scikit-leranのKerasClassifierでValueError: The estimator KerasClassifier should be a classifier.

scikit-learnのStackingClassifierでKerasClassifierを使った時に表題のエラー。

ValueError: The estimator KerasClassifier should be a classifier.

sklearnのモデルには、classifierやregressorなどを指定する属性として_estimator_typeを持っている。

This distinction between classifiers and regressors is implemented using the _estimator_type attribute, which takes a string value. It should be "classifier" for classifiers and "regressor" for regressors and "clusterer" for clustering methods, to work as expected.
https://scikit-learn.org/stable/developers/develop.html

そのため、KerasClassifierに対して明示的にclassifierであることを示せばOK。

nn = KerasClassifier()
nn._estimator_type = "classifier"

参考

Developing scikit-learn estimators

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