Python argparse skip if not int

Question

I would like to have 3 optional positional arguments (int, int, then str).

What I want:

$ ./args.py 0 100 vid
start=0
end=100
video='vid'

$ ./args.py 0 100
start=0
end=100
video=None

$ ./args.py 0
start=0
end=None
video=None

$ ./args.py vid
start=None
end=None
video='vid'

$ ./args.py
start=None
end=None
video=None

What I have tried:

#!/usr/bin/python3
import argparse

parser = argparse.ArgumentParser()

parser.add_argument('start', type=int, nargs='?')
parser.add_argument('end',   type=int, nargs='?')
parser.add_argument('video', type=str, nargs='?')

print(parser.parse_args())

The problem with that code is:

$ ./args.py vid
usage: args.py [-h] [start] [end] [video]
args.py: error: argument start: invalid int value: 'vid'

argparse knows that the value 'vid' is not an integer, so I would like it to "skip" the two first arguments start and end since they don't match.

When I make the video argument mandatory, it works a bit better:

parser.add_argument('start', type=int, nargs='?')
parser.add_argument('end',   type=int, nargs='?')
parser.add_argument('video', type=str)

Demo:

# Fine!
$ ./args.py 0 100 vid
start=0
end=100
video='vid'

# Fine!
$ ./args.py vid
start=None
end=None
video='vid'

# Not fine
./args.py 
usage: args.py [-h] [start] [end] video
args.py: error: the following arguments are required: video

argparse raises an error when the type doesn't match; it doesn't skip that argument or go on to try another. Positional values like this are assigned strictly on position, not on value. — hpaulj
– hpaulj, Commented Dec 27, 2018 at 17:24

Him · Accepted Answer · 2018-12-27 12:55:24Z

1

There is no built-in way to do this cleanly, I think. Thus, you'll just have to gather all of the options into a list and parse them out yourself.

e.g.

parser.add_argument('start_end_video', nargs='*')
args=parser.parse_args()
if len(args.start_end_video) == 1:
   video = args.start_end_video
elif len(args.start_end_video) == 3:
   start, end, video = args.start_end_video

etc.

answered Dec 27, 2018 at 12:55

Him

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Comments

RoadRunner · Accepted Answer · 2018-12-27 13:08:24Z

I think you should add what your arguments are called:

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-s', '--start', type=int)
parser.add_argument('-e', '--end', type=int)
parser.add_argument('-v', '--video', type=str)

args = parser.parse_args()
for arg in vars(args):
    print(arg, '=', getattr(args, arg))

That way you can specify the arguments and no confusion can occur:

$ ./args.py -s 0 -e 100 -v vid
start = 0
end = 100
video = vid
$ ./args.py -s 0 -e 100
start = 0
end = 100
video = None
$ ./args.py -s 0
start = 0
end = None
video = None
$ ./args.py -v vid
start = None
end = None
video = vid
$ ./args.py
start = None
end = None
video = None
$ ./args.py vid
usage: args.py [-h] [-s START] [-e END] [-v VIDEO]
args.py: error: unrecognized arguments: vid

Note: I have included short aliases in the arguments above. E.g. You can call -s instead of --start.

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