argparse: setting optional argument with value of mandatory argument

As far as I know, there is no way to do this that is more clean than:

ns = parser.parse_args()ns.extra_file = ns.extra_file if ns.extra_file else ns.filename

(just like you propose in your question).

You could probably do some custom action gymnastics similar to this, but I really don't think that would be worthwhile (or "pythonic").

This has slightly different semantics than your original set of options, but may work for you:

parse.add_argument('filename', action='append', dest='filenames')parse.add_argument('--extra-file', '-f', action='append', dest='filenames')args = parse.parse_args()

This would replace args.filename with a list args.filenames of at least one file, with -f appending its argument to that list. Since it's possible to specify -f on the command line before the positional argument, you couldn't on any particular order of the input files in args.filenames.

Another option would be to dispense with the -f option and make filename a multi-valued positional argument:

parse.add_argument('filenames', nargs='+')

Again, args.filenames would be a list of at least one filename. This would also interfere if you have other positional arguments for your script.