import struct
from collections import namedtuple

class Struct(object):

    def __init__(self, typename, *lst, **kwds):
        expected_size = kwds.pop('expected_size', None)
        if kwds:
            raise TypeError, 'unexpected keyword arguments: %s' % (kwds.keys())
        names = [name for name, fmt in lst if name]
        self.T = namedtuple(typename, names)

        fmts = [fmt for name, fmt in lst]
        fmt = ''.join(fmts)
        self.struct = struct.Struct(fmt)
        self.size = self.struct.size
        if expected_size is not None:
            assert self.size == expected_size

    def unpack(self, s):
        return self.T(*self.struct.unpack(s))

    def pack(self, tup):
        return self.struct.pack(*tup)