How to use the fbtftp.constants function in fbtftp

"""
        Deals with incoming RRQ packets. This is called by `run_once` when data
        is available on the listening socket.
        This method deals with extracting all the relevant information from the
        request (like file, transfer mode, path, and options).
        If all is good it will run the `get_handler` method, which returns a
        `BaseHandler` object. `BaseHandler` is a subclass of a
        `multiprocessing.Process` class so calling `start()` on it will cause
        a `fork()`.
        """
        data, peer = self._listener.recvfrom(constants.DEFAULT_BLKSIZE)
        code = struct.unpack('!H', data[:2])[0]
        if code != constants.OPCODE_RRQ:
            logging.warning(
                'unexpected TFTP opcode %d, expected %d' %
                (code, constants.OPCODE_RRQ)
            )
            return

        # extract options
        tokens = list(filter(bool, data[2:].decode('latin-1').split('\x00')))
        if len(tokens) < 2 or len(tokens) % 2 != 0:
            logging.error(
                'Received malformed packet, ignoring '
                '(tokens length: {tl})'.format(tl=len(tokens))
            )
            return

        path = tokens[0]
        options = collections.OrderedDict([
            ('mode', tokens[1].lower()),
            ('default_timeout', self._timeout),