How to use top-level arguments with subparsers in argparse

Question

In Python's argparse, how do you implement top-level arguments while still using commands implemented as subparsers?

I'm trying to implement a --version argument to show the program's version number, but argparse is giving me error: too few arguments because I'm not specifying a sub-command for one of the subparsers.

My code:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument(
    '-v', '--version',
    help='Show version.',
    action='store_true',
    default=False
)
subparsers = parser.add_subparsers(
    dest="command",
)
list_parser = subparsers.add_parser('list')
parser.parse_args(['--version'])

the output:

usage: myscript.py [-h] [-v] {list} ...
myscript.py: error: too few arguments

Answer 1

If you only need version to work, you can do this:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument(
    '-v', '--version',
    action='version',
    version='%(prog)s 1.0',
)

Subparsers won't bother any more; the special version action is processed and exits the script before the parser looks for subcommands.

Answer 2

The subparsers is a kind of positional argument . So normally that's required (just as though you'd specified add_argument('foo') ).

skyline's suggestion works because action='version' is an action class that exits after displaying its information, just like the default -h .

There is bug/feature in the latest argparse that makes subparsers optional. Depending on how that is resolved, it may be possible in the future to give the add_subparsers command a required=False parameter. But the intended design is that subparsers will be required, unless a flagged argument (like '-h') short circuits the parsing.