code: def get_by_index(self, index):
          
          if index >= len(self._datasets):
              raise DataInvalidIndex('A dataset with index {} does not exist'.format(index))
  
          return self._datasets[index]
docs: Return a dataset by its index.
  
          Args:
              index (int): The index of the dataset that should be returned.
  
          Raises:
              DataInvalidIndex: If the index does not represent a valid dataset.