code: def _get_kdjk(cls, df, n_days):
          
          rsv_column = 'rsv_{}'.format(n_days)
          k_column = 'kdjk_{}'.format(n_days)
          df[k_column] = list(cls._calc_kd(df.get(rsv_column)))
docs: 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