Argparse required options with variable parameters?

Question

A program I'm writing requires 1 of 3 inputs.

2 of the inputs require an additional parameter. 1 of the inputs requires 2 additional parameters. Only 1 of these can be input at a time.

How can I ensure this using the argparse module?

Please see the code below for what I've tried so far.

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-d',  
      default='temp.file')

parser.add_argument('command',
      choices=['g', 's', 'm'])

args = parser.parse_args()

>>> prog.py g #  requires 1 param.
>>> prog.py s #  requires 1 param.
>>> prog.py m #  requires 2 params.

Answer 1

g , s , and m are subcommands, which can be implemented using sub parsers.

p = ArgumentParser()
p.add_argument('-d', default='temp.file')

subparsers = p.add_subparsers()
g_parser = subparsers.add_parser('g')
g_parser.add_argument('g_foo')

s_parser = subparsers.add_parser('s')
s_parser.add_argument('s_foo')

m_parser = subparsers.add_parser('m')
m_parser.add_argument('m_foo')
m_parser.add_argument('m_bar')

When you call p.parse_args , then g_foo et al. will only appear in the result when the appropriate subcommand is used. For example:

>>> p.parse_args(['g', '3'])
Namespace(d='temp.file', g_foo='3')
>>> p.parse_args(['m', '4', '5'])
Namespace(d='temp.file', m_foo='4', m_bar='5')

Answer 2

Looks like a good use for a mutually exclusive group

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-d',  
      default='temp.file')
mx = parser.add_mutually_exclusive_group(required=True)
mx.add_argument('-g')
mx.add_argument('-s')
mx.add_argument('-m', nargs=2)

args = parser.parse_args()
print(args)

Sample runs:

1316:~/mypy$ python3 stack56926264.py -h
usage: stack56926264.py [-h] [-d D] (-g G | -s S | -m M M)

optional arguments:
  -h, --help  show this help message and exit
  -d D
  -g G
  -s S
  -m M M

1527:~/mypy$ python3 stack56926264.py -g foo
Namespace(d='temp.file', g='foo', m=None, s=None)

1528:~/mypy$ python3 stack56926264.py -s bar
Namespace(d='temp.file', g=None, m=None, s='bar')

1528:~/mypy$ python3 stack56926264.py -m 1 2
Namespace(d='temp.file', g=None, m=['1', '2'], s=None)

and catching some errors:

1528:~/mypy$ python3 stack56926264.py -m 1 2 -s bar -d afile
usage: stack56926264.py [-h] [-d D] (-g G | -s S | -m M M)
stack56926264.py: error: argument -s: not allowed with argument -m

1528:~/mypy$ python3 stack56926264.py -m 1 
usage: stack56926264.py [-h] [-d D] (-g G | -s S | -m M M)
stack56926264.py: error: argument -m: expected 2 arguments

1530:~/mypy$ python3 stack56926264.py -d afile
usage: stack56926264.py [-h] [-d D] (-g G | -s S | -m M M)
stack56926264.py: error: one of the arguments -g -s -m is required

Answer 3

What you could do is check for yourself that the arguments are in line with your logic:

parser = argparse.ArgumentParser()

parser.add_argument('-d',  
                    default='temp.file')

parser.add_argument('command',
                    choices=['g', 's', 'm'])

parser.add_argument('additionals',
                    nargs='+')

args = parser.parse_args()

if args.command in ('g', 's') and len(args.additionals) != 1:
    print("Commands g and s require 1 argument")
elif args.command == 'm' and len(args.additionals) != 2:
    print("Command m require 2 arguments")

Another way to go around this is to seperate those commands to different arguments with the corresponding number of additional arguments they require:

parser = argparse.ArgumentParser()

parser.add_argument('-d',  
                    default='temp.file')

parser.add_argument('-g',
                    nargs=1)

parser.add_argument('-s',
                    nargs=1)

parser.add_argument('-m',
                    nargs=2)

args = parser.parse_args()

Read about nargs for how to use multiple arguments for one action.