diff --git a/docker/optuna_example.py b/docker/optuna_example.py
index 3b75e90c..66cda2df 100644
--- a/docker/optuna_example.py
+++ b/docker/optuna_example.py
@@ -6,7 +6,7 @@ import optuna
 def objective(trial):
     x = trial.suggest_float("x", -100, 100)
     y = trial.suggest_categorical("y", [-1, 0, 1])
-    return x ** 2 + y
+    return x**2 + y
 
 
 if __name__ == "__main__":
diff --git a/optuna_dashboard/app.py b/optuna_dashboard/app.py
index a320de63..70094be5 100644
--- a/optuna_dashboard/app.py
+++ b/optuna_dashboard/app.py
@@ -27,8 +27,8 @@ from bottle import static_file
 import optuna
 from optuna.exceptions import DuplicatedStudyError
 from optuna.storages import BaseStorage
-from optuna.storages import RedisStorage
 from optuna.storages import RDBStorage
+from optuna.storages import RedisStorage
 from optuna.study import Study
 from optuna.study import StudyDirection
 from optuna.study import StudySummary
@@ -38,6 +38,7 @@ from optuna.trial import TrialState
 from . import serializer
 from .search_space import get_search_space
 
+
 if typing.TYPE_CHECKING:
     from _typeshed.wsgi import WSGIApplication
 
diff --git a/python_tests/wsgi_client.py b/python_tests/wsgi_client.py
index d3170e6a..2a0a860a 100644
--- a/python_tests/wsgi_client.py
+++ b/python_tests/wsgi_client.py
@@ -8,6 +8,7 @@ from typing import Union
 
 from bottle import Bottle
 
+
 if typing.TYPE_CHECKING:
     from _typeshed.wsgi import WSGIEnvironment
 
diff --git a/visual_regression_test.py b/visual_regression_test.py
index 6e01881b..2da2ab31 100644
--- a/visual_regression_test.py
+++ b/visual_regression_test.py
@@ -94,7 +94,7 @@ def create_dummy_storage() -> optuna.storages.InMemoryStorage:
     def objective_multi(trial: optuna.Trial) -> Tuple[float, float]:
         x = trial.suggest_float("x", 0, 5)
         y = trial.suggest_float("y", 0, 3)
-        v0 = 4 * x ** 2 + 4 * y ** 2
+        v0 = 4 * x**2 + 4 * y**2
         v1 = (x - 5) ** 2 + (y - 5) ** 2
         return v0, v1
 
@@ -110,13 +110,13 @@ def create_dummy_storage() -> optuna.storages.InMemoryStorage:
         if category == "foo":
             x = trial.suggest_float("x1", 0, 5)
             y = trial.suggest_float("y1", 0, 3)
-            v0 = 4 * x ** 2 + 4 * y ** 2
+            v0 = 4 * x**2 + 4 * y**2
             v1 = (x - 5) ** 2 + (y - 5) ** 2
             return v0, v1
         else:
             x = trial.suggest_float("x2", 0, 5)
             y = trial.suggest_float("y2", 0, 3)
-            v0 = 2 * x ** 2 + 2 * y ** 2
+            v0 = 2 * x**2 + 2 * y**2
             v1 = (x - 2) ** 2 + (y - 3) ** 2
             return v0, v1
 
@@ -130,7 +130,7 @@ def create_dummy_storage() -> optuna.storages.InMemoryStorage:
     def objective_prune_without_report(trial: optuna.Trial) -> float:
         x = trial.suggest_float("x", -15, 30)
         y = trial.suggest_float("y", -15, 30)
-        v = x ** 2 + y ** 2
+        v = x**2 + y**2
         if v > 100:
             raise optuna.TrialPruned()
         return v