def test_contextManager(self):
        """
        The null reserved file descriptor is a null context manager.
        """
        self.assertFalse(self.nullReservedFD.available())
        with self.nullReservedFD:
            self.assertFalse(self.nullReservedFD.available())
        self.assertFalse(self.nullReservedFD.available())

        The null reserved file descriptor is a null context manager.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_tcp.py