sklearn_grid_search_partial.py

out = parallel(delayed(_fit_and_score)(clone(base_estimator),
                                       X, y,
                                       train=train, test=test,
                                       parameters=parameters,
                                       **fit_and_score_kwargs)
               for parameters, (train, test)
               in product(candidate_params,
                          cv.split(X, y, groups)))