code: def get_r_df(self):
          ''' getter '''
          if isinstance(self.__r_df, pd.DataFrame) is False and self.__r_df is not None:
              raise TypeError("The type of `__r_df` must be `pd.DataFrame`.")
          return self.__r_df get_r_df(self):
          ''' getter '''
          if isinstance(self.__r_df, pd.DataFrame) is False and self.__r_df is not None:
              raise TypeError("The type of `__r_df` must be `pd.DataFrame`.")
          return self.__r_df
docs: getter