Add arguments to argparse depending on value of initial arguments

Question

I know similar questions have been asked before but I could not really find an exact one (also, I could not understand the 'nargs' or 'subparse' concept and how it would apply to my case)

I want something like:

parser = argparse.ArgumentParser()
parser.add_argument('-mode', choices=['download', 'upload'], required=True)
parser.add_argument('-d', required=True)
args = parser.parse_args()
if args.mode == 'download':
    parser.add_argument('-f', required=True)

args = parser.parse_args()

so the -f argument is required only when the -mode is download otherwise it should not be parsed

Answer 1

Generally, you should avoid making --options required. In this case, I would suggest the following:

1. Replace --mode with a subcommand.
2. Replace -d with a positional argument
3. Replace -f with a 2nd positional argument for the download command only.

If you really want to, you can keep -d and -f instead of making them positional arguments.

Your calls would look like this:

./script.py upload d-argument
./script.py download d-argument f-argument

or

./script.py upload -d foo
./script.py download -d foo -f bar

The code to implement is a little tedious, but fairly straightforward.

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()
upload_parser = subparsers.add_parser('upload')
download_parser = subparsers.add_parser('download')
upload_parser.set_defaults(cmd='upload')
download_parser.set_defaults(cmd='download')
for p in [ upload_parser, download_parser ]:
    p.add_argument("d")
    # p.add_argument("-d", required=True)
download_parser.add_argument("f")
# download_parser.add_argument("-f", required=True)


args = parser.parse_args()
if args.cmd == 'upload':
    # handle upload with args.d
elif args.cmd == 'download':
    # handle download with args.d and args.f

Note the calls to set_defaults used to mark in the final parsed arguments which command was used.