Q:

argparse optional subparser (for --version)

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

A:

According to documentation, --version with action='version' (and not with action='store_true') prints automatically the version number:
parser.add_argument('--version', action='version', version='%(prog)s 2.0')

A:

FWIW, I ran into this also, and ended up "solving" it by not using subparsers (I already had my own system for printing help, so didn't lose anything there).
Instead, I do this:
parser.add_argument("command", nargs="?",
                    help="name of command to execute")

args, subcommand_args = parser.parse_known_args()

...and then the subcommand creates its own parser (similar to a subparser) which operates only on subcommand_args.

A:

This seems to implement the basic idea of an optional subparser.  We parse the standard arguments that apply to all subcommands.  Then, if anything is left, we invoke the parser on the rest.  The primary arguments are a parent of the subcommand so the -h appears correctly.  I plan to enter an interactive prompt if no subcommands are present.
import argparse

p1 = argparse.ArgumentParser( add_help = False )    
p1.add_argument( ‘–flag1′ )

p2 = argparse.ArgumentParser( parents = [ p1 ] )
s = p2.add_subparsers()
p = s.add_parser( ‘group’ )
p.set_defaults( group=True )

( init_ns, remaining ) = p1.parse_known_args( )

if remaining:
    p2.parse_args( args = remaining, namespace=init_ns )
else:
    print( ‘Enter interactive loop’ )

print( init_ns )

