scikit-learn
diff --git a/‎doc/whats_new/v1.2.rst‎
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diff --git a/‎sklearn/base.py‎
Lines changed: 11 additions & 0 deletions b/‎sklearn/base.py‎
Lines changed: 11 additions & 0 deletions
diff --git a/‎sklearn/tests/test_base.py‎
Lines changed: 45 additions & 0 10BC0 deletions b/‎sklearn/tests/test_base.py‎
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@@ -12,6 +12,13 @@ Version 1.2.1
 Changelog
 ---------
 
+:mod:`sklearn.base`
+...................
+
+- |Fix| Fix a regression in `BaseEstimator.__getstate__` that would prevent
+  certain estimators to be pickled when using Python 3.11. :pr:`25188` by
+  :user:`Benjamin Bossan <BenjaminBossan>`.
+
 :mod:`sklearn.utils`
 ....................
 
 
@@ -271,9 +271,20 @@ def __repr__(self, N_CHAR_MAX=700):
         return repr_
 
     def __getstate__(self):
+        if getattr(self, "__slots__", None):
+            raise TypeError(
+                "You cannot use `__slots__` in objects inheriting from "
+                "`sklearn.base.BaseEstimator`."
+            )
+
         try:
             state = super().__getstate__()
+            if state is None:
+                # For Python 3.11+, empty instance (no `__slots__`,
+                # and `__dict__`) will return a state equal to `None`.
+                state = self.__dict__.copy()
         except AttributeError:
+            # Python < 3.11
             state = self.__dict__.copy()
 
         if type(self).__module__.startswith("sklearn."):
 
@@ -675,3 +675,48 @@ def test_clone_keeps_output_config():
     ss_clone = clone(ss)
     config_clone = _get_output_config("transform", ss_clone)
     assert config == config_clone
+
+
+class _Empty:
+    pass
+
+
+class EmptyEstimator(_Empty, BaseEstimator):
+    pass
+
+
+@pytest.mark.parametrize("estimator", [BaseEstimator(), EmptyEstimator()])
+def test_estimator_empty_instance_dict(estimator):
+    """Check that ``__getstate__`` returns an empty ``dict`` with an empty
+    instance.
+
+    Python 3.11+ changed behaviour by returning ``None`` instead of raising an
+    ``AttributeError``. Non-regression test for gh-25188.
+    """
+    state = estimator.__getstate__()
+    expected = {"_sklearn_version": sklearn.__version__}
+    assert state == expected
+
+    # this should not raise
+    pickle.loads(pickle.dumps(BaseEstimator()))
+
+
+def test_estimator_getstate_using_slots_error_message():
+    """Using a `BaseEstimator` with `__slots__` is not supported."""
+
+    class WithSlots:
+        __slots__ = ("x",)
+
+    class Estimator(BaseEstimator, WithSlots):
+        pass
+
+    msg = (
+        "You cannot use `__slots__` in objects inheriting from "
+        "`sklearn.base.BaseEstimator`"
+    )
+
+    with pytest.raises(TypeError, match=msg):
+        Estimator().__getstate__()
+
+    with pytest.raises(TypeError, match=msg):
+        pickle.dumps(Estimator())