Python - 类工厂

Question

我正在使用类来设计网络对话，我有这段代码：

class Conversation(object):
    __metaclass__ = ABCMeta

    def __init__(self, received=0, sent=0):
        self.sent = sent
        self.received = received

    def __str__(self):
        return "<Conversation: received=" + str(self.received) + " sent=" + str(self.sent) + " >"

    def __eq__(self, other):
        return isinstance(other, Conversation)

    @classmethod
    def filter(cls, pkt):
        return True

    @abstractmethod
    def is_sent(self, pkt):
        raise NotImplementedError

    def add(self, pkt):
        if self.is_sent(pkt):
            self.sent += 1
        else:
            self.received += 1

class IPConversation(Conversation):
    def __init__(self, a_ip, b_ip, received=0, sent=0):
        super(IPConversation, self).__init__(received, sent)
        self.a_ip = a_ip
        self.b_ip = b_ip

    def __str__(self):
        return "<IPConversation: " + self.a_ip + " >>> " + self.b_ip

    def __eq__(self, other):
        if super(IPConversation, self).__eq__(other):
            a = (self.a_ip, self.b_ip)
            b = (other.ip_a, other.ip_b)
            return a == b or a == b[::-1]
        return False

    @classmethod
    def filter(cls, pkt):
        return super(IPConversation, cls).filter(pkt) and 'IP' in pkt
    def is_sent(self, pkt):
        return self.a_ip == pkt['IP'].src

然后我考虑创建IPv6类，它将与IP类相同，但唯一的区别是 classmethod 中“IP”前面的pkt中的“IPv6” 。我怎么能用元类来做呢？