> conn = MySQLConn() > start_thread1(conn) > start_thread2(conn): > while True: > if os.fork() == 0: # child > raise Exception('doom') # triggers destructor There is no guarantee here that the lock will be held at the time of the fork. So even if we ensure that a lock acquired before the fork stayed lock, we won't necessarily get a deadlock. More importantly, you should never fork without ensuring that you exit with os._exit() or os.exec*(). So your example should be something like conn = MySQLConn() start_thread1(conn) start_thread2(conn): while True: if os.fork() == 0: # child try: raise Exception('doom') # does NOT trigger destructor except: sys.excepthook(*sys.exc_info()) os._exit(1) else: os._exit(0) With this hard exit the destructor never runs.