FIX EmptyRequest.get defaults to Bunch of METHODS

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 +239,28 @@ class InvalidObject:
         process_routing(InvalidObject(), "fit", groups=my_groups)
 
 
+def test_process_routing_empty_params_get_with_default():
+    empty_params = {}
+    routed_params = process_routing(ConsumingClassifier(), "fit", **empty_params)
+
+    # Behaviour should be an empty dictionary returned for each method when retrieved.
+    for method in METHODS:
+        # This behaviour should be equivalent with using `get` with no default
+        params_for_method = routed_params[method]
+
+        # An empty dictionary for each method
+        assert isinstance(params_for_method, dict)
+        assert set(params_for_method.keys()) == set(METHODS)
+
+        default_params_for_method = routed_params.get(method)
+        assert default_params_for_method == params_for_method
+
+        # However, with a default, should return that instead.
+        assert routed_params.get(method, default="default") == "default"
+        assert routed_params.get(method, default=[]) == []
+        assert routed_params.get(method, default=None) is None
+
+
 def test_simple_metadata_routing():
     # Tests that metadata is properly routed
 

sklearn/utils/_metadata_requests.py

@@ -115,6 +115,9 @@
 
 METHODS = SIMPLE_METHODS + list(COMPOSITE_METHODS.keys())
 
+# Used as a sentinel value to indicate nothing was passed.
+_MISSING = object()
+
 
 def _routing_enabled():
     """Return whether metadata routing is enabled.
@@ -1530,7 +1533,10 @@ 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 {}
+                if not default:
+                    return Bunch(**{method: dict() for method in METHODS})
+
+                return default
 
             def __getitem__(self, name):
                 return Bunch(**{method: dict() for method in METHODS})