If you are using python in your pipeline, might as well go all in! from pathlib import Path all_possible_filenames = {f'{i:04}A.csv' for i in range(1,10)}

If you are using python in your pipeline, might as well go all in! from pathlib import Path all_possible_filenames = {f'{i:04}A.csv' for i in range(1,10)} cur_dir_filenames = {Path('.').iterdir()} missing_filenames = all_possible_filenames - cur_dir_filenames print(*missing_filenames, sep='\n')