'Return a Message representation or raise a KeyError.'

def get_message(self, key):

    (start, stop) = self._lookup(key)
    self._file.seek(start)
    from_line = self._file.readline().replace(os.linesep, '')
    string = self._file.read((stop - self._file.tell()))
    msg = self._message_factory(string.replace(os.linesep, '\n'))
    msg.set_from(from_line[5:])
    return msg
