Default sub-command, or handling no sub-command with argparse

How can I have a default sub-command, or handle the case where no sub-command is given using argparse?

import argparse

a = argparse.ArgumentParser()
b = a.add_subparsers()
b.add_parser('hi')
a.parse_args()

Here I'd like a command to be selected, or the arguments to be handled based only on the next highest level of parser (in this case the top-level parser).

joiner@X:~/src> python3 default_subcommand.py
usage: default_subcommand.py [-h] {hi} ...
default_subcommand.py: error: too few arguments

It seems I've stumbled on the solution eventually myself.

If the command is optional, then this makes the command an option. In my original parser configuration, I had a package command that could take a range of possible steps, or it would perform all steps if none was given. This makes the step a choice:

parser = argparse.ArgumentParser()

command_parser = subparsers.add_parser('command')
command_parser.add_argument('--step', choices=['prepare', 'configure', 'compile', 'stage', 'package'])

...other command parsers

parsed_args = parser.parse_args()

if parsed_args.step is None:
    do all the steps...