[python] reset storage in record evaluation callback each time before starting training (#4885)

* Update test_sklearn.py

* Update python_package.yml

* Update python_package.yml

* Update callback.py

* Update callback.py

python-package/lightgbm/callback.py

@@ -128,15 +128,15 @@ def record_evaluation(eval_result: Dict[str, Dict[str, List[Any]]]) -> Callable:
     """
     if not isinstance(eval_result, dict):
         raise TypeError('eval_result should be a dictionary')
-    eval_result.clear()
 
     def _init(env: CallbackEnv) -> None:
+        eval_result.clear()
         for data_name, eval_name, _, _ in env.evaluation_result_list:
             eval_result.setdefault(data_name, collections.OrderedDict())
             eval_result[data_name].setdefault(eval_name, [])
 
     def _callback(env: CallbackEnv) -> None:
-        if not eval_result:
+        if env.iteration == env.begin_iteration:
             _init(env)
         for data_name, eval_name, result, _ in env.evaluation_result_list:
             eval_result[data_name][eval_name].append(result)
@@ -221,7 +221,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
     best_score_list: list = []
     cmp_op = []
     enabled = True
-    inited = False
     first_metric = ''
 
     def _init(env: CallbackEnv) -> None:
@@ -230,7 +229,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
         nonlocal best_score_list
         nonlocal cmp_op
         nonlocal enabled
-        nonlocal inited
         nonlocal first_metric
         enabled = not any(env.params.get(boost_alias, "") == 'dart' for boost_alias
                           in _ConfigAliases.get("boosting"))
@@ -249,7 +247,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
         best_iter = []
         best_score_list = []
         cmp_op = []
-        inited = True
         first_metric = ''
 
         n_metrics = len(set(m[1] for m in env.evaluation_result_list))
@@ -293,7 +290,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
     def _final_iteration_check(env: CallbackEnv, eval_name_splitted: List[str], i: int) -> None:
         nonlocal best_iter
         nonlocal best_score_list
-        nonlocal inited
         if env.iteration == env.end_iteration - 1:
             if verbose:
                 best_score_str = '\t'.join([_format_eval_result(x) for x in best_score_list[i]])
@@ -301,7 +297,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
                           f'Best iteration is:\n[{best_iter[i] + 1}]\t{best_score_str}')
                 if first_metric_only:
                     _log_info(f"Evaluated only: {eval_name_splitted[-1]}")
-            inited = False
             raise EarlyStopException(best_iter[i], best_score_list[i])
 
     def _callback(env: CallbackEnv) -> None:
@@ -310,9 +305,8 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
         nonlocal best_score_list
         nonlocal cmp_op
         nonlocal enabled
-        nonlocal inited
         nonlocal first_metric
-        if not inited:
+        if env.iteration == env.begin_iteration:
             _init(env)
         if not enabled:
             return
@@ -336,7 +330,6 @@ def early_stopping(stopping_rounds: int, first_metric_only: bool = False, verbos
                     _log_info(f"Early stopping, best iteration is:\n[{best_iter[i] + 1}]\t{eval_result_str}")
                     if first_metric_only:
                         _log_info(f"Evaluated only: {eval_name_splitted[-1]}")
-                inited = False
                 raise EarlyStopException(best_iter[i], best_score_list[i])
             _final_iteration_check(env, eval_name_splitted, i)
     _callback.order = 30  # type: ignore

tests/python_package_test/test_sklearn.py

@@ -294,20 +294,23 @@ def test_stacking_regressor():
 def test_grid_search():
     X, y = load_iris(return_X_y=True)
     y = y.astype(str)  # utilize label encoder at it's max power
-    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.1,
-                                                        random_state=42)
-    X_train, X_val, y_train, y_val = train_test_split(X_train, y_train, test_size=0.1,
-                                                      random_state=42)
+    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.1, random_state=42)
+    X_train, X_val, y_train, y_val = train_test_split(X_train, y_train, test_size=0.1, random_state=42)
     params = dict(subsample=0.8,
                   subsample_freq=1)
     grid_params = dict(boosting_type=['rf', 'gbdt'],
                        n_estimators=[4, 6],
                        reg_alpha=[0.01, 0.005])
-    fit_params = dict(eval_set=[(X_val, y_val)],
-                      eval_metric=constant_metric,
-                      callbacks=[lgb.early_stopping(2)])
-    grid = GridSearchCV(estimator=lgb.LGBMClassifier(**params), param_grid=grid_params,
-                        cv=2)
+    evals_result = {}
+    fit_params = dict(
+        eval_set=[(X_val, y_val)],
+        eval_metric=constant_metric,
+        callbacks=[
+            lgb.early_stopping(2),
+            lgb.record_evaluation(evals_result)
+        ]
+    )
+    grid = GridSearchCV(estimator=lgb.LGBMClassifier(**params), param_grid=grid_params, cv=2)
     grid.fit(X_train, y_train, **fit_params)
     score = grid.score(X_test, y_test)  # utilizes GridSearchCV default refit=True
     assert grid.best_params_['boosting_type'] in ['rf', 'gbdt']
@@ -319,6 +322,7 @@ def test_grid_search():
     assert grid.best_estimator_.best_score_['valid_0']['error'] == 0
     assert score >= 0.2
     assert score <= 1.
+    assert evals_result == grid.best_estimator_.evals_result_
 
 
 def test_random_search():