How to use the tftpy.TftpStates.TftpStateExpectDAT function in tftpy

self.sendError(TftpErrors.IllegalTftpOp)
                raise TftpException("There is no block zero!")
            log.warning("Dropping duplicate block %d" % pkt.blocknumber)
            self.context.metrics.add_dup(pkt)
            log.debug("ACKing block %d again, just in case", pkt.blocknumber)
            self.sendACK(pkt.blocknumber)

        else:
            # FIXME: should we be more tolerant and just discard instead?
            msg = "Whoa! Received future block %d but expected %d" \
                % (pkt.blocknumber, self.context.next_block)
            log.error(msg)
            raise TftpException(msg)

        # Default is to ack
        return TftpStateExpectDAT(self.context)

# Now check the packet type and dispatch it properly.
        if isinstance(pkt, TftpPacketOACK):
            log.info("Received OACK from server")
            try:
                self.handleOACK(pkt)
            except TftpException as err:
                log.error("Failed to negotiate options: %s" % str(err))
                self.sendError(TftpErrors.FailedNegotiation)
                raise
            else:
                log.debug("Sending ACK to OACK")

                self.sendACK(blocknumber=0)

                log.debug("Changing state to TftpStateExpectDAT")
                return TftpStateExpectDAT(self.context)

        elif isinstance(pkt, TftpPacketDAT):
            # If there are any options set, then the server didn't honour any
            # of them.
            log.info("Received DAT from server")
            if self.context.options:
                log.info("Server ignored options, falling back to defaults")
                self.context.options = { 'blksize': DEF_BLKSIZE }
            return self.handleDat(pkt)

        # Every other packet type is a problem.
        elif isinstance(pkt, TftpPacketACK):
            # Umm, we ACK, the server doesn't.
            self.sendError(TftpErrors.IllegalTftpOp)
            raise TftpException("Received ACK from server while in download")

# Now check the packet type and dispatch it properly.
        if isinstance(pkt, TftpPacketOACK):
            log.info("Received OACK from server")
            try:
                self.handleOACK(pkt)
            except TftpException as err:
                log.error("Failed to negotiate options: %s" % str(err))
                self.sendError(TftpErrors.FailedNegotiation)
                raise
            else:
                log.debug("Sending ACK to OACK")

                self.sendACK(blocknumber=0)

                log.debug("Changing state to TftpStateExpectDAT")
                return TftpStateExpectDAT(self.context)

        elif isinstance(pkt, TftpPacketDAT):
            # If there are any options set, then the server didn't honour any
            # of them.
            log.info("Received DAT from server")
            if self.context.options:
                log.info("Server ignored options, falling back to defaults")
                self.context.options = { 'blksize': DEF_BLKSIZE }
            return self.handleDat(pkt)

        # Every other packet type is a problem.
        elif isinstance(pkt, TftpPacketACK):
            # Umm, we ACK, the server doesn't.
            self.sendError(TftpErrors.IllegalTftpOp)
            raise TftpException("Received ACK from server while in download")

self.sendError(TftpErrors.IllegalTftpOp)
                raise TftpException("There is no block zero!")
            log.warning("Dropping duplicate block %d" % pkt.blocknumber)
            self.context.metrics.add_dup(pkt)
            log.debug("ACKing block %d again, just in case", pkt.blocknumber)
            self.sendACK(pkt.blocknumber)

        else:
            # FIXME: should we be more tolerant and just discard instead?
            msg = "Whoa! Received future block %d but expected %d" \
                % (pkt.blocknumber, self.context.next_block)
            log.error(msg)
            raise TftpException(msg)

        # Default is to ack
        return TftpStateExpectDAT(self.context)

if self.context.vfs.exists(path):
            logger.warning(
                "File %s exists already, overwriting..."
                % (self.context.file_to_transfer)
            )
        self.make_subdirs()
        self.context.fileobj = self.context.vfs.open(path, "wb")
        # Options negotiation.
        if sendoack:
            logger.debug("Sending OACK to client")
            self.sendOACK()
        else:
            logger.debug("No requested options, expecting transfer to begin...")
            self.sendACK()
        self.context.next_block = 1
        return TftpStateExpectDAT(self.context)