def test_missingCommand(self):
        """
        L{imap4.IMAP4Server.parse_command} sends a C{BAD} response to
        a line that includes a tag but no command.
        """
        self.server.parse_command(b"001")

        self.assertEqual(self.transport.value(),
                         b"001 BAD Missing command\r\n")

        self.server.connectionLost(
            failure.Failure(error.ConnectionDone("Done")),
        )

        L{imap4.IMAP4Server.parse_command} sends a C{BAD} response to
        a line that includes a tag but no command.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py