def test_requestNegotiation(self):
        """
        L{telnet.Telnet.requestNegotiation} formats the feature byte and the
        payload bytes into the subnegotiation format and sends them.

        See RFC 855.
        """
        transport = proto_helpers.StringTransport()
        self.protocol.makeConnection(transport)
        self.protocol.requestNegotiation(b'\x01', b'\x02\x03')
        self.assertEqual(
            transport.value(),
            # IAC SB feature bytes IAC SE
            b'\xff\xfa\x01\x02\x03\xff\xf0')

        L{telnet.Telnet.requestNegotiation} formats the feature byte and the
        payload bytes into the subnegotiation format and sends them.

        See RFC 855.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_telnet.py