diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index dfa54af5..ae9c1c6d 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -47,21 +47,10 @@ $ docker run -it --rm -p 8080:8080 -v `PWD`:/app -w /app sqlite:///db.sqlite3
 
 ```
 $ pip install -e .
-$ optuna-dashboard sqlite:///db.sqlite3
-```
-
-<details>
-
-<summary>Environment variables for development</summary>
-
-If you set `OPTUNA_DASHBOARD_DEBUG=1`, the server will automatically restart when the source codes are changed.
-
-```
 $ OPTUNA_DASHBOARD_DEBUG=1 optuna-dashboard sqlite:///db.sqlite3
 ```
 
-</details>
-
+Note that `OPTUNA_DASHBOARD_DEBUG=1` makes the server will automatically restart when the source codes are changed.
 
 ## Running tests, lint checks and formatters
 
@@ -102,7 +91,7 @@ $ pip install -r requirements.txt
 $ flake8
 $ black --check .
 $ isort . --check
-$ mypy .
+$ mypy optuna_dashboard python_tests
 ```
 
 or
@@ -118,10 +107,11 @@ $ tox -e flake8 -e black -e mypy
 $ npm run fmt
 ```
 
-### Auto-formatting Python files (by black)
+### Auto-formatting Python files
 
 ```
 $ black .
+$ isort .
 ```
 
 
diff --git a/python_tests/test_cached_extra_study_property.py b/python_tests/test_cached_extra_study_property.py
index 09e91c22..f1aa92fe 100644
--- a/python_tests/test_cached_extra_study_property.py
+++ b/python_tests/test_cached_extra_study_property.py
@@ -5,6 +5,7 @@ import warnings
 
 import optuna
 from optuna import create_trial
+from optuna.distributions import BaseDistribution
 from optuna.distributions import UniformDistribution
 from optuna.exceptions import ExperimentalWarning
 from optuna.trial import TrialState
@@ -18,7 +19,7 @@ class _CachedExtraStudyPropertySearchSpaceTestCase(TestCase):
         warnings.simplefilter("ignore", category=ExperimentalWarning)
 
     def test_same_distributions(self) -> None:
-        distributions = [
+        distributions: List[Dict[str, BaseDistribution]] = [
             {
                 "x0": UniformDistribution(low=0, high=10),
                 "x1": UniformDistribution(low=0, high=10),
@@ -49,7 +50,7 @@ class _CachedExtraStudyPropertySearchSpaceTestCase(TestCase):
         self.assertEqual(len(cached_extra_study_property.union), 2)
 
     def test_different_distributions(self) -> None:
-        distributions = [
+        distributions: List[Dict[str, BaseDistribution]] = [
             {
                 "x0": UniformDistribution(low=0, high=10),
                 "x1": UniformDistribution(low=0, high=10),
@@ -80,7 +81,7 @@ class _CachedExtraStudyPropertySearchSpaceTestCase(TestCase):
         self.assertEqual(len(cached_extra_study_property.union), 3)
 
     def test_dynamic_search_space(self) -> None:
-        distributions = [
+        distributions: List[Dict[str, BaseDistribution]] = [
             {
                 "x0": UniformDistribution(low=0, high=10),
                 "x1": UniformDistribution(low=0, high=10),
@@ -143,7 +144,7 @@ class _CachedExtraStudyPropertyIntermediateTestCase(TestCase):
         self.assertFalse(cached_extra_study_property.has_intermediate_values)
 
     def test_some_trials_has_no_intermediate_value(self) -> None:
-        intermediate_values = [
+        intermediate_values: List[Dict] = [
             {0: 0.3, 1: 1.2},
             {},
             {0: 0.3, 1: 1.2},
@@ -163,7 +164,7 @@ class _CachedExtraStudyPropertyIntermediateTestCase(TestCase):
         self.assertTrue(cached_extra_study_property.has_intermediate_values)
 
     def test_all_trials_has_intermediate_value(self) -> None:
-        intermediate_values = [{0: 0.3, 1: 1.2}, {0: 0.3, 1: 1.2}]
+        intermediate_values: List[Dict] = [{0: 0.3, 1: 1.2}, {0: 0.3, 1: 1.2}]
         trials = [
             create_trial(
                 state=TrialState.COMPLETE,