def _queryTest(self, filter):
        """
        Invoke L{Resolver._query} and verify that it sends the correct DNS
        query.  Deliver a canned response to the query and return whatever the
        L{Deferred} returned by L{Resolver._query} fires with.

        @param filter: The value to pass for the C{filter} parameter to
            L{Resolver._query}.
        """
        reactor = MemoryReactor()
        resolver = Resolver([], reactor=reactor)
        d = resolver._query(
            Query(b'foo.example.com', A, IN), [('1.1.2.3', 1053)], (30,),
            filter)

        # A UDP port should have been started.
        portNumber, transport = reactor.udpPorts.popitem()

        # And a DNS packet sent.
        [(packet, address)] = transport._sentPackets

        message = Message()
        message.fromStr(packet)

        # It should be a query with the parameters used above.
        self.assertEqual(message.queries, [Query(b'foo.example.com', A, IN)])
        self.assertEqual(message.answers, [])
        self.assertEqual(message.authority, [])
        self.assertEqual(message.additional, [])

        response = []
        d.addCallback(response.append)
        self.assertEqual(response, [])

        # Once a reply is received, the Deferred should fire.
        del message.queries[:]
        message.answer = 1
        message.answers.append(RRHeader(
            b'foo.example.com', payload=Record_A('5.8.13.21')))
        transport._protocol.datagramReceived(
            message.toStr(), ('1.1.2.3', 1053))
        return response[0]

        Invoke L{Resolver._query} and verify that it sends the correct DNS
        query.  Deliver a canned response to the query and return whatever the
        L{Deferred} returned by L{Resolver._query} fires with.

        @param filter: The value to pass for the C{filter} parameter to
            L{Resolver._query}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/test/test_rootresolve.py