How can I define global options with sub-parsers in python argparse?

I'm trying to figure out how to add global option in a sub-parser scenario with pythons arparse library.

Right now my code looks like this:

def parseArgs(self):
    parent_parser = argparse.ArgumentParser(add_help=False)
    parent_parser.add_argument('--debug', default=False, required=False,
        action='store_true', dest="debug", help='debug flag')

    main_parser = argparse.ArgumentParser()
    main_parser.add_argument('--debug', default=False, required=False,
        action='store_true', dest="debug", help='debug flag')

    service_subparsers = main_parser.add_subparsers(title="category",
        dest="category")
    agent_parser = service_subparsers.add_parser("agent",
        help="agent commands", parents=[parent_parser])
    return main_parser.parse_args()

This works for the command line ./test --help and the --debug option is listed as global:

usage: test [-h] [--debug] {agent} ...

optional arguments:
  -h, --help  show this help message and exit
  --debug     debug flag

category:
  {agent}
    agent     agent commands

However when I trigger the agent sub-parser with the command line ./test agent --help the --debug option is now no longer listed as a global option but as an option for the sub-parser. Also it must now specified as ./test agent --debug and ./test --debug agent no longer works:

usage: test agent [-h] [--debug]

optional arguments:
  -h, --help  show this help message and exit
  --debug     debug flag

What I'd like to be able to do is define --debug is global so that it can always be specified for all sub-parsers and appropriately listed as such in the help output.