import sys
import threading
import weakref
import gc


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

done = False

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

def my_thread2():
    while not done:
        gc.collect()

t1 = threading.Thread(target=my_thread)
t2 = threading.Thread(target=my_thread2)
sys.setswitchinterval(1e-6)
t1.start()
t2.start()
t1.join()
t2.join()