#!/usr/bin/env python
from __future__ import print_function
from argparse import ArgumentParser


def action(args):
    print(args)

if __name__ == '__main__':
    std = ArgumentParser(add_help=False)
    std.add_argument('standard')

    ap = ArgumentParser()
    sp = ap.add_subparsers()

    cmd = sp.add_parser('subcommand', parents=[std], description='Do subcommand')
    cmd.add_argument('arg')
    cmd.set_defaults(do=action)

    args = ap.parse_args()
    args.do(args)

me@computer$ python test.py
usage: test.py [-h] {subcommand} ...
test.py: error: too few arguments

me@computer$ python3 test.py
Traceback (most recent call last):
  File "test.py", line 21, in <module>
    args.do(args)
AttributeError: 'Namespace' object has no attribute 'do'

ap = ArgumentParser()
sp = ap.add_subparsers(dest='parser')  # dest needed for error message
sp.required=True   # force 'required' testing

AttributeError: 'Namespace' object has no attribute 'do'