code: def get_field(self, field, idx):
          
          ret = []
          scalar = False
  
          # TODO: ensure idx is unique in this Group
  
          if isinstance(idx, (int, float, str)):
              scalar = True
              idx = [idx]
  
          models = [self._idx_model[i] for i in idx]
  
          for i, m in zip(idx, models):
              ret.append(self.system.__dict__[m].get_field(field, idx=i))
  
          if scalar is True:
              return ret[0]
          else:
              return ret
docs: Return the field ``field`` of elements ``idx`` in the group
  
          :param field: field name
          :param idx: element idx
          :return: values of the requested field