Creating mutually inclusive positional arguments with argparse

Question

I'm trying to build a command line interface with Python's argparse module. I want two positional arguments where one depends on the other (mutually inclusive). Here is what I want:

prog [arg1 [arg2]]

Here's what I have so far:

prog [arg1] [arg2]

Which is produced by:

parser = argparse.ArgumentParser()
parser.add_argument('arg1', nargs='?')
parser.add_argument('arg2', nargs='?')

How do I get from there to having a mutually inclusive arg2?

Answer 1

Module argparse doesn't have options for creating mutually inclusive arguments. However it's simple to write it by yourself.
Start with adding both arguments as optional:

parser.add_argument('arg1', nargs='?')
parser.add_argument('arg2', nargs='?')

After parsing arguments check if arg1 is set and arg2 is not:

args = parser.parse_args()
if args.arg1 and not args.arg2:

(this may be more tricky if you change default value from None for not used arguments to something different)

Then use parser.error() function to display normal argparse error message:

parser.error('the following arguments are required: arg2')

Finally change usage: message to show that arg2 depends on arg1:

parser = argparse.ArgumentParser(usage='%(prog)s [arg1 [arg2]]')

A complete script:

import argparse

parser = argparse.ArgumentParser(usage='%(prog)s [arg1 [arg2]]')
parser.add_argument('arg1', nargs='?')
parser.add_argument('arg2', nargs='?')

args = parser.parse_args()

if args.arg1 and not args.arg2:
    parser.error('the following arguments are required: arg2')