from weakref import WeakValueDictionary
import _thread, gc, time

def f():
    while True:
        time.sleep(0.01)
        gc.collect()

class C: pass
class D: pass
d = WeakValueDictionary()

_thread.start_new_thread(f, ())

while True:
    x = d.setdefault(10, D())
    assert x is not None   # we never put None in there!
    tp = x.__class__
    del x
    if tp is D:
        c = C(); c.cycle = c
        d[10] = c
        c = None