code: def set_default_by_index(self, index):
          
          if index >= len(self._datasets):
              raise DataInvalidIndex('A dataset with index {} does not exist'.format(index))
  
          self._default_index = index
docs: Set the default dataset by its index.
  
          After changing the default dataset, all calls without explicitly specifying the
          dataset by index or alias will be redirected to this dataset.
  
          Args:
              index (int): The index of the dataset that should be made the default.
  
          Raises:
              DataInvalidIndex: If the index does not represent a valid dataset.