def test_isNotInIOThread(self):
        """
        The reactor registers itself as the I/O thread when it runs so that
        L{twisted.python.threadable.isInIOThread} returns C{False} if it is
        called in a different thread than the reactor is running in.
        """
        results = []
        reactor = self.buildReactor()
        def check():
            results.append(isInIOThread())
            reactor.callFromThread(reactor.stop)
        reactor.callInThread(check)
        self.runReactor(reactor)
        self.assertEqual([False], results)

        The reactor registers itself as the I/O thread when it runs so that
        L{twisted.python.threadable.isInIOThread} returns C{False} if it is
        called in a different thread than the reactor is running in.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_threads.py