@classmethod
def _get_kdjk(cls, df, n_days):
    """ Get the K of KDJ

        K ＝ 2/3 × (prev. K) +1/3 × (curr. RSV)
        2/3 and 1/3 are the smooth parameters.
        :param df: data
        :param n_days: calculation range
        :return: None
        """
    rsv_column = 'rsv_{}'.format(n_days)
    k_column = 'kdjk_{}'.format(n_days)
    df[k_column] = list(cls._calc_kd(df.get(rsv_column)))
