code: def _add_saved_data(self, data_list, data_filter, raise_download_errors):
          
          filtered_download_list = self.download_list if data_filter is None else \
              [self.download_list[index] for index in data_filter]
          for i, request in enumerate(filtered_download_list):
              if request.return_data and data_list[i] is None:
                  if os.path.exists(request.get_file_path()):
                      data_list[i] = read_data(request.get_file_path())
                  elif raise_download_errors:
                      raise DownloadFailedException('Failed to download data from {}.\n No previously downloaded data '
                                                    'exists in file {}.'.format(request.url, request.get_file_path()))
          return data_list
docs: Adds already saved data that was not redownloaded to the requested data list.