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In Python 3.5.2, with a positional argument with nargs='*', running the program with no arguments gives an error like this:
usage: caffeinate [-h] [-V] COMMAND [ARGUMENT [ARGUMENT ...]]
caffeinate: error: the following arguments are required: COMMAND, ARGUMENT
Here it is clear from the (correct, though redundant) syntax that "ARGUMENT" is optional, but the error message claims, incorrectly, that it is required.
The add_argument calls used were:
parser.add_argument('COMMAND', help='command to run')
parser.add_argument('ARGUMENT', nargs='*', help='arguments to COMMAND')
parser.add_argument('-V', '--version', action='version', version=PROGRAM_NAME + ' ' + VERSION)