Python Magic ...

output

usage: test.py foo [-h] [-a {bar}]
test.py foo: error: argument -a/--argument: invalid choice: ' bar' (choose from 'bar')
Exception - short
usage: test.py [-h] {foo} ...
test.py: error: unrecognized arguments: --argument bar
Exception - long
bar

test.py

from argparse import ArgumentParser

def foo(v):
    print v.argument
    pass

def main(*args):
    parser = ArgumentParser()
    subparsers = parser.add_subparsers()
    foo_parser = subparsers.add_parser('foo')
    foo_parser.add_argument('-a', '--argument', choices=['bar'], help='An argument with short and long forms')
    foo_parser.set_defaults(func=foo)

    try:
        parsed = parser.parse_args(['foo', '-a bar'])
        parsed.func(parsed)
    except:
        print "Exception - short"
    
    try:
        parsed = parser.parse_args(['foo', '--argument bar'])
        parsed.func(parsed)
    except:
        print "Exception - long"

    try:
        parsed = parser.parse_args(['foo', '--argument', 'bar'])
        parsed.func(parsed)
    except:
        print "Exception - long/split"

if __name__ == '__main__':
    main()