code: def _get_kdjd(cls, df, n_days):
          
          k_column = 'kdjk_{}'.format(n_days)
          d_column = 'kdjd_{}'.format(n_days)
          df[d_column] = list(cls._calc_kd(df.get(k_column)))
docs: Get the D of KDJ
  
          D = 2/3 × (prev. D) +1/3 × (curr. K)
          2/3 and 1/3 are the smooth parameters.
          :param df: data
          :param n_days: calculation range
          :return: None