Handle GET_TXN request

def handle_get_txn_req(self, request: Request, frm: str):
        """
        Handle GET_TXN request
        """
        ledger_id = request.operation.get(f.LEDGER_ID.nm, DOMAIN_LEDGER_ID)
        if ledger_id not in self.ledger_to_req_handler:
            self.send_nack_to_client((request.identifier, request.reqId),
                                     'Invalid ledger id {}'.format(ledger_id),
                                     frm)
            return

        seq_no = request.operation.get(DATA)
        self.send_ack_to_client((request.identifier, request.reqId), frm)
        ledger = self.getLedger(ledger_id)

        try:
            txn = self.getReplyFromLedger(ledger, seq_no)
        except KeyError:
            txn = None

        if txn is None:
            logger.debug(
                "{} can not handle GET_TXN request: ledger doesn't "
                "have txn with seqNo={}".format(self, str(seq_no)))

        result = {
            f.IDENTIFIER.nm: request.identifier,
            f.REQ_ID.nm: request.reqId,
            TXN_TYPE: request.operation[TXN_TYPE],
            DATA: None
        }

        if txn:
            result[DATA] = txn.result
            result[f.SEQ_NO.nm] = get_seq_no(txn.result)

        self.transmitToClient(Reply(result), frm)