diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index ceb8f60a..4dd2b3d2 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import json
 from unittest import TestCase
 
diff --git a/python_tests/test_note.py b/python_tests/test_note.py
index f69de768..f4cfa1d2 100644
--- a/python_tests/test_note.py
+++ b/python_tests/test_note.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from unittest import TestCase
 from unittest.mock import patch
 
diff --git a/python_tests/test_serializers.py b/python_tests/test_serializers.py
index 11de8080..131013c9 100644
--- a/python_tests/test_serializers.py
+++ b/python_tests/test_serializers.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from unittest import TestCase
 
 from optuna_dashboard._serializer import serialize_attrs
diff --git a/python_tests/wsgi_client.py b/python_tests/wsgi_client.py
index 36d48dfd..3a6b078e 100644
--- a/python_tests/wsgi_client.py
+++ b/python_tests/wsgi_client.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import io
 import typing
 from typing import Optional