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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]