[BUG] ensure BaseMetaObject resets at set_params; add 'test_set_params_resets_fitted_state' test to trigger error

skbase/base/_meta.py

@@ -233,10 +233,21 @@ def _set_params(self, attr: str, **params):
         Self
             Instance of self.
         """
+        if not params:
+            return self
+
+        # Track whether we handle any params locally.
+        # This is needed because we pop params before calling super(),
+        # and if we pop ALL params, super() will return early without
+        # calling reset(), which would be inconsistent with BaseObject behavior.
+        # See issue #412.
+        params_handled_locally = False
+
         # Ensure strict ordering of parameter setting:
         # 1. All steps
         if attr in params:
             setattr(self, attr, params.pop(attr))
+            params_handled_locally = True
         # 2. Step replacement
         items = getattr(self, attr)
         names = []
@@ -245,8 +256,16 @@ def _set_params(self, attr: str, **params):
         for name in list(params.keys()):
             if "__" not in name and name in names:
                 self._replace_object(attr, name, params.pop(name))
+                params_handled_locally = True
         # 3. Step parameters and other initialisation arguments
         super().set_params(**params)  # type: ignore
+
+        # If we handled params locally and super() got nothing (empty dict),
+        # it would have returned early without calling reset().
+        # To maintain consistency with BaseObject, we must call reset() ourselves.
+        if params_handled_locally and not params:
+            self.reset()
+
         return self
 
     def _replace_object(self, attr, name, new_val) -> None:

skbase/tests/test_meta.py

@@ -168,3 +168,40 @@ def test_metaestimator_composite(long_steps):
 
     meta_est.set_params(bar__b="something else")
     assert meta_est.get_params()["bar__b"] == "something else"
+
+
+def test_set_params_resets_fitted_state():
+    """Test that set_params calls reset, removing fitted state.
+
+    Regression test for issue #412.
+    BaseMetaObject should call reset() during set_params() to clear fitted state,
+    maintaining consistency with BaseObject behavior.
+    """
+    steps = [("foo", ComponentDummy(42)), ("bar", ComponentDummy(24))]
+    meta_obj = MetaObjectTester(steps=steps)
+
+    # Add some fitted state (simulating a fit operation)
+    meta_obj.fitted_attr_ = "should be removed after set_params"
+    meta_obj.another_fitted_ = 123
+
+    # Test 1: Setting the named object parameter should trigger reset
+    new_steps = [("foo", ComponentDummy(99))]
+    meta_obj.set_params(steps=new_steps)
+
+    # Fitted state should be gone after set_params
+    assert not hasattr(
+        meta_obj, "fitted_attr_"
+    ), "fitted_attr_ should be removed by reset() during set_params(steps=...)"
+    assert not hasattr(
+        meta_obj, "another_fitted_"
+    ), "another_fitted_ should be removed by reset() during set_params(steps=...)"
+
+    # Test 2: Replacing individual step should also trigger reset
+    meta_obj = MetaObjectTester(steps=steps)
+    meta_obj.fitted_attr_ = "should be removed"
+
+    meta_obj.set_params(foo=ComponentDummy(77))
+
+    assert not hasattr(
+        meta_obj, "fitted_attr_"
+    ), "fitted_attr_ should be removed by reset() during set_params(foo=...)"