Merge pull request optuna#4923 from optuna/fix-lightgbm-checks-integr…

…ation

Fix the `checks-integration` errors on LightGBMTuner

optuna/integration/lightgbm.py

@@ -1,15 +1,20 @@
+from __future__ import annotations
+
 import sys
-from typing import List
-from typing import Optional
+from typing import TYPE_CHECKING
 
 import optuna
 from optuna._imports import try_import
 from optuna.integration import _lightgbm_tuner as tuner
 
 
+if TYPE_CHECKING:
+    from lightgbm.basic import _LGBM_BoosterEvalMethodResultType
+    from lightgbm.callback import CallbackEnv
+
+
 with try_import() as _imports:
     import lightgbm as lgb
-    from lightgbm.callback import CallbackEnv
 
 # Attach lightgbm API.
 if _imports.is_successful():
@@ -88,27 +93,35 @@ def __init__(
         self._report_interval = report_interval
 
     def _find_evaluation_result(
-        self, target_valid_name: str, env: "CallbackEnv"
-    ) -> Optional[List]:
-        for evaluation_result in env.evaluation_result_list:
+        self, target_valid_name: str, env: CallbackEnv
+    ) -> _LGBM_BoosterEvalMethodResultType | None:
+        evaluation_result_list = env.evaluation_result_list
+        if evaluation_result_list is None:
+            return None
+
+        for evaluation_result in evaluation_result_list:
             valid_name, metric, current_score, is_higher_better = evaluation_result[:4]
             # The prefix "valid " is added to metric name since LightGBM v4.0.0.
             if valid_name != target_valid_name or (
                 metric != "valid " + self._metric and metric != self._metric
             ):
                 continue
-
             return evaluation_result
 
         return None
 
-    def __call__(self, env: "CallbackEnv") -> None:
+    def __call__(self, env: CallbackEnv) -> None:
         if (env.iteration + 1) % self._report_interval == 0:
             # If this callback has been passed to `lightgbm.cv` function,
             # the value of `is_cv` becomes `True`. See also:
-            # https://github.com/Microsoft/LightGBM/blob/v2.2.2/python-package/lightgbm/engine.py#L329
+            # https://github.com/microsoft/LightGBM/blob/v4.1.0/python-package/lightgbm/engine.py#L533
             # Note that `5` is not the number of folds but the length of sequence.
-            is_cv = len(env.evaluation_result_list) > 0 and len(env.evaluation_result_list[0]) == 5
+            evaluation_result_list = env.evaluation_result_list
+            is_cv = (
+                evaluation_result_list is not None
+                and len(evaluation_result_list) > 0
+                and len(evaluation_result_list[0]) == 5
+            )
             if is_cv:
                 target_valid_name = "cv_agg"
             else: