Merge pull request optuna#4930 from xadrianzetx/metric-names-getter2

Add `metric_names` getter to study

optuna/study/_dataframe.py

@@ -8,7 +8,6 @@
 
 import optuna
 from optuna._imports import try_import
-from optuna.study.study import _SYSTEM_ATTR_METRIC_NAMES
 from optuna.trial._state import TrialState
 
 
@@ -41,9 +40,7 @@ def _create_records_and_aggregate_column(
     column_agg: DefaultDict[str, Set] = collections.defaultdict(set)
     non_nested_attr = ""
 
-    metric_names = study._storage.get_study_system_attrs(study._study_id).get(
-        _SYSTEM_ATTR_METRIC_NAMES
-    )
+    metric_names = study.metric_names
 
     records = []
     for trial in study.get_trials(deepcopy=False):

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,
@@ -1075,9 +1088,7 @@ def _log_completed_trial(self, trial: trial_module.FrozenTrial) -> None:
         if not _logger.isEnabledFor(logging.INFO):
             return
 
-        metric_names = self._storage.get_study_system_attrs(self._study_id).get(
-            _SYSTEM_ATTR_METRIC_NAMES
-        )
+        metric_names = self.metric_names
 
         if len(trial.values) > 1:
             trial_values: list[float] | dict[str, float]

optuna/visualization/_pareto_front.py

@@ -11,7 +11,6 @@
 from optuna.exceptions import ExperimentalWarning
 from optuna.study import Study
 from optuna.study._multi_objective import _get_pareto_front_trials_by_trials
-from optuna.study.study import _SYSTEM_ATTR_METRIC_NAMES
 from optuna.trial import FrozenTrial
 from optuna.trial import TrialState
 from optuna.visualization._plotly_imports import _imports
@@ -343,9 +342,7 @@ def _infer_n_targets(
         )
 
     if target_names is None:
-        metric_names = study._storage.get_study_system_attrs(study._study_id).get(
-            _SYSTEM_ATTR_METRIC_NAMES
-        )
+        metric_names = study.metric_names
         if metric_names is None:
             target_names = [f"Objective {i}" for i in range(n_targets)]
         else:

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