Restricting the number of hyperparameters for an existing component

The following example demonstrates how to replace an existing component with a new component, implementing the same classifier, but with different hyperparameters .

from ConfigSpace.configuration_space import ConfigurationSpace
from ConfigSpace.hyperparameters import UniformIntegerHyperparameter, UniformFloatHyperparameter

from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split

import autosklearn.classification
import autosklearn.pipeline.components.classification
from autosklearn.pipeline.components.classification \
    import AutoSklearnClassificationAlgorithm
from autosklearn.pipeline.constants import DENSE, UNSIGNED_DATA, PREDICTIONS, SPARSE

Subclass auto-sklearn’s random forest classifier

# This classifier only has one of the hyperparameter's of auto-sklearn's
# default parametrization (``max_features``). Instead, it also
# tunes the number of estimators (``n_estimators``).

class CustomRandomForest(AutoSklearnClassificationAlgorithm):

    def __init__(self, n_estimators, max_features, random_state=None):
        self.n_estimators = n_estimators
        self.max_features = max_features
        self.random_state = random_state

    def fit(self, X, y):
        from sklearn.ensemble import RandomForestClassifier

        self.n_estimators = int(self.n_estimators)

        if self.max_features not in ("sqrt", "log2", "auto"):
            max_features = int(X.shape[1] ** float(self.max_features))
        else:
            max_features = self.max_features

        self.estimator = RandomForestClassifier(
            n_estimators=self.n_estimators,
            max_features=max_features,
            random_state=self.random_state,
        )
        self.estimator.fit(X, y)
        return self

    def predict(self, X):
        if self.estimator is None:
            raise NotImplementedError()
        return self.estimator.predict(X)

    def predict_proba(self, X):
        if self.estimator is None:
            raise NotImplementedError()
        return self.estimator.predict_proba(X)

    @staticmethod
    def get_properties(dataset_properties=None):
        return {
            'shortname': 'RF',
            'name': 'Random Forest Classifier',
            'handles_regression': False,
            'handles_classification': True,
            'handles_multiclass': True,
            'handles_multilabel': True,
            'handles_multioutput': False,
            'is_deterministic': True,
            'input': (DENSE, SPARSE, UNSIGNED_DATA),
            'output': (PREDICTIONS,)
        }

    @staticmethod
    def get_hyperparameter_search_space(dataset_properties=None):
        cs = ConfigurationSpace()

        # The maximum number of features used in the forest is calculated as m^max_features, where
        # m is the total number of features, and max_features is the hyperparameter specified below.
        # The default is 0.5, which yields sqrt(m) features as max_features in the estimator. This
        # corresponds with Geurts' heuristic.
        max_features = UniformFloatHyperparameter("max_features", 0., 1., default_value=0.5)
        n_estimators = UniformIntegerHyperparameter("n_estimators", 10, 1000, default_value=100)

        cs.add_hyperparameters([max_features, n_estimators])
        return cs


# Add custom random forest classifier component to auto-sklearn.
autosklearn.pipeline.components.classification.add_classifier(CustomRandomForest)
cs = CustomRandomForest.get_hyperparameter_search_space()
print(cs)

Out:

Configuration space object:
  Hyperparameters:
    max_features, Type: UniformFloat, Range: [0.0, 1.0], Default: 0.5
    n_estimators, Type: UniformInteger, Range: [10, 1000], Default: 100

Data Loading

X, y = load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y)

Fit Random forest classifier to the data

clf = autosklearn.classification.AutoSklearnClassifier(
    time_left_for_this_task=30,
    per_run_time_limit=10,
    # Here we exclude auto-sklearn's default random forest component
    exclude={
        'classifier': ['random_forest']
    },
    # Bellow two flags are provided to speed up calculations
    # Not recommended for a real implementation
    initial_configurations_via_metalearning=0,
    smac_scenario_args={'runcount_limit': 1},
)
clf.fit(X_train, y_train)

Out:

AutoSklearnClassifier(exclude={'classifier': ['random_forest']},
                      initial_configurations_via_metalearning=0,
                      per_run_time_limit=10,
                      smac_scenario_args={'runcount_limit': 1},
                      time_left_for_this_task=30)