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

-----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 1

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

class TftpPacketWRQ(TftpPacketInitial):
    """
::

            2 bytes    string   1 byte     string   1 byte
            -----------------------------------------------
    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:

-----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 1

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

class TftpPacketWRQ(TftpPacketInitial):
    """
::

            2 bytes    string   1 byte     string   1 byte
            -----------------------------------------------
    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:

shortbuf = subbuf[:tlength+1]
        log.debug("about to unpack buffer with fmt: %s", fmt)
        log.debug("unpacking buffer: %s", repr(shortbuf))
        mystruct = struct.unpack(fmt, shortbuf)

        tftpassert(len(mystruct) == 2, "malformed packet")
        self.filename = mystruct[0].decode('ascii')
        self.mode = mystruct[1].decode('ascii').lower() # force lc - bug 17
        log.debug("set filename to %s", self.filename)
        log.debug("set mode to %s", self.mode)

        self.options = self.decode_options(subbuf[tlength+1:])
        log.debug("options dict is now %s", self.options)
        return self

class TftpPacketRRQ(TftpPacketInitial):
    """
::

            2 bytes    string   1 byte     string   1 byte
            -----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 1

    def __str__(self):
        s = 'RRQ packet: filename = %s' % self.filename
        s += ' mode = %s' % self.mode
        if self.options:

shortbuf = subbuf[:tlength+1]
        log.debug("about to unpack buffer with fmt: %s", fmt)
        log.debug("unpacking buffer: %s", repr(shortbuf))
        mystruct = struct.unpack(fmt, shortbuf)

        tftpassert(len(mystruct) == 2, "malformed packet")
        self.filename = mystruct[0].decode('ascii')
        self.mode = mystruct[1].decode('ascii').lower() # force lc - bug 17
        log.debug("set filename to %s", self.filename)
        log.debug("set mode to %s", self.mode)

        self.options = self.decode_options(subbuf[tlength+1:])
        log.debug("options dict is now %s", self.options)
        return self

class TftpPacketRRQ(TftpPacketInitial):
    """
::

            2 bytes    string   1 byte     string   1 byte
            -----------------------------------------------
    RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
    WRQ     -----------------------------------------------
    """
    def __init__(self):
        TftpPacketInitial.__init__(self)
        self.opcode = 1

    def __str__(self):
        s = 'RRQ packet: filename = %s' % self.filename
        s += ' mode = %s' % self.mode
        if self.options: