FIX EmptyRequest.get defaults to Bunch of METHODS (#28371)

sklearn/metrics/tests/test_score_objects.py

@@ -1490,3 +1490,18 @@ def test_make_scorer_deprecation(deprecated_params, new_params, warn_msg):
     assert deprecated_roc_auc_scorer(classifier, X, y) == pytest.approx(
         roc_auc_scorer(classifier, X, y)
     )
+
+
+@pytest.mark.parametrize("enable_metadata_routing", [True, False])
+def test_metadata_routing_multimetric_metadata_routing(enable_metadata_routing):
+    """Test multimetric scorer works with and without metadata routing enabled when
+    there is no actual metadata to pass.
+
+    Non-regression test for https://github.com/scikit-learn/scikit-learn/issues/28256
+    """
+    X, y = make_classification(n_samples=50, n_features=10, random_state=0)
+    estimator = EstimatorWithFitAndPredict().fit(X, y)
+
+    multimetric_scorer = _MultimetricScorer(scorers={"acc": get_scorer("accuracy")})
+    with config_context(enable_metadata_routing=enable_metadata_routing):
+        multimetric_scorer(estimator, X, y)

sklearn/tests/test_metadata_routing.py

@@ -239,6 +239,22 @@ class InvalidObject:
         process_routing(InvalidObject(), "fit", groups=my_groups)
 
 
+@pytest.mark.parametrize("method", METHODS)
+@pytest.mark.parametrize("default", [None, "default", []])
+def test_process_routing_empty_params_get_with_default(method, default):
+    empty_params = {}
+    routed_params = process_routing(ConsumingClassifier(), "fit", **empty_params)
+
+    # Behaviour should be an empty dictionary returned for each method when retrieved.
+    params_for_method = routed_params[method]
+    assert isinstance(params_for_method, dict)
+    assert set(params_for_method.keys()) == set(METHODS)
+
+    # No default to `get` should be equivalent to the default
+    default_params_for_method = routed_params.get(method, default=default)
+    assert default_params_for_method == params_for_method
+
+
 def test_simple_metadata_routing():
     # Tests that metadata is properly routed
 

sklearn/utils/_metadata_requests.py

@@ -1082,8 +1082,12 @@ def _serialize(self):
 
     def __iter__(self):
         if self._self_request:
-            yield "$self_request", RouterMappingPair(
-                mapping=MethodMapping.from_str("one-to-one"), router=self._self_request
+            yield (
+                "$self_request",
+                RouterMappingPair(
+                    mapping=MethodMapping.from_str("one-to-one"),
+                    router=self._self_request,
+                ),
             )
         for name, route_mapping in self._route_mappings.items():
             yield (name, route_mapping)
@@ -1530,7 +1534,7 @@ def process_routing(_obj, _method, /, **kwargs):
         # an empty dict on routed_params.ANYTHING.ANY_METHOD.
         class EmptyRequest:
             def get(self, name, default=None):
-                return default if default else {}
+                return Bunch(**{method: dict() for method in METHODS})
 
             def __getitem__(self, name):
                 return Bunch(**{method: dict() for method in METHODS})