def test_disallowedPort(self):
        """
        If a port number is initially selected which cannot be bound, the
        L{CannotListenError} is handled and another port number is attempted.
        """
        ports = []

        class FakeReactor(object):
            def listenUDP(self, port, *args, **kwargs):
                ports.append(port)
                if len(ports) == 1:
                    raise CannotListenError(None, port, None)

        resolver = client.Resolver(servers=[('example.com', 53)])
        resolver._reactor = FakeReactor()

        resolver._connectedProtocol()
        self.assertEqual(len(set(ports)), 2)

        If a port number is initially selected which cannot be bound, the
        L{CannotListenError} is handled and another port number is attempted.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/test/test_client.py