How to use the tftpy.TftpPacketTypes.TftpPacket function in tftpy

-----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 2

    def __str__(self):
        s = 'WRQ packet: filename = %s' % self.filename
        s += ' mode = %s' % self.mode
        if self.options:
            s += '\n    options = %s' % self.options
        return s

class TftpPacketDAT(TftpPacket):
    """
::

            2 bytes    2 bytes       n bytes
            ---------------------------------
    DATA  | 03    |   Block #  |    Data    |
            ---------------------------------
    """
    def __init__(self):
        TftpPacket.__init__(self)
        self.opcode = 3
        self.blocknumber = 0
        self.data = None

    def __str__(self):
        s = 'DAT packet: block %s' % self.blocknumber

return self

    def decode(self):
        """Decode self.buffer into instance variables. It returns self for
        easy method chaining."""
        # We know the first 2 bytes are the opcode. The second two are the
        # block number.
        (self.blocknumber,) = struct.unpack(str("!H"), self.buffer[2:4])
        log.debug("decoding DAT packet, block number %d", self.blocknumber)
        log.debug("should be %d bytes in the packet total", len(self.buffer))
        # Everything else is data.
        self.data = self.buffer[4:]
        log.debug("found %d bytes of data", len(self.data))
        return self

class TftpPacketACK(TftpPacket):
    """
::

            2 bytes    2 bytes
            -------------------
    ACK   | 04    |   Block #  |
            --------------------
    """
    def __init__(self):
        TftpPacket.__init__(self)
        self.opcode = 4
        self.blocknumber = 0

    def __str__(self):
        return 'ACK packet: block %d' % self.blocknumber

return self

    def decode(self):
        """Decode self.buffer into instance variables. It returns self for
        easy method chaining."""
        # We know the first 2 bytes are the opcode. The second two are the
        # block number.
        (self.blocknumber,) = struct.unpack(str("!H"), self.buffer[2:4])
        log.debug("decoding DAT packet, block number %d", self.blocknumber)
        log.debug("should be %d bytes in the packet total", len(self.buffer))
        # Everything else is data.
        self.data = self.buffer[4:]
        log.debug("found %d bytes of data", len(self.data))
        return self

class TftpPacketACK(TftpPacket):
    """
::

            2 bytes    2 bytes
            -------------------
    ACK   | 04    |   Block #  |
            --------------------
    """
    def __init__(self):
        TftpPacket.__init__(self)
        self.opcode = 4
        self.blocknumber = 0

    def __str__(self):
        return 'ACK packet: block %d' % self.blocknumber

log.debug("encoding ACK: opcode = %d, block = %d",
            self.opcode, self.blocknumber)
        self.buffer = struct.pack(str("!HH"), self.opcode, self.blocknumber)
        return self

    def decode(self):
        if len(self.buffer) > 4:
            log.debug("detected TFTP ACK but request is too large, will truncate")
            log.debug("buffer was: %s", repr(self.buffer))
            self.buffer = self.buffer[0:4]
        self.opcode, self.blocknumber = struct.unpack(str("!HH"), self.buffer)
        log.debug("decoded ACK packet: opcode = %d, block = %d",
            self.opcode, self.blocknumber)
        return self

class TftpPacketERR(TftpPacket):
    """
::

            2 bytes  2 bytes        string    1 byte
            ----------------------------------------
    ERROR | 05    |  ErrorCode |   ErrMsg   |   0  |
            ----------------------------------------

    Error Codes

    Value     Meaning

    0         Not defined, see error message (if any).
    1         File not found.
    2         Access violation.
    3         Disk full or allocation exceeded.

-----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 2

    def __str__(self):
        s = 'WRQ packet: filename = %s' % self.filename
        s += ' mode = %s' % self.mode
        if self.options:
            s += '\n    options = %s' % self.options
        return s

class TftpPacketDAT(TftpPacket):
    """
::

            2 bytes    2 bytes       n bytes
            ---------------------------------
    DATA  | 03    |   Block #  |    Data    |
            ---------------------------------
    """
    def __init__(self):
        TftpPacket.__init__(self)
        self.opcode = 3
        self.blocknumber = 0
        self.data = None

    def __str__(self):
        s = 'DAT packet: block %s' % self.blocknumber