def test_connectionTracking(self):
        """
        The C{connectionMade} and C{connectionLost} methods of
        L{DNSServerFactory} cooperate to keep track of all L{DNSProtocol}
        objects created by a factory which are connected.
        """
        protoA, protoB = object(), object()
        factory = server.DNSServerFactory()
        factory.connectionMade(protoA)
        self.assertEqual(factory.connections, [protoA])
        factory.connectionMade(protoB)
        self.assertEqual(factory.connections, [protoA, protoB])
        factory.connectionLost(protoA)
        self.assertEqual(factory.connections, [protoB])
        factory.connectionLost(protoB)
        self.assertEqual(factory.connections, [])

        The C{connectionMade} and C{connectionLost} methods of
        L{DNSServerFactory} cooperate to keep track of all L{DNSProtocol}
        objects created by a factory which are connected.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/test/test_server.py