def _lookup(self, name, cls, type, timeout=None):
        """
        Determine a response to a particular DNS query.

        @param name: The name which is being queried and for which to lookup a
            response.
        @type name: L{bytes}

        @param cls: The class which is being queried.  Only I{IN} is
            implemented here and this value is presently disregarded.
        @type cls: L{int}

        @param type: The type of records being queried.  See the types defined
            in L{twisted.names.dns}.
        @type type: L{int}

        @param timeout: All processing is done locally and a result is
            available immediately, so the timeout value is ignored.

        @return: A L{Deferred} that fires with a L{tuple} of three sets of
            response records (to comprise the I{answer}, I{authority}, and
            I{additional} sections of a DNS response) or with a L{Failure} if
            there is a problem processing the query.
        """
        cnames = []
        results = []
        authority = []
        additional = []
        default_ttl = max(self.soa[1].minimum, self.soa[1].expire)

        domain_records = self.records.get(name.lower())

        if domain_records:
            for record in domain_records:
                if record.ttl is not None:
                    ttl = record.ttl
                else:
                    ttl = default_ttl

                if (record.TYPE == dns.NS and
                        name.lower() != self.soa[0].lower()):
                    # NS record belong to a child zone: this is a referral.  As
                    # NS records are authoritative in the child zone, ours here
                    # are not.  RFC 2181, section 6.1.
                    authority.append(
                        dns.RRHeader(
                            name, record.TYPE, dns.IN, ttl, record, auth=False
                        )
                    )
                elif record.TYPE == type or type == dns.ALL_RECORDS:
                    results.append(
                        dns.RRHeader(
                            name, record.TYPE, dns.IN, ttl, record, auth=True
                        )
                    )
                if record.TYPE == dns.CNAME:
                    cnames.append(
                        dns.RRHeader(
                            name, record.TYPE, dns.IN, ttl, record, auth=True
                        )
                    )
            if not results:
                results = cnames

            # Sort of https://tools.ietf.org/html/rfc1034#section-4.3.2 .
            # See https://twistedmatrix.com/trac/ticket/6732
            additionalInformation = self._additionalRecords(
                results, authority, default_ttl)
            if cnames:
                results.extend(additionalInformation)
            else:
                additional.extend(additionalInformation)

            if not results and not authority:
                # Empty response. Include SOA record to allow clients to cache
                # this response. RFC 1034, sections 3.7 and 4.3.4, and RFC 2181
                # section 7.1.
                authority.append(
                    dns.RRHeader(
                        self.soa[0], dns.SOA, dns.IN, ttl, self.soa[1],
                        auth=True
                    )
                )
            return defer.succeed((results, authority, additional))
        else:
            if dns._isSubdomainOf(name, self.soa[0]):
                # We may be the authority and we didn't find it.
                # XXX: The QNAME may also be in a delegated child zone. See
                # #6581 and #6580
                return defer.fail(
                    failure.Failure(dns.AuthoritativeDomainError(name))
                )
            else:
                # The QNAME is not a descendant of this zone. Fail with
                # DomainError so that the next chained authority or
                # resolver will be queried.
                return defer.fail(failure.Failure(error.DomainError(name)))

        Determine a response to a particular DNS query.

        @param name: The name which is being queried and for which to lookup a
            response.
        @type name: L{bytes}

        @param cls: The class which is being queried.  Only I{IN} is
            implemented here and this value is presently disregarded.
        @type cls: L{int}

        @param type: The type of records being queried.  See the types defined
            in L{twisted.names.dns}.
        @type type: L{int}

        @param timeout: All processing is done locally and a result is
            available immediately, so the timeout value is ignored.

        @return: A L{Deferred} that fires with a L{tuple} of three sets of
            response records (to comprise the I{answer}, I{authority}, and
            I{additional} sections of a DNS response) or with a L{Failure} if
            there is a problem processing the query.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/authority.py