FIX: clone behavior for estimator types (scikit-learn#12585)

Fixes scikit-learn#12521

Author: phausamann

sklearn/base.py

@@ -48,7 +48,7 @@ def clone(estimator, safe=True):
     # XXX: not handling dictionaries
     if estimator_type in (list, tuple, set, frozenset):
         return estimator_type([clone(e, safe=safe) for e in estimator])
-    elif not hasattr(estimator, 'get_params'):
+    elif not hasattr(estimator, 'get_params') or isinstance(estimator, type):
         if not safe:
             return copy.deepcopy(estimator)
         else:

sklearn/tests/test_base.py

@@ -167,6 +167,15 @@ def test_clone_sparse_matrices():
         assert_array_equal(clf.empty.toarray(), clf_cloned.empty.toarray())
 
 
+def test_clone_estimator_types():
+    # Check that clone works for parameters that are types rather than
+    # instances
+    clf = MyEstimator(empty=MyEstimator)
+    clf2 = clone(clf)
+
+    assert clf.empty is clf2.empty
+
+
 def test_repr():
     # Smoke test the repr of the base estimator.
     my_estimator = MyEstimator()