sklearn.model_selection
.RandomizedSearchCV¶

class
sklearn.model_selection.
RandomizedSearchCV
(estimator, param_distributions, *, n_iter=10, scoring=None, n_jobs=None, refit=True, cv=None, verbose=0, pre_dispatch='2*n_jobs', random_state=None, error_score=nan, return_train_score=False)[source]¶ Randomized search on hyper parameters.
RandomizedSearchCV implements a “fit” and a “score” method. It also implements “score_samples”, “predict”, “predict_proba”, “decision_function”, “transform” and “inverse_transform” if they are implemented in the estimator used.
The parameters of the estimator used to apply these methods are optimized by crossvalidated search over parameter settings.
In contrast to GridSearchCV, not all parameter values are tried out, but rather a fixed number of parameter settings is sampled from the specified distributions. The number of parameter settings that are tried is given by n_iter.
If all parameters are presented as a list, sampling without replacement is performed. If at least one parameter is given as a distribution, sampling with replacement is used. It is highly recommended to use continuous distributions for continuous parameters.
Read more in the User Guide.
New in version 0.14.
 Parameters
 estimatorestimator object.
A object of that type is instantiated for each grid point. This is assumed to implement the scikitlearn estimator interface. Either estimator needs to provide a
score
function, orscoring
must be passed. param_distributionsdict or list of dicts
Dictionary with parameters names (
str
) as keys and distributions or lists of parameters to try. Distributions must provide arvs
method for sampling (such as those from scipy.stats.distributions). If a list is given, it is sampled uniformly. If a list of dicts is given, first a dict is sampled uniformly, and then a parameter is sampled using that dict as above. n_iterint, default=10
Number of parameter settings that are sampled. n_iter trades off runtime vs quality of the solution.
 scoringstr, callable, list, tuple or dict, default=None
Strategy to evaluate the performance of the crossvalidated model on the test set.
If
scoring
represents a single score, one can use:a single string (see The scoring parameter: defining model evaluation rules);
a callable (see Defining your scoring strategy from metric functions) that returns a single value.
If
scoring
represents multiple scores, one can use:a list or tuple of unique strings;
a callable returning a dictionary where the keys are the metric names and the values are the metric scores;
a dictionary with metric names as keys and callables a values.
See Specifying multiple metrics for evaluation for an example.
If None, the estimator’s score method is used.
 n_jobsint, default=None
Number of jobs to run in parallel.
None
means 1 unless in ajoblib.parallel_backend
context.1
means using all processors. See Glossary for more details.Changed in version v0.20:
n_jobs
default changed from 1 to None refitbool, str, or callable, default=True
Refit an estimator using the best found parameters on the whole dataset.
For multiple metric evaluation, this needs to be a
str
denoting the scorer that would be used to find the best parameters for refitting the estimator at the end.Where there are considerations other than maximum score in choosing a best estimator,
refit
can be set to a function which returns the selectedbest_index_
given thecv_results
. In that case, thebest_estimator_
andbest_params_
will be set according to the returnedbest_index_
while thebest_score_
attribute will not be available.The refitted estimator is made available at the
best_estimator_
attribute and permits usingpredict
directly on thisRandomizedSearchCV
instance.Also for multiple metric evaluation, the attributes
best_index_
,best_score_
andbest_params_
will only be available ifrefit
is set and all of them will be determined w.r.t this specific scorer.See
scoring
parameter to know more about multiple metric evaluation.Changed in version 0.20: Support for callable added.
 cvint, crossvalidation generator or an iterable, default=None
Determines the crossvalidation splitting strategy. Possible inputs for cv are:
None, to use the default 5fold cross validation,
integer, to specify the number of folds in a
(Stratified)KFold
,An iterable yielding (train, test) splits as arrays of indices.
For integer/None inputs, if the estimator is a classifier and
y
is either binary or multiclass,StratifiedKFold
is used. In all other cases,KFold
is used. These splitters are instantiated withshuffle=False
so the splits will be the same across calls.Refer User Guide for the various crossvalidation strategies that can be used here.
Changed in version 0.22:
cv
default value if None changed from 3fold to 5fold. verboseint
Controls the verbosity: the higher, the more messages.
 pre_dispatchint, or str, default=None
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 fastrunning jobs, to avoid delays due to ondemand 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’
 random_stateint, RandomState instance or None, default=None
Pseudo random number generator state used for random uniform sampling from lists of possible values instead of scipy.stats distributions. Pass an int for reproducible output across multiple function calls. See Glossary.
 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. This parameter does not affect the refit step, which will always raise the error.
 return_train_scorebool, default=False
If
False
, thecv_results_
