(id, None)
