parser.add_argument('-l', '--logfile',
                    nargs='?',
                    const='tranfer.log',
                    default=False,
                    help='save log of the transfer')

parser.add_argument('left_path',
                    action='store',
                    help='first path')
parser.add_argument('right_path',
                    action='store',
                    help='second path')