[MRG+2] Clone estimator for each parameter value in validation_curve (s…

…cikit-learn#9119)

doc/whats_new.rst

@@ -389,6 +389,10 @@ Bug fixes
      classes, and some values proposed in the docstring could raise errors.
      :issue:`5359` by `Tom Dupre la Tour`_.
 
+   - Fixed a bug where :func:`model_selection.validation_curve`
+     reused the same estimator for each parameter value.
+     :issue:`7365` by `Aleksandr Sandrovskii <Sundrique>`.
+
 API changes summary
 -------------------
 

sklearn/learning_curve.py

@@ -348,7 +348,7 @@ def validation_curve(estimator, X, y, param_name, param_range, cv=None,
     parallel = Parallel(n_jobs=n_jobs, pre_dispatch=pre_dispatch,
                         verbose=verbose)
     out = parallel(delayed(_fit_and_score)(
-        estimator, X, y, scorer, train, test, verbose,
+        clone(estimator), X, y, scorer, train, test, verbose,
         parameters={param_name: v}, fit_params=None, return_train_score=True)
         for train, test in cv for v in param_range)
 

sklearn/model_selection/_validation.py

@@ -988,7 +988,7 @@ def validation_curve(estimator, X, y, param_name, param_range, groups=None,
     parallel = Parallel(n_jobs=n_jobs, pre_dispatch=pre_dispatch,
                         verbose=verbose)
     out = parallel(delayed(_fit_and_score)(
-        estimator, X, y, scorer, train, test, verbose,
+        clone(estimator), X, y, scorer, train, test, verbose,
         parameters={param_name: v}, fit_params=None, return_train_score=True)
         # NOTE do not change order of iteration to allow one time cv splitters
         for train, test in cv.split(X, y, groups) for v in param_range)

sklearn/model_selection/tests/test_validation.py

@@ -133,6 +133,21 @@ def _is_training_data(self, X):
         return X is self.X_subset
 
 
+class MockEstimatorWithSingleFitCallAllowed(MockEstimatorWithParameter):
+    """Dummy classifier that disallows repeated calls of fit method"""
+
+    def fit(self, X_subset, y_subset):
+        assert_false(
+            hasattr(self, 'fit_called_'),
+            'fit is called the second time'
+        )
+        self.fit_called_ = True
+        return super(type(self), self).fit(X_subset, y_subset)
+
+    def predict(self, X):
+        raise NotImplementedError
+
+
 class MockClassifier(object):
     """Dummy classifier to test the cross-validation"""
 
@@ -852,6 +867,18 @@ def test_validation_curve():
     assert_array_almost_equal(test_scores.mean(axis=1), 1 - param_range)
 
 
+def test_validation_curve_clone_estimator():
+    X, y = make_classification(n_samples=2, n_features=1, n_informative=1,
+                               n_redundant=0, n_classes=2,
+                               n_clusters_per_class=1, random_state=0)
+
+    param_range = np.linspace(1, 0, 10)
+    _, _ = validation_curve(
+        MockEstimatorWithSingleFitCallAllowed(), X, y,
+        param_name="param", param_range=param_range, cv=2
+    )
+
+
 def test_validation_curve_cv_splits_consistency():
     n_samples = 100
     n_splits = 5

sklearn/tests/test_learning_curve.py

@@ -12,6 +12,7 @@
 from sklearn.utils.testing import assert_equal
 from sklearn.utils.testing import assert_array_equal
 from sklearn.utils.testing import assert_array_almost_equal
+from sklearn.utils.testing import assert_false
 from sklearn.datasets import make_classification
 
 with warnings.catch_warnings():
@@ -93,6 +94,18 @@ def score(self, X=None, y=None):
         return None
 
 
+class MockEstimatorWithSingleFitCallAllowed(MockEstimatorWithParameter):
+    """Dummy classifier that disallows repeated calls of fit method"""
+
+    def fit(self, X_subset, y_subset):
+        assert_false(
+            hasattr(self, 'fit_called_'),
+            'fit is called the second time'
+        )
+        self.fit_called_ = True
+        return super(type(self), self).fit(X_subset, y_subset)
+
+
 def test_learning_curve():
     X, y = make_classification(n_samples=30, n_features=1, n_informative=1,
                                n_redundant=0, n_classes=2,
@@ -284,3 +297,15 @@ def test_validation_curve():
 
     assert_array_almost_equal(train_scores.mean(axis=1), param_range)
     assert_array_almost_equal(test_scores.mean(axis=1), 1 - param_range)
+
+
+def test_validation_curve_clone_estimator():
+    X, y = make_classification(n_samples=2, n_features=1, n_informative=1,
+                               n_redundant=0, n_classes=2,
+                               n_clusters_per_class=1, random_state=0)
+
+    param_range = np.linspace(1, 0, 10)
+    _, _ = validation_curve(
+        MockEstimatorWithSingleFitCallAllowed(), X, y,
+        param_name="param", param_range=param_range, cv=2
+    )