Python argparse - Mutually exclusive group with default if no argument is given

Question

I'm writing a Python script to process a machine-readable file and output a human-readable report on the data contained within.
I would like to give the option of outputting the data to stdout (-s) (by default) or to a txt (-t) or csv (-c) file. I would like to have a switch for the default behaviour, as many commands do.

In terms of Usage:, I'd like to see something like script [-s | -c | -t] input file, and have -s be the default if no arguments are passed.

I currently have (for the relevant args, in brief):

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-s', '--stdout', action='store_true')
group.add_argument('-c', '--csv', action='store_true')
group.add_argument('-t', '--txt', action='store_true')
args = parser.parse_args()

if not any((args.stdout, args.csv, args.txt)):
    args.stdout = True

So if none of -s, -t, or -c are set, stdout (-s) is forced to True, exactly as if -s had been passed.

Is there a better way to achieve this? Or would another approach entirely be generally considered 'better' for some reason?

Note: I'm using Python 3.5.1/2 and I'm not worried about compatibility with other versions, as there is no plan to share this script with others at this point. It's simply to make my life easier.

I know this doesn't work for _every_ case, but this sounds exactly like what `choices` option is for: `parser.add_argument('--output', default='stdout', choices=['stdout', 'csv', 'txt'])` — Mr. Llama, Dec 01 '21 at 20:14

Robᵩ · Accepted Answer · 2017-04-12T18:20:22.783

You could have each of your actions update the same variable, supplying stdout as the default value for that variable.

Consider this program:

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument(
    '-s', '--stdout', action='store_const', dest='type', const='s', default='s')
group.add_argument(
    '-c', '--csv', action='store_const', dest='type', const='c')
group.add_argument(
    '-t', '--txt', action='store_const', dest='type', const='t')
args = parser.parse_args()
print args

Your code could look like:

if args.type == 's':
    ofile = sys.stdout
elif args.type == 'c':
    ofile = ...
...

First alternative:

Rather than arbitrarily choose one of the .add_argument()s to specify the default type, you can use parser.set_defaults() to specify the default type.

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-s', '--stdout', action='store_const', dest='type', const='s')
group.add_argument('-c', '--csv', action='store_const', dest='type', const='c')
group.add_argument('-t', '--txt', action='store_const', dest='type', const='t')
parser.set_defaults(type='s')
args = parser.parse_args()
print args

Second alternative:

Rather than specify the type as an enumerated value, you could store a callable into the type, and then invoke the callable:

import argparse

def do_stdout():
    # do everything that is required to support stdout
    print("stdout!")
    return
def do_csv():
    # do everything that is required to support CSV file
    print("csv!")
    return
def do_text():
    # do everything that is required to support TXT file
    print("text!")
    return

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-s', '--stdout', action='store_const', dest='type', const=do_stdout)
group.add_argument('-c', '--csv', action='store_const', dest='type', const=do_csv)
group.add_argument('-t', '--txt', action='store_const', dest='type', const=do_text)
parser.set_defaults(type=do_stdout)
args = parser.parse_args()
print args
args.type()

The idea here is that each of the actions writes to the same variable. You could specify the default outside of the argument defintions too with: `if args.type is None or args.type =='s': ofile = sys.stdout` — theorifice, Jul 21 '16 at 15:31

score -2 · Answer 2 · answered Jul 21 '16 at 15:33

You can "cheat" with sys.argv :

import sys


def main():
    if len(sys.argv) == 2 and sys.argv[1] not in ['-s', '-c', '-t', '-h']:
        filename = sys.argv[1]
        print "mode : stdout", filename
    else:
        parser = argparse.ArgumentParser()
        group = parser.add_mutually_exclusive_group()
        group.add_argument('-s', '--stdout')
        group.add_argument('-c', '--csv')
        group.add_argument('-t', '--txt')
        args = parser.parse_args()
        if args.stdout:
            print "mode stdout :",  args.stdout
        if args.csv:
            print "mode csv :",  args.csv
        if args.txt:
            print "mode txt :",  args.txt

if __name__ == "__main__":
    main()

Python argparse - Mutually exclusive group with default if no argument is given

2 Answers2

First alternative:

Second alternative: