Python argparse conditional requirements

Question

How do I set up argparse as follows:

if -2 is on the command line, no other arguments are required
if -2 is not on the command line, -3 and -4 arguments are requ         


        

Answer 1

I think it is pretty hard to achieve that (including a nice help message) while only using the standard argparse functions. You can however easily test it yourself after parsing the arguments. You can describe the extra requirements in the epilogue or so. Note that it is unusual to use numbers as options, I had to use dest='two', since args.2 is not valid syntax.

#!/usr/bin/env python

import argparse

parser = argparse.ArgumentParser(
   description='bla bla',
   epilog='Note: arguments -3 and -4 are required when -2 is missing')

parser.add_argument('-2', dest='two', action='store_true')
parser.add_argument('-3', dest='three')
parser.add_argument('-4', dest='four')
parser.add_argument('-5', dest='five')

args = parser.parse_args()

if not args.two and (args.three is None or args.four is None):
    parser.error('arguments -3 and -4 are required when -2 is missing')

print 'Good:', args

With these results:

[~]: ./test.py -h
usage: test.py [-h] [-2] [-3 THREE] [-4 FOUR] [-5 FIVE]

bla bla

optional arguments:
  -h, --help  show this help message and exit
  -2
  -3 THREE
  -4 FOUR
  -5 FIVE

Note: arguments -3 and -4 are required when -2 is missing

[~]: ./test.py -2
Good: Namespace(five=None, four=None, three=None, two=True)
[~]: ./test.py -3 a -4 b
Good: Namespace(five=None, four='b', three='a', two=False)
[~]: ./test.py -3 a
usage: test.py [-h] [-2] [-3 THREE] [-4 FOUR] [-5 FIVE]
test.py: error: arguments -3 and -4 are required when -2 is missing
[~]: ./test.py -2 -5 c
Good: Namespace(five='c', four=None, three=None, two=True)
[~]: ./test.py -2 -3 a
Good: Namespace(five=None, four=None, three='a', two=True)