class History(object):
    """
    This holds all the response and request objects for a
    session. A server using this should call "clear" after
    each request cycle in order to keep it from clogging
    memory.
    """
    size = 20
    requests = []
    responses = []
    _instance = None

    @classmethod
    def instance(cls):
        if not cls._instance:
            cls._instance = cls()
        return cls._instance

    def add_response(self, response_obj):
        if self.size == 0:
            return
        self.responses.append(response_obj)
        if self.size > 0:
            self.responses = self.responses[0 - self.size:]

    def add_request(self, request_obj):
        if self.size == 0:
            return
        self.requests.append(request_obj)
        if self.size > 0:
            self.requests = self.requests[0 - self.size:]

    @property
    def request(self):
        if len(self.requests) == 0:
            return None
        else:
            return self.requests[-1]

    @property
    def response(self):
        if len(self.responses) == 0:
            return None
        else:
            return self.responses[-1]

    def clear(self):
        del self.requests[:]
        del self.responses[:]