code: def _get_candidates(self):
          
          candidates = np.where(self.dpp_vector == 0)
          return None if len(candidates[0]) == 0 else candidates[0]
docs: Finds the pipelines that are not yet tried.
  
          Returns:
              np.array: Indices corresponding to columns in ``dpp_matrix`` that haven't been tried on
                  ``X``. ``None`` if all pipelines have been tried on X.