code: def sweep(self, mode, speed=None):
          
          sweep_modes = ['UP', 'DOWN', 'PAUSE', 'ZERO', 'LIMIT']
          sweep_speed = ['SLOW', 'FAST', None]
          if not mode in sweep_modes:
              raise ValueError('Invalid sweep mode.')
          if not speed in sweep_speed:
              raise ValueError('Invalid sweep speed.')
          if speed is None:
              self._write('SWEEP {0}'.format(mode))
          else:
              self._write('SWEEP {0} {1}'.format(mode, speed))
docs: Starts the output current sweep.
  
          :param mode: The sweep mode. Valid entries are `'UP'`, `'DOWN'`,
              `'PAUSE'`or `'ZERO'`. If in shim mode, `'LIMIT'` is valid as well.
          :param speed: The sweeping speed. Valid entries are `'FAST'`, `'SLOW'`
              or `None`.