RuntimeError: "Cannot clone object ..." when using clone with an implementation of BaseEstimator that copies objects in get_params method

Description

RuntimeError thrown when using sklearn.base.clone with estimators whose implementation of get_params returns copies instead of references.

Sanity check at the end of the clone function fails when the implementation of the estimator used, copies parameters during its initialisation or in the get_params method.

Either the documentation of __init__ and get_params in BaseEstimator should indicate that parameters should never be copied or the sanity check in clone should be more flexible.

In the case that implementations of estimators should not copy parameters, an issue should be created in the Keras project regarding the 'get_params`method of the classes used for integration with scikit-learn.

Steps/Code to Reproduce

from keras.wrappers.scikit_learn import KerasClassifier
from keras.models import Sequential
from keras.layers import Dense
 
from sklearn.base import clone
 
def create_keras_classifier_model(n_classes):
    """Keras multinomial logistic regression creation model
 
    Args:
        n_classes(int): Number of classes to be classified
 
    Returns:
        Compiled keras model
 
    """
    # create model
    model = Sequential()
    model.add(Dense(n_classes, activation="softmax"))
    # Compile model
    model.compile(
        loss="categorical_crossentropy", optimizer="adam", metrics=["accuracy"]
    )
    return model
 
estimator = KerasClassifier(build_fn=create_keras_classifier_model, n_classes=2, class_weight={0: 1, 1:3})
 
clone(estimator)

Expected Results

No error is thrown.

Actual Results

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-1-520f4ee6e745> in <module>
     26 estimator = KerasClassifier(build_fn=create_keras_classifier_model, n_classes=2, class_weight={0: 1, 1:3})
    27
---> 28 clone(estimator)
 
/usr/local/anaconda/envs/ivan/lib/python3.6/site-packages/sklearn/base.py in clone(estimator, safe)
     73             raise RuntimeError('Cannot clone object %s, as the constructor '
     74                                'either does not set or modifies parameter %s' %
---> 75                                (estimator, name))
     76     return new_object
     77
 
RuntimeError: Cannot clone object <keras.wrappers.scikit_learn.KerasClassifier object at 0x7f7504148f28>, as the constructor either does not set or modifies parameter class_weight

Versions

System: python: 3.6.9 |Anaconda, Inc.| (default, Jul 30 2019, 19:07:31) [GCC 7.3.0] executable: /usr/local/anaconda/envs/ivan/bin/python machine: Linux-3.10.0-957.21.3.el7.x86_64-x86_64-with-redhat-7.6-Maipo

BLAS: macros: SCIPY_MKL_H=None, HAVE_CBLAS=None lib_dirs: /usr/local/anaconda/envs/ivan/lib cblas_libs: blas, cblas, lapack, pthread, blas, cblas, lapack, blas, cblas, lapack

Python deps: pip: 19.2.3 setuptools: 41.4.0 sklearn: 0.20.3 numpy: 1.17.3 scipy: 1.3.1 Cython: 0.29.13 pandas: 0.24.2

[ashishsangwan18]

This is sklearn bug. You should reduce the version of sklearn:

conda install scikit-learn==0.21.2

[wchill]

@cmarmo I’ve confirmed it’s still an issue in 1.0.2, opened #22857 with an example.