If I understand correctly, this is meant to mirror add_mutually_exclusive_group(). One thing I noticed: the exclusive group rejects a required=True member at setup time (ValueError: mutually exclusive arguments must be optional), but the inclusive group accepts it. I'm wondering how add_mutually_inclusive_group() should behave in that case?

For example, with a non-required group where one member is required=True:

>>> g = p.add_mutually_inclusive_group()
>>> g.add_argument("--foo")
_StoreAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None, deprecated=False)
>>> g.add_argument("--bar", required=True)
_StoreAction(option_strings=['--bar'], dest='bar', nargs=None, const=None, default=None, type=None, choices=None, required=True, help=None, metavar=None, deprecated=False)
>>> p.parse_args([])
usage: [-h] [--foo FOO & --bar BAR]
: error: the following arguments are required: --bar

Since the group is all-or-nothing, I'd have expected nothing (empty args) to be accepted, but --bar being required makes that impossible. Should this combination be guarded against (like the exclusive group does) or is it intended? Apologies if I'm missing something.