def test_rport(self):
        """
        An rport setting of None should insert the parameter with no value.
        """
        v = sip.Via("foo.bar", rport=None)
        self.assertEqual(v.toString(), "SIP/2.0/UDP foo.bar:5060;rport")
        self.assertTrue(v.rportRequested)
        self.assertIsNone(v.rportValue)

        An rport setting of None should insert the parameter with no value.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_sip.py