attribute will not include training scores. Computing training scores is used to get insights on how different parameter settings impact the overfitting/underfitting tradeoff. However computing the scores on the training set can be computationally expensive and is not strictly required to select the parameters that yield the best generalization performance.New in version 0.19.
Changed in version 0.21: Default value was changed from
True
toFalse
 Attributes
 cv_results_dict of numpy (masked) ndarrays
A dict with keys as column headers and values as columns, that can be imported into a pandas
DataFrame
.For instance the below given table
param_kernel
param_gamma
split0_test_score
…
rank_test_score
‘rbf’
0.1
0.80
…
1
‘rbf’
0.2
0.84
…
3
‘rbf’
0.3
0.70
…
2
will be represented by a
cv_results_
dict of:{ 'param_kernel' : masked_array(data = ['rbf', 'rbf', 'rbf'], mask = False), 'param_gamma' : masked_array(data = [0.1 0.2 0.3], mask = False), 'split0_test_score' : [0.80, 0.84, 0.70], 'split1_test_score' : [0.82, 0.50, 0.70], 'mean_test_score' : [0.81, 0.67, 0.70], 'std_test_score' : [0.01, 0.24, 0.00], 'rank_test_score' : [1, 3, 2], 'split0_train_score' : [0.80, 0.92, 0.70], 'split1_train_score' : [0.82, 0.55, 0.70], 'mean_train_score' : [0.81, 0.74, 0.70], 'std_train_score' : [0.01, 0.19, 0.00], 'mean_fit_time' : [0.73, 0.63, 0.43], 'std_fit_time' : [0.01, 0.02, 0.01], 'mean_score_time' : [0.01, 0.06, 0.04], 'std_score_time' : [0.00, 0.00, 0.00], 'params' : [{'kernel' : 'rbf', 'gamma' : 0.1}, ...], }
NOTE
The key
'params'
is used to store a list of parameter settings dicts for all the parameter candidates.The
mean_fit_time
,std_fit_time
,mean_score_time
andstd_score_time
are all in seconds.For multimetric evaluation, the scores for all the scorers are available in the
cv_results_
dict at the keys ending with that scorer’s name ('_<scorer_name>'
) instead of'_score'
shown above. (‘split0_test_precision’, ‘mean_train_precision’ etc.) best_estimator_estimator
Estimator that was chosen by the search, i.e. estimator which gave highest score (or smallest loss if specified) on the left out data. Not available if
refit=False
.For multimetric evaluation, this attribute is present only if
refit
is specified.See
refit
parameter for more information on allowed values. best_score_float
Mean crossvalidated score of the best_estimator.
For multimetric evaluation, this is not available if
refit
isFalse
. Seerefit
parameter for more information.This attribute is not available if
refit
is a function. best_params_dict
Parameter setting that gave the best results on the hold out data.
For multimetric evaluation, this is not available if
refit
isFalse
. Seerefit
parameter for more information. best_index_int
The index (of the
cv_results_
arrays) which corresponds to the best candidate parameter setting.The dict at
search.cv_results_['params'][search.best_index_]
gives the parameter setting for the best model, that gives the highest mean score (search.best_score_
).For multimetric evaluation, this is not available if
refit
isFalse
. Seerefit
parameter for more information. scorer_function or a dict
Scorer function used on the held out data to choose the best parameters for the model.
For multimetric evaluation, this attribute holds the validated
scoring
dict which maps the scorer key to the scorer callable. n_splits_int
The number of crossvalidation splits (folds/iterations).
 refit_time_float
Seconds used for refitting the best model on the whole dataset.
This is present only if
refit
is not False.New in version 0.20.
 multimetric_bool
Whether or not the scorers compute several metrics.
See also
GridSearchCV
Does exhaustive search over a grid of parameters.
ParameterSampler
A generator over parameter settings, constructed from param_distributions.
Notes
The parameters selected are those that maximize the score of the heldout data, according to the scoring parameter.
If
n_jobs
was set to a value higher than one, the data is copied for each parameter setting(and notn_jobs
times). This is done for efficiency reasons if individual jobs take very little time, but may raise errors if the dataset is large and not enough memory is available. A workaround in this case is to setpre_dispatch
. Then, the memory is copied onlypre_dispatch
many times. A reasonable value forpre_dispatch
is2 * n_jobs
.Examples
>>> from sklearn.datasets import load_iris >>> from sklearn.linear_model import LogisticRegression >>> from sklearn.model_selection import RandomizedSearchCV >>> from scipy.stats import uniform >>> iris = load_iris() >>> logistic = LogisticRegression(solver='saga', tol=1e2, max_iter=200, ... random_state=0) >>> distributions = dict(C=uniform(loc=0, scale=4), ... penalty=['l2', 'l1']) >>> clf = RandomizedSearchCV(logistic, distributions, random_state=0) >>> search = clf.fit(iris.data, iris.target) >>> search.best_params_ {'C': 2..., 'penalty': 'l1'}
Methods
Call decision_function on the estimator with the best found parameters.
fit
(X[, y, groups])Run fit with all sets of parameters.
get_params
([deep])Get parameters for this estimator.
Call inverse_transform on the estimator with the best found params.
predict
(X)Call predict on the estimator with the best found parameters.
Call predict_log_proba on the estimator with the best found parameters.
Call predict_proba on the estimator with the best found parameters.
score
(X[, y])Returns the score on the given data, if the estimator has been refit.
Call score_samples on the estimator with the best found parameters.
set_params
(**params)Set the parameters of this estimator.
transform
(X)Call transform on the estimator with the best found parameters.

