with signature ``scorer(estimator, X, y)`` which should return only a single value. Similar to :func:`cross_validate` but only a single metric is permitted. If `None`, the estimator's default scorer (if available) is used. cv : int, cross-validation generator or an iterable, default=None Determines the cross-validation splitting strategy. Possible inputs for cv are: - `None`, to use the default 5-fold cross validation, - int, to specify the number of folds in a `(Stratified)KFold`, - :term:`CV splitter`, - An iterable that generates (train, test) splits as arrays of indices. For `int`/`None` inputs, if the estimator is a classifier and `y` is either binary or multiclass, :class:`StratifiedKFold` is used. In all other cases, :class:`KFold` is used. These splitters are instantiated with `shuffle=False` so the splits will be the same across calls. Refer :ref:`User Guide ` for the various cross-validation strategies that can be used here. .. versionchanged:: 0.22 `cv` default value if `None` changed from 3-fold to 5-fold. n_jobs : int, default=None Number of jobs to run in parallel. Training the estimator and computing the score are parallelized over the cross-validation splits. ``None`` means 1 unless in a :obj:`joblib.parallel_backend` context. ``-1`` means using all processors. See :term:`Glossary ` for more details. verbose : int, default=0 The verbosity level. params : dict, default=None Parameters to pass to the underlying estimator's ``fit``, the scorer, and the CV splitter. .. versionadded:: 1.4 pre_dispatch : int or str, default='2*n_jobs' Controls the number of jobs that get dispatched during parallel execution. Reducing this number can be useful to avoid an explosion of memory consumption when more jobs get dispatched than CPUs can process. This parameter can be: - ``None``, in which case all the jobs are immediately created and spawned. Use this for lightweight and fast-running jobs, to avoid delays due to on-demand spawning of the jobs - An int, giving the exact number of total jobs that are spawned - A str, giving an expression as a function of n_jobs, as in '2*n_jobs' error_score : 'raise' or numeric, default=np.nan Value to assign to the score if an error occurs in estimator fitting. If set to 'raise', the error is raised. If a numeric value is given, FitFailedWarning is raised. .. versionadded:: 0.20 Returns ------- scores : ndarray of float of shape=(len(list(cv)),) Array of scores of the estimator for each run of the cross validation. See Also -------- cross_validate : To run cross-validation on multiple metrics and also to return train scores, fit times and score times. cross_val_predict : Get predictions from each split of cross-validation for diagnostic purposes. sklearn.metrics.make_scorer : Make a scorer from a performance metric or loss function. Examples -------- >>> from sklearn import datasets, linear_model >>> from sklearn.model_selection import cross_val_score >>> diabetes = datasets.load_diabetes() >>> X = diabetes.data[:150] >>> y = diabetes.target[:150] >>> lasso = linear_model.Lasso() >>> print(cross_val_score(lasso, X, y, cv=3)) [0.3315057 0.08022103 0.03531816] ©