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

argparse does not have mutually inclusive arguments, which is what you are talking about. You should instead make neither -d nor -f be required, but include in the usage message the fact that either both or neither must be specified, then check to make sure that both -d and -f are specified if either is. — 2rs2ts
– 2rs2ts, Commented Sep 9, 2013 at 22:49
You could use subparsers, so that download becomes a subcommand with it's own set of (required) flags. — miku
– miku, Commented Sep 9, 2013 at 22:51

chepner · Accepted Answer · 2013-09-09 22:59:04Z

8

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

Replace --mode with a subcommand.
Replace -d with a positional argument
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.

answered Sep 9, 2013 at 22:59

chepner

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2 Comments

Wtower Over a year ago

Great, thanks. Not sure if this happens only for my my python version (3.5), but I need to check if cmd in args in case the user calls the script without cmd.

chepner Over a year ago

If your script is called with no positional arguments, argparse itself will raise an error. Otherwise, the first argument will be used for cmd.

alecxe · Accepted Answer · 2013-09-09 22:52:41Z

6

What about this?

import argparse

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

args = parser.parse_args()
if args.mode == 'download' and not args.f:
    parser.error('-f argument is required in "download" mode.')

DEMO:

$ python test.py -mode=upload -d 10
$ python test.py -mode=download -d 10
usage: test.py [-h] -mode {download,upload} -d D [-f F]
test.py: error: -f argument is required in "download" mode.
$ python test.py -mode=download -d 10 -f 10

answered Sep 9, 2013 at 22:52

alecxe

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