decision_function
(X)[source]¶ Call decision_function on the estimator with the best found parameters.
Only available if
refit=True
and the underlying estimator supportsdecision_function
. Parameters
 Xindexable, length n_samples
Must fulfill the input assumptions of the underlying estimator.

fit
(X, y=None, *, groups=None, **fit_params)[source]¶ Run fit with all sets of parameters.
 Parameters
 Xarraylike of shape (n_samples, n_features)
Training vector, where n_samples is the number of samples and n_features is the number of features.
 yarraylike of shape (n_samples, n_output) or (n_samples,), default=None
Target relative to X for classification or regression; None for unsupervised learning.
 groupsarraylike of shape (n_samples,), default=None
Group labels for the samples used while splitting the dataset into train/test set. Only used in conjunction with a “Group” cv instance (e.g.,
GroupKFold
). **fit_paramsdict of str > object
Parameters passed to the
fit
method of the estimator

get_params
(deep=True)[source]¶ Get parameters for this estimator.
 Parameters
 deepbool, default=True
If True, will return the parameters for this estimator and contained subobjects that are estimators.
 Returns
 paramsdict
Parameter names mapped to their values.

inverse_transform
(Xt)[source]¶ Call inverse_transform on the estimator with the best found params.
Only available if the underlying estimator implements
inverse_transform
andrefit=True
. Parameters
 Xtindexable, length n_samples
Must fulfill the input assumptions of the underlying estimator.

predict
(X)[source]¶ Call predict on the estimator with the best found parameters.
Only available if
refit=True
and the underlying estimator supportspredict
. Parameters
 Xindexable, length n_samples
Must fulfill the input assumptions of the underlying estimator.

predict_log_proba
(X)[source]¶ Call predict_log_proba on the estimator with the best found parameters.
Only available if
refit=True
and the underlying estimator supportspredict_log_proba
. Parameters
 Xindexable, length n_samples
Must fulfill the input assumptions of the underlying estimator.

predict_proba
(X)[source]¶ Call predict_proba on the estimator with the best found parameters.
Only available if
refit=True
and the underlying estimator supportspredict_proba
. Parameters
 Xindexable, length n_samples
Must fulfill the input assumptions of the underlying estimator.

score
(X, y=None)[source]¶ Returns the score on the given data, if the estimator has been refit.
This uses the score defined by
scoring
where provided, and thebest_estimator_.score
method otherwise. Parameters
 Xarraylike of shape (n_samples, n_features)
Input data, where n_samples is the number of samples and n_features is the number of features.
 yarraylike of shape (n_samples, n_output) or (n_samples,), default=None
Target relative to X for classification or regression; None for unsupervised learning.
 Returns
 scorefloat

score_samples
(X)[source]¶ Call score_samples on the estimator with the best found parameters.
Only available if
refit=True
and the underlying estimator supportsscore_samples
.New in version 0.24.
 Parameters
 Xiterable
Data to predict on. Must fulfill input requirements of the underlying estimator.
 Returns
 y_scorendarray of shape (n_samples,)

set_params
(**params)[source]¶ Set the parameters of this estimator.
The method works on simple estimators as well as on nested objects (such as
Pipeline
). The latter have parameters of the form<component>__<parameter>
so that it’s possible to update each component of a nested object. Parameters
 **paramsdict
Estimator parameters.
 Returns
 selfestimator instance
Estimator instance.