Argparse optional arguments with default values

Question

I'm working on something where I need to use argparse. Here's the code I got a problem with:

parser = argparse.ArgumentParser()
parser.add_argument('--create n file', dest='create',
                    nargs=2, default=(1, 'world1.json'),
                    help='Create a party of n player with mission parameters in file')

I'm trying to find a way to either set both n and file to another value, or set only one of them. n is an int and file a str.

Here's what I would like to get, using the following command:

Command	Expected result
python mission.py --create 2	create = [2, 'world1.json']
python mission.py --create world2.json	create = [1, 'world2.json']
python mission.py --create 3 world2.json	create = [3, 'world2.json']

When --create is used (with or without specifying n/file), I'll need to start a function using the list as arguments.

I've tried multiple things and read argparse documentation more than once but can't find a way to do it.

Change to add_argument('--create', nargs='+'). After parsing decide for yourself what to do with the 1 or more strings in the args.create list. argparse isn't set up decode a mix of int and str types. — hpaulj
– hpaulj, Commented Nov 28, 2017 at 3:46
Save yourself a lot of work, use the begins library. pypi.python.org/pypi/begins/0.9 — SteveJ
– SteveJ, Commented Nov 28, 2017 at 5:09
@stevej, How does begins help with this spec? How do you express this requirement as a function's arguments? — hpaulj
– hpaulj, Commented Nov 28, 2017 at 5:24
On further thought, I think I can come up with a custom type function and Action class that does what you want, even replacing default values piecemeal. But it will be just as easy to do that conversion after parsing. Why don't you do that, and I can then suggest how to make it a part of the parser? — hpaulj
– hpaulj, Commented Nov 28, 2017 at 7:49
Edited my question to add what my goal is using --create. @hpaulj using nargs='+' doesn't work for what i need. Since user could just use --create alone to make a party using the default list [1, 'world1.json]. After that what you suggest me to do is to just decode the list after? — Fabien Bauer
– Fabien Bauer, Commented Nov 28, 2017 at 14:42

Pierre de Buyl · Accepted Answer · 2017-11-28 14:58:43Z

1

The code below returns the expected results for the listed usecases. I decided to use an extra function to handle the argument, as the program must accept either an int or a string for the first argument passed.

I use a "try" block to see whether the single argument can be parsed as an int before proceeding.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--create n file', dest='create', nargs='+', default=(1,'world1.json'),
                    help='Create a party of n player with mission parameters in file')
args = parser.parse_args()

def get_n_file(arg):
    if len(arg)==1:
        try:
            i = int(arg[0])
            result = int(arg[0]), 'world'+str(arg[0])+'.json'
        except:
            s = arg[0]
            result = 1, s
        return result
    elif len(arg)==2:
        return int(arg[0]), arg[1]

print(args.create)

n, f = get_n_file(args.create)

print(n, f)

answered Nov 28, 2017 at 14:58

Pierre de Buyl

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1 Comment

wovano Over a year ago

You should never use a bare except. This should probably be except ValueError:.

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