def test_stopThreadPool(self):
        """
        When the reactor stops, L{ReactorBase._stopThreadPool} drops the
        reactor's direct reference to its internal threadpool and removes
        the associated startup and shutdown triggers.

        This is the case of the thread pool being created before the reactor
        is run.
        """
        reactor = self.buildReactor()
        threadpool = ref(reactor.getThreadPool())
        reactor.callWhenRunning(reactor.stop)
        self.runReactor(reactor)
        gc.collect()
        self.assertIsNone(threadpool())

        When the reactor stops, L{ReactorBase._stopThreadPool} drops the
        reactor's direct reference to its internal threadpool and removes
        the associated startup and shutdown triggers.

        This is the case of the thread pool being created before the reactor
        is run.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_threads.py