The following code:

group = parser.add_mutually_exclusive_group()
group.add_argument('--install-only', action='store_true',
                    help='just install the program, do not run it')
group.add_argument('args', metavar='ARGUMENT', nargs='*', default=None,
                    help='arguments to PROGRAM')

gives the following error:

    group.add_argument('args', metavar='ARGUMENT', nargs='*',
  File "/usr/lib/python3.8/argparse.py", line 1398, in add_argument
    return self._add_action(action)
  File "/usr/lib/python3.8/argparse.py", line 1621, in _add_action
    raise ValueError(msg)
ValueError: mutually exclusive arguments must be optional

But the 'args' argument *is* optional, as there can be 0 of them.

Give the positional a non-none default:

e.g.

    group.add_argument('args', metavar='ARGUMENT', nargs='*', default=[],
                     help='arguments to PROGRAM')

Since a '*' or '?' positional works with an empty list of arguments, it is "always seen".  It requires some special handling to allow it to work in the mutually exclusive context.  It's a tricky piece of code that might not be well documented (if at all).

Thanks for the hint; could this be documented, please?