testsubcommand.py

import sys
import argparse
import subprocess

parser = argparse.ArgumentParser()

parser.add_argument('command', choices=['userpath'])
parser.add_argument('--verbose')
parser.add_argument('--flag', action='store_true', default=False)

args, subcommand_args = parser.parse_known_args()

if args.command == 'userpath':

	if len(subcommand_args) == 0:
		print('pipx vendors in the userpath command. run all commands with'
			  'pipx userpath ...')
		print('see the userpath help below:\n')
		subprocess.run([sys.executable, '-m', 'userpath', '--help'])
		exit(1)

	subprocess.run([sys.executable, '-m', 'userpath', *subcommand_args], check=True)