diff --git a/python_tests/artifact/test_backend.py b/python_tests/artifact/test_backend.py
index 56fdf6f5f..38333fe9e 100644
--- a/python_tests/artifact/test_backend.py
+++ b/python_tests/artifact/test_backend.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import base64
 import json
 import tempfile
diff --git a/python_tests/artifact/test_backoff.py b/python_tests/artifact/test_backoff.py
index 5d99c1b79..cbcdafad1 100644
--- a/python_tests/artifact/test_backoff.py
+++ b/python_tests/artifact/test_backoff.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import io
 import uuid
 
diff --git a/python_tests/artifact/test_boto3.py b/python_tests/artifact/test_boto3.py
index 9a48fb3e0..be82abfc8 100644
--- a/python_tests/artifact/test_boto3.py
+++ b/python_tests/artifact/test_boto3.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import io
 from unittest import TestCase
 
diff --git a/python_tests/artifact/test_file_system.py b/python_tests/artifact/test_file_system.py
index ceaaf1251..07f1a890e 100644
--- a/python_tests/artifact/test_file_system.py
+++ b/python_tests/artifact/test_file_system.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import io
 import tempfile
 from unittest import TestCase
diff --git a/python_tests/artifact/test_prefix.py b/python_tests/artifact/test_prefix.py
index 87287d4ce..39cdeb7fa 100644
--- a/python_tests/artifact/test_prefix.py
+++ b/python_tests/artifact/test_prefix.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import io
 import uuid
 
diff --git a/python_tests/preferential/samplers/test_gp.py b/python_tests/preferential/samplers/test_gp.py
index e5df97279..fba0023a0 100644
--- a/python_tests/preferential/samplers/test_gp.py
+++ b/python_tests/preferential/samplers/test_gp.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import sys
 from unittest.mock import patch
 
diff --git a/python_tests/streamlit/test_streamlit_helper.py b/python_tests/streamlit/test_streamlit_helper.py
index 8f3248206..7745634da 100644
--- a/python_tests/streamlit/test_streamlit_helper.py
+++ b/python_tests/streamlit/test_streamlit_helper.py
@@ -1,6 +1,7 @@
+from __future__ import annotations
+
 import itertools
 from typing import Sequence
-from typing import Union
 
 import optuna
 from optuna_dashboard import ChoiceWidget
@@ -60,7 +61,7 @@ def test_render_trial_note_without_note() -> None:
 
 @pytest.mark.parametrize("widgets", widgets_combinations_for_user_attr)
 def test_render_user_attr_form_widgets(
-    widgets: Sequence[Union[ChoiceWidget, SliderWidget, TextInputWidget]],
+    widgets: Sequence[ChoiceWidget | SliderWidget | TextInputWidget],
 ) -> None:
     study = optuna.create_study()
     register_user_attr_form_widgets(study, widgets)  # type: ignore
@@ -77,7 +78,7 @@ def test_render_user_attr_form_widgets(
 
 @pytest.mark.parametrize("widgets", widgets_combinations_for_objective)
 def test_render_objective_form_widgets(
-    widgets: Sequence[Union[ChoiceWidget, SliderWidget, TextInputWidget]],
+    widgets: Sequence[ChoiceWidget | SliderWidget | TextInputWidget],
 ) -> None:
     study = optuna.create_study(directions=["maximize"] * len(widgets))
     register_objective_form_widgets(study, widgets)  # type: ignore
diff --git a/python_tests/wsgi_client.py b/python_tests/wsgi_client.py
index 84fe8aac9..0bf527bfa 100644
--- a/python_tests/wsgi_client.py
+++ b/python_tests/wsgi_client.py
@@ -2,8 +2,6 @@
 
 import io
 import typing
-from typing import Optional
-from typing import Union
 
 from bottle import Bottle
 from optuna_dashboard._storage import trials_cache
@@ -58,9 +56,9 @@ def send_request(
     app: Bottle,
     path: str,
     method: str,
-    body: Union[str, bytes] = b"",
-    queries: Optional[dict[str, str]] = None,
-    headers: Optional[dict[str, str]] = None,
+    body: str | bytes = b"",
+    queries: dict[str, str] | None = None,
+    headers: dict[str, str] | None = None,
     content_type: str = "text/plain; charset=utf-8",
 ) -> tuple[int, list[tuple[str, str]], bytes]:
     status: str = ""