https://github.com/python/cpython/pull/2642 fixes the warning. I tested the change with test_handle_called_with_mp_queue-bug.patch: no more warning. Sorry, I don't know multiprocessing to understand the purpose of the removed test. I would like to really make sure that a Queue object doesn't "leak" a thread when I close .close() + .join_thread(). It's surprising that .join_thread() doesn't join anything and leave a thread running in the background. Even if in the common case, when the system load is low, the thread quits quickly thanks to .close().