Add metric_names getter to complement get_metric_names

optuna/study/study.py

@@ -333,6 +333,19 @@ def system_attrs(self) -> dict[str, Any]:
 
         return copy.deepcopy(self._storage.get_study_system_attrs(self._study_id))
 
+    @property
+    @experimental_func("3.4.0")
+    def metric_names(self) -> list[str] | None:
+        """Return metric names.
+
+        .. note::
+            Use :meth:`~optuna.study.Study.set_metric_names` to set the metric names first.
+
+        Returns:
+            A list with names for each dimension of the returned values of the objective function.
+        """
+        return self._storage.get_study_system_attrs(self._study_id).get(_SYSTEM_ATTR_METRIC_NAMES)
+
     def optimize(
         self,
         func: ObjectiveFuncType,

tests/study_tests/test_study.py

@@ -1601,3 +1601,18 @@ def test_set_invalid_metric_names() -> None:
     study = create_study(directions=["minimize", "minimize"])
     with pytest.raises(ValueError):
         study.set_metric_names(metric_names)
+
+
+def test_get_metric_names() -> None:
+    study = create_study()
+    assert study.metric_names is None
+    study.set_metric_names(["v0"])
+    assert study.metric_names == ["v0"]
+    study.set_metric_names(["v1"])
+    assert study.metric_names == ["v1"]
+
+
+def test_get_metric_names_experimental_warning() -> None:
+    study = create_study()
+    with pytest.warns(ExperimentalWarning):
+        study.metric_names