scikit-learn
diff --git a/‎sklearn/model_selection/_search.py
Lines changed: 4 additions & 2 deletions b/‎sklearn/model_selection/_search.py
Lines changed: 4 additions & 2 deletions
diff --git a/‎sklearn/model_selection/_validation.py
Lines changed: 8 additions & 1 deletion b/‎sklearn/model_selection/_validation.py
Lines changed: 8 additions & 1 deletion
diff --git a/‎sklearn/model_selection/tests/test_search.py
Lines changed: 19 additions & 1 deletion b/‎sklearn/model_selection/tests/test_search.py
Lines changed: 19 additions & 1 deletion
@@ -727,8 +727,10 @@ def evaluate_candidates(candidate_params):
             self.best_params_ = results["params"][self.best_index_]
 
         if self.refit:
-            self.best_estimator_ = clone(base_estimator).set_params(
-                **self.best_params_)
+            # we clone again after setting params in case some
+            # of the params are estimators as well.
+            self.best_estimator_ = clone(clone(base_estimator).set_params(
+                **self.best_params_))
             refit_start_time = time.time()
             if y is not None:
                 self.best_estimator_.fit(X, y, **fit_params)
 
@@ -494,7 +494,14 @@ def _fit_and_score(estimator, X, y, scorer, train, test, verbose,
 
     train_scores = {}
     if parameters is not None:
-        estimator.set_params(**parameters)
+        # clone after setting parameters in case any parameters
+        # are estimators (like pipeline steps)
+        # because pipeline doesn't clone steps in fit
+        cloned_parameters = {}
+        for k, v in parameters.items():
+            cloned_parameters[k] = clone(v, safe=False)
+
+        estimator = estimator.set_params(**cloned_parameters)
 
     start_time = time.time()
 
 
@@ -63,7 +63,7 @@
 from sklearn.metrics import roc_auc_score
 from sklearn.impute import SimpleImputer
 from sklearn.pipeline import Pipeline
-from sklearn.linear_model import Ridge, SGDClassifier
+from sklearn.linear_model import Ridge, SGDClassifier, LinearRegression
 
 from sklearn.model_selection.tests.common import OneTimeSplitter
 
@@ -198,6 +198,24 @@ def test_grid_search():
     assert_raises(ValueError, grid_search.fit, X, y)
 
 
+def test_grid_search_pipeline_steps():
+    # check that parameters that are estimators are cloned before fitting
+    pipe = Pipeline([('regressor', LinearRegression())])
+    param_grid = {'regressor': [LinearRegression(), Ridge()]}
+    grid_search = GridSearchCV(pipe, param_grid, cv=2)
+    grid_search.fit(X, y)
+    regressor_results = grid_search.cv_results_['param_regressor']
+    assert isinstance(regressor_results[0], LinearRegression)
+    assert isinstance(regressor_results[1], Ridge)
+    assert not hasattr(regressor_results[0], 'coef_')
+    assert not hasattr(regressor_results[1], 'coef_')
+    assert regressor_results[0] is not grid_search.best_estimator_
+    assert regressor_results[1] is not grid_search.best_estimator_
+    # check that we didn't modify the parameter grid that was passed
+    assert not hasattr(param_grid['regressor'][0], 'coef_')
+    assert not hasattr(param_grid['regressor'][1], 'coef_')
+
+
 def check_hyperparameter_searcher_with_fit_params(klass, **klass_kwargs):
     X = np.arange(100).reshape(10, 10)
     y = np.array([0] * 5 + [1] * 5)