Fix local thread and its tests

package/samplers/auto_sampler/_sampler.py

@@ -85,15 +85,18 @@ def __init__(
         constraints_func: Callable[[FrozenTrial], Sequence[float]] | None = None,
     ) -> None:
         self._rng = LazyRandomState(seed)
-        seed_for_random_sampler = self._rng.rng.randint(_MAXINT32)
-        sampler: BaseSampler = RandomSampler(seed=seed_for_random_sampler)
         self._thread_local_sampler = _ThreadLocalSampler()
-        self._thread_local_sampler._sampler = sampler
         self._constraints_func = constraints_func
 
     @property
     def _sampler(self) -> BaseSampler:
-        assert self._sampler is not None
+        if self._thread_local_sampler._sampler is None:
+            # NOTE(nabenabe): Do not do this process in the __init__ method because the
+            # substitution at the init does not update attributes in self._thread_local_sampler
+            # in each thread.
+            seed_for_random_sampler = self._rng.rng.randint(_MAXINT32)
+            self._sampler = RandomSampler(seed=seed_for_random_sampler)
+
         return self._thread_local_sampler._sampler
 
     @_sampler.setter

package/samplers/auto_sampler/tests/test_auto_sampler.py

@@ -135,3 +135,15 @@ def test_choose_tpe_with_conditional_params() -> None:
     assert ["RandomSampler"] + ["GPSampler"] * 15 + ["TPESampler"] * (
         n_trials - 16
     ) == sampler_names
+
+
+def test_multi_thread() -> None:
+    n_trials = 30
+    auto_sampler = AutoSampler()
+    auto_sampler._N_COMPLETE_TRIALS_FOR_CMAES = 10
+    study = optuna.create_study(sampler=auto_sampler)
+    study.optimize(objective, n_trials=n_trials)
+    sampler_names = _get_used_sampler_names(study)
+    assert "RandomSampler" in sampler_names
+    assert "GPSampler" in sampler_names
+    assert "CmaEsSampler" in sampler_names