argparse optional subparser (for --version)

Question

I have the following code (using Python 2.7):

# shared command line options, like --version or --verbose
parser_shared = argparse.ArgumentParser(add_help=False)
parser_shared.add_argument('--version', action='store_true')

# the main parser, inherits from `parser_shared`
parser = argparse.ArgumentParser(description='main', parents=[parser_shared])

# several subcommands, which can't inherit from the main parser, since
# it would expect subcommands ad infinitum
subparsers = parser.add_subparsers('db', parents=[parser_shared])

...

args = parser.parse_args()

Now I would like to be able to call this program e.g. with the --version appended to the normal program or some subcommand:

$ prog --version
0.1

$ prog db --version
0.1

Basically, I need to declare optional subparsers. I'm aware that this isn't really supported, but are there any workarounds or alternatives?

Edit: The error message I am getting:

$ prog db --version
# works fine

$ prog --version
usage: ....
prog: error: too few arguments

Answer 1

Although @eumiro's answer address the --version option, it can only do so because that is a special case for optparse. To allow general invocations of:

 prog
 prog --verbose
 prog --verbose main
 prog --verbose db 

and have prog --version work the same as prog --verbose main (and prog main --verbose) you can add a method to Argumentparser and call that with the name of the default subparser, just before invoking parse_args():

import argparse
import sys

def set_default_subparser(self, name, args=None):
    """default subparser selection. Call after setup, just before parse_args()
    name: is the name of the subparser to call by default
    args: if set is the argument list handed to parse_args()

    , tested with 2.7, 3.2, 3.3, 3.4
    it works with 2.6 assuming argparse is installed
    """
    subparser_found = False
    for arg in sys.argv[1:]:
        if arg in ['-h', '--help']:  # global help if no subparser
            break
    else:
        for x in self._subparsers._actions:
            if not isinstance(x, argparse._SubParsersAction):
                continue
            for sp_name in x._name_parser_map.keys():
                if sp_name in sys.argv[1:]:
                    subparser_found = True
        if not subparser_found:
            # insert default in first position, this implies no
            # global options without a sub_parsers specified
            if args is None:
                sys.argv.insert(1, name)
            else:
                args.insert(0, name)

argparse.ArgumentParser.set_default_subparser = set_default_subparser

def do_main(args):
    print 'main verbose', args.verbose

def do_db(args):
    print 'db verbose:', args.verbose

parser = argparse.ArgumentParser()
parser.add_argument('--verbose', action='store_true')
parser.add_argument('--version', action='version', version='%(prog)s 2.0')
subparsers = parser.add_subparsers()
sp = subparsers.add_parser('main')
sp.set_defaults(func=do_main)
sp.add_argument('--verbose', action='store_true')
sp = subparsers.add_parser('db')
sp.set_defaults(func=do_db)

parser.set_default_subparser('main')
args = parser.parse_args()

if hasattr(args, 'func'):
    args.func(args)

The set_default_subparser() method is part of the ruamel.std.argparse package.