Bug report

state_group = parser.add_mutually_exclusive_group()
state_group.add_argument('--param1', nargs='?', const='default', metavar='NAME',
                         help=SUPPRESS)
state_group.add_argument('--param2', nargs='?', const='default', metavar='NAME',
                         help=SUPPRESS)

will cause an assert in
assert ' '.join(opt_parts) == opt_usage
of argparse.py

because the empty group [] is converted into two spaces
e.g.
[group1] [] [group 2]
gets converted to this by the opt_usage = format(optionals, groups) code above the assert:
[group1] [group 2]
and the two spaces don't match the single space of the assert

Your environment

• CPython versions tested on: 3.10.4
• Operating system and architecture: OSX, arm64