'Start. Multiple calls have no effect.
Not safe to call from multiple threads at once.'

def open(self):

    with self._lock:
        if self._thread_will_exit:
            try:
                self._thread.join()
            except ReferenceError:
                pass
        self._thread_will_exit = False
        self._stopped = False
    started = False
    try:
        started = (self._thread and self._thread.is_alive())
    except ReferenceError:
        pass
    if (not started):
        thread = threading.Thread(target=self._run, name=self._name)
        thread.daemon = True
        self._thread = weakref.proxy(thread)
        _register_executor(self)
        thread.start()
