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 required

For example,

-2 [good]
-3 a -4 b [good]
-3 a [not good, -4 required]
-2 -5 c [good]
-2 -3 a [good]

There are a number of similar questions here, but either they don't address this situation or I don't understand.

Python 2.7 if that matters.

Answer 1

A subparser (as suggested in comments) might work.

Another alternative (since mutually_exclusive_group can't quite do this) is just to code it manually, as it were:

import argparse

def main():
    parser = argparse.ArgumentParser()

    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:
        if args.three is None or args.four is None:
            parser.error('without -2, *both* -3 <a> *and* -4 <b> are required')

    print args
    return 0

Adding a little driver to this:

import sys
sys.exit(main())

and run with your examples, it seems to do the right thing; here are two runs:

$ python mxgroup.py -2; echo $?
Namespace(five=None, four=None, three=None, two=True)
0
$ python mxgroup.py -3 a; echo $?
usage: mxgroup.py [-h] [-2] [-3 THREE] [-4 FOUR] [-5 FIVE]
mxgroup.py: error: without -2, *both* -3 <a> *and* -4 <b> are required
2
$ 

Answer 2

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)