import threading, time

class SlowEq:
    def __init__(self, i):
        self. i = i
    def __hash__(self):
        return hash(self.i)
    def __eq__(self, other):
        time.sleep(0.001)
        return self.i == other.i

A = set(SlowEq(i) for i in range(100))
B = frozenset(SlowEq(i) for i in range(100))
C = set([B])

stop = False

def set_test():
    global stop
    while not stop:
        if A not in C:
            stop = True
            raise RuntimeError('A not in C')

print('A in C?', A in C)
print()

thread = threading.Thread(target=set_test)
thread.start()

set_test()