获取下一个可用的代理
        :return: proxy_host, proxy_port, proxy_username, proxy_password

def get_proxy(self):
        """
        获取下一个可用的代理
        :return: proxy_host, proxy_port, proxy_username, proxy_password
        """
        if self.proxy_list is None or len(self.proxy_list) <= 0:
            return None, None, None, None
        self.proxy_index += 1
        self.proxy_index = self.proxy_index % len(self.proxy_list)
        item = self.proxy_list[self.proxy_index]
        if len(item) == 2:
            return item[0], item[1], None, None
        else:
            return item[0], item[1], item[2], item[3]