How to use the nifgen.Waveform.__members__.keys function in nifgen

def _main(argsv):
    supported_waveforms = list(nifgen.Waveform.__members__.keys())[:-1]  # no support for user-defined waveforms in example
    parser = argparse.ArgumentParser(description='Generates the standard function.', formatter_class=argparse.ArgumentDefaultsHelpFormatter)
    parser.add_argument('-n', '--resource-name', default='PXI1Slot2', help='Resource name of a National Instruments Function Generator')
    parser.add_argument('-w', '--waveform', default=supported_waveforms[0], choices=supported_waveforms, type=str.upper, help='Standard waveform')
    parser.add_argument('-f', '--frequency', default=1000, type=float, help='Frequency (Hz)')
    parser.add_argument('-a', '--amplitude', default=1.0, type=float, help='Amplitude (Vpk-pk)')
    parser.add_argument('-o', '--offset', default=0.0, type=float, help='DC offset (V)')
    parser.add_argument('-p', '--phase', default=0.0, type=float, help='Start phase (deg)')
    parser.add_argument('-t', '--time', default=5.0, type=float, help='Generation time (s)')
    parser.add_argument('-op', '--option-string', default='', type=str, help='Option string')
    args = parser.parse_args(argsv)
    example(args.resource_name, args.option_string, args.waveform, args.frequency, args.amplitude, args.offset, args.phase, args.time)