Bugfix: BaseEstimator __getstate__ in Python 3.11

Since Python 3.11, objects have a __getstate__ method by default:

python/cpython#70766

Therefore, the exception in BaseEstimator.__getstate__ will no longer be
raised, thus not falling back on using the object's __dict__:

https://github.com/scikit-learn/scikit-learn/blob/dc580a8ef5ee2a8aea80498388690e2213118efd/sklearn/base.py#L274-L280

If the instance dict of the object is empty, the return value will,
however, be None. Therefore, the line below calling state.items()
results in an error.

In this bugfix, it is checked if the state is None and if it is, the
object's __dict__ is used (which should always be empty).

Not addressed in this PR is how to deal with slots (see also discussion
in scikit-learn#10079). When there are __slots__, __getstate__ will actually return
a tuple, as documented here:

https://docs.python.org/3/library/pickle.html#object.__getstate__

The user would thus still get an indiscriptive error message.

sklearn/base.py

@@ -273,7 +273,11 @@ def __repr__(self, N_CHAR_MAX=700):
     def __getstate__(self):
         try:
             state = super().__getstate__()
+            if state is None:
+                state = self.__dict__.copy()
         except AttributeError:
+            # TODO: Remove once Python < 3.11 is dropped, as there will never be
+            # an AttributeError
             state = self.__dict__.copy()
 
         if type(self).__module__.startswith("sklearn."):

sklearn/tests/test_base.py

@@ -675,3 +675,27 @@ def test_clone_keeps_output_config():
     ss_clone = clone(ss)
     config_clone = _get_output_config("transform", ss_clone)
     assert config == config_clone
+
+
+def test_parent_object_empty_instance_dict():
+    # Since Python 3.11, Python objects have a __getstate__ method by default
+    # that returns None if the instance dict is empty
+    class Empty:
+        pass
+
+    class Estimator(Empty, BaseEstimator):
+        pass
+
+    state = Estimator().__getstate__()
+    expected = {"_sklearn_version": sklearn.__version__}
+    assert state == expected
+
+
+def test_base_estimator_empty_instance_dict():
+    # Since Python 3.11, Python objects have a __getstate__ method by default
+    # that returns None if the instance dict is empty
+
+    # this should not raise
+    state = BaseEstimator().__getstate__()
+    expected = {"_sklearn_version": sklearn.__version__}
+    assert state == expected