def _add_saved_data(self, data_list, data_filter, raise_download_errors):
        """
        Adds already saved data that was not redownloaded to the requested data list.
        """
        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
Adds already saved data that was not redownloaded to the requested data list.