FIX EmptyRequest.get defaults to Bunch of METHODS

sklearn/metrics/tests/test_score_objects.py

@@ -1490,3 +1490,20 @@ 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)
     )
+
+
+def test_metadata_routing_multimetric_without_metadata_works_with_and_without_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=True):
+        multimetric_scorer(estimator, X, y)
+
+    with config_context(enable_metadata_routing=False):
+        multimetric_scorer(estimator, X, y)

sklearn/tests/test_metadata_routing.py

@@ -15,7 +15,9 @@
     BaseEstimator,
     clone,
 )
+from sklearn.dummy import DummyClassifier
 from sklearn.linear_model import LinearRegression
+from sklearn.metrics._scorer import _MultimetricScorer, get_scorer
 from sklearn.tests.metadata_routing_common import (
     ConsumingClassifier,
     ConsumingRegressor,
@@ -990,3 +992,18 @@ def fit(self, X, y, metadata=None):
         NotImplementedError, match="Estimator has not implemented metadata routing yet."
     ):
         MetaRegressor(estimator=Estimator()).fit(X, y, metadata=my_groups)
+
+
+def test_metadata_routing_multimetric_behaves_equally_with_and_without_routing():
+    """Test multimetric scorer works with and without metadata routing enabled.
+
+    Non-regression test for https://github.com/scikit-learn/scikit-learn/issues/28256
+    """
+
+    multimetric_scorer = _MultimetricScorer(scorers={"acc": get_scorer("accuracy")})
+    estimator = DummyClassifier().fit(X, y)
+    with config_context(enable_metadata_routing=True):
+        multimetric_scorer(estimator, X, y)
+
+    with config_context(enable_metadata_routing=False):
+        multimetric_scorer(estimator, X, y)

sklearn/utils/_metadata_requests.py

@@ -1530,7 +1530,10 @@
         # 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})