def pickServer(self):
        """
        Return the address of a nameserver.

        TODO: Weight servers for response time so faster ones can be
        preferred.
        """
        if not self.servers and not self.dynServers:
            return None
        serverL = len(self.servers)
        dynL = len(self.dynServers)

        self.index += 1
        self.index %= (serverL + dynL)
        if self.index < serverL:
            return self.servers[self.index]
        else:
            return self.dynServers[self.index - serverL]

        Return the address of a nameserver.

        TODO: Weight servers for response time so faster ones can be
        preferred.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/client.py