import sys
import time
import threading
import weakref


class A:
    def __init__(self):
        self.cycle = self

def my_thread():
    d = weakref.WeakValueDictionary()
    while 1:
        a = A()
        d[42] = a
        assert 42 in d
        # 'a' is alive at least until here, so the assert above should
        # always pass---but it does not.

threading.Thread(target=my_thread).start()
threading.Thread(target=my_thread).start()

if hasattr(sys, 'setswitchinterval'):
    sys.setswitchinterval(1e-6)  # Python 3.x
else:
    sys.setcheckinterval(10)     # Python 2.7
time.sleep(10)