import pickle
import io
import gc
import weakref

class MyPickler(pickle.Pickler):
    def persistent_id(self, obj):
        return None

def test_cycle():
    pickler = MyPickler(io.BytesIO())
    r = weakref.ref(pickler)
    del pickler
    if r() is not None:
        print("Oops, still here")
    gc.collect()
    if r() is None:
        print("Now it's gone")

def test_break_cycle():
    pickler = MyPickler(io.BytesIO())
    r = weakref.ref(pickler)
    # Note: the following line doesn't break the cycle
    #pickler.persistent_id = lambda obj: None
    pickle.Pickler.persistent_id.__set__(pickler, lambda obj: None)
    del pickler
    if r() is None:
        print("breaking persistent_id cycle worked")

if __name__ == '__main__':
    test_cycle()
    test_break_cycle()