def _get_candidates(self):
        """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.
        """
        candidates = np.where(self.dpp_vector == 0)
        return None if len(candidates[0]) == 0 else candidates[0]
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.