mutex_group.add_argument('--show', action='store_const', dest='mutex',
    const='show')
mutex_group.add_argument('--insert', action='store_const', dest='mutex',
    const='insert')
mutex_group.add_argument('--delete', action='store_const', dest='mutex',
    const='delete')
parser.set_defaults(mutex='show')
args = parser.parse_args()
print(args)
Set the default to false if another mutually exclusive argument is true