python argparse - add action to subparser with no arguments?

Question

I am adding subparsers to my parser to simulate subcommands functionality (for example code see: Simple command line application in python - parse user input?). Now I want t

Answer 1

The documentation for subcommands gives two examples of how to identify the subparser.

https://docs.python.org/dev/library/argparse.html#sub-commands

One is to give the add_subparsers a dest:

def do_quit(args):
    # action
    quit()

parser = ArgumentParser()
subparser = parser.add_subparsers(dest='cmd')
....
subparser.add_parser('quit')
...
args = parser.parse_args()
print args.cmd   # displays 'quit'
if args.cmd == 'quit':
   do_quit(args)

the other is to use set_defaults to link the subparser with a function:

parser = ArgumentParser()
subparsers = parser.add_subparsers()
...
parser_quit = subparsers.add_parser('quit')
parser_quit.set_defaults(func=do_quit)
...
args = parser.parse_args()
args.func(args)

---

On further thought, here's a way, using a custom Action. It is like _HelpAction (which is used by a -h). It's called by a positional argument with nargs=0 (or '?'). Such an argument is always called, even though there are no strings to match it (or rather, 0 strings match it). This a logical, but somewhat obscure, consequence of how positionals are handled.

class QuitAction(argparse.Action):
    def __call__(self, parser, *args, **kwargs):
        parser.exit(message="QUITTING\n")

p=argparse.ArgumentParser()
sp=p.add_subparsers(dest='cmd')
p1=sp.add_parser('quit')
p1.add_argument('foo', action=QuitAction, nargs='?', help=argparse.SUPPRESS)
p.parse_args(['quit'])

producing (when run in Ipython):

QUITTING
An exception has occurred, use %tb to see the full traceback.    
SystemExit: 0