以字典形式返回分级A数据
        :param fields:利率范围，形如['+3.0%', '6.0%']
        :param min_volume:最小交易量，单位万元
        :param min_discount:最小折价率, 单位%
        :param ignore_nodown:是否忽略无下折品种,默认 False
        :param forever: 是否选择永续品种,默认 False

def funda(
        self,
        fields=None,
        min_volume=0,
        min_discount=0,
        ignore_nodown=False,
        forever=False,
    ):
        """以字典形式返回分级A数据
        :param fields:利率范围，形如['+3.0%', '6.0%']
        :param min_volume:最小交易量，单位万元
        :param min_discount:最小折价率, 单位%
        :param ignore_nodown:是否忽略无下折品种,默认 False
        :param forever: 是否选择永续品种,默认 False
        """
        if fields is None:
            fields = []

        # 添加当前的ctime
        self.__funda_url = self.__funda_url.format(ctime=int(time.time()))
        # 请求数据
        rep = requests.get(self.__funda_url)
        # 获取返回的json字符串
        fundajson = json.loads(rep.text)
        # 格式化返回的json字符串
        data = self.formatfundajson(fundajson)
        # 过滤小于指定交易量的数据
        if min_volume:
            data = {
                k: data[k]
                for k in data
                if float(data[k]["funda_volume"]) > min_volume
            }
        if len(fields):
            data = {
                k: data[k]
                for k in data
                if data[k]["coupon_descr_s"] in "".join(fields)
            }
        if ignore_nodown:
            data = {
                k: data[k]
                for k in data
                if data[k]["fund_descr"].find("无下折") == -1
            }
        if forever:
            data = {
                k: data[k]
                for k in data
                if data[k]["funda_left_year"].find("永续") != -1
            }
        if min_discount:
            data = {
                k: data[k]
                for k in data
                if float(data[k]["funda_discount_rt"][:-1]) > min_discount
            }

        self.__funda = data
        return self.__funda