My first steps into python metaprogramming. Not sure if this really makes sense, but at least it was reasonable for me.

metatest1.py

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

from struct import pack, unpack
from binascii import hexlify, unhexlify

class HWAddr(object):
    fmt = "6s"

    def __init__(self, addr="FF:FF:FF:FF:FF:FF"):
        self.addr = addr

    def asc2hex(self):
        return unhexlify(self.addr.replace(':',''))

class MetaClass(type):
    def __new__(cls, clsname, clsbase, clsdict):
        tmp = type.__new__(cls, clsname, clsbase, clsdict)
        tmp.__hdr__ = []
        for key, val in clsdict.iteritems():
            if isinstance(val, HWAddr):
                tmp.__hdr__.append((val.asc2hex(), val.fmt))
        tmp.__fmt__ = getattr(tmp, '__byte_order__', '>') + ''.join([val[1] for val in tmp.__hdr__])
        return tmp

class Packet(object):
    __metaclass__ = MetaClass

    def __str__(self):
        return pack(self.__fmt__, *[ hdr[0] for hdr in self.__hdr__ ])

class Ethernet(Packet):
    src = HWAddr("AA:BB:CC:DD:EE:FF")
    dst = HWAddr("11:22:33:44:55:66")

if __name__ == '__main__':
    print Ethernet()