Fix

package/samplers/ctpe/sampler.py

@@ -9,7 +9,6 @@
 from optuna.logging import get_logger
 from optuna.samplers import TPESampler
 from optuna.samplers._tpe.parzen_estimator import _ParzenEstimator
-from optuna.samplers._tpe.sampler import _split_trials
 from optuna.study import Study
 from optuna.study import StudyDirection
 from optuna.trial import FrozenTrial
@@ -45,6 +44,11 @@ def __init__(
         use_min_bandwidth_discrete: bool = True,
         constraints_func: Callable[[FrozenTrial], Sequence[float]] | None = None,
     ):
+        if constraints_func is None:
+            raise ValueError(
+                f"{self.__class__.__name__} must take constraints_func, but got None."
+            )
+
         gamma = GammaFunc(strategy=gamma_strategy, beta=gamma_beta)
         weights = WeightFunc(strategy=weight_strategy)
         super().__init__(
@@ -136,7 +140,7 @@ def _sample(
         )
 
         n_below_feasible = self._gamma(len(trials))
-        below_trials, above_trials = _split_trials(
+        below_trials, above_trials = _split_trials_for_ctpe(
             study, trials, n_below_feasible, is_feasible=np.all(constraints_vals <= 0, axis=-1)
         )
         mpes_below.append(
@@ -145,6 +149,7 @@ def _sample(
         mpes_above.append(
             self._build_parzen_estimator(study, search_space, above_trials, handle_below=False)
         )
+        quantiles.append(len(below_trials) / len(trials))
 
         _samples_below: dict[str, list[_ParzenEstimator]] = {
             param_name: [] for param_name in search_space