remove evals_result argument from train() function

examples/python-guide/notebooks/interactive_plot_example.ipynb

@@ -148,8 +148,10 @@
     "                valid_sets=[lgb_train, lgb_test],\n",
     "                feature_name=[f'f{i + 1}' for i in range(X_train.shape[-1])],\n",
     "                categorical_feature=[21],\n",
-    "                evals_result=evals_result,\n",
-    "                callbacks=[lgb.log_evaluation(10)])"
+    "                callbacks=[\n",
+    "                    lgb.log_evaluation(10),\n",
+    "                    lgb.record_evaluation(evals_result)\n",
+    "                ])"
    ]
   },
   {

examples/python-guide/plot_example.py

@@ -36,14 +36,18 @@
 
 print('Starting training...')
 # train
-gbm = lgb.train(params,
-                lgb_train,
-                num_boost_round=100,
-                valid_sets=[lgb_train, lgb_test],
-                feature_name=[f'f{i + 1}' for i in range(X_train.shape[-1])],
-                categorical_feature=[21],
-                evals_result=evals_result,
-                callbacks=[lgb.log_evaluation(10)])
+gbm = lgb.train(
+    params,
+    lgb_train,
+    num_boost_round=100,
+    valid_sets=[lgb_train, lgb_test],
+    feature_name=[f'f{i + 1}' for i in range(X_train.shape[-1])],
+    categorical_feature=[21],
+    callbacks=[
+        lgb.log_evaluation(10),
+        lgb.record_evaluation(evals_result)
+    ]
+)
 
 print('Plotting metrics recorded during training...')
 ax = lgb.plot_metric(evals_result, metric='l1')

python-package/lightgbm/callback.py

@@ -128,18 +128,24 @@ 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()
+    inited = False
 
     def _init(env: CallbackEnv) -> None:
+        nonlocal inited
+        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, [])
+        inited = True
 
     def _callback(env: CallbackEnv) -> None:
-        if not eval_result:
+        nonlocal inited
+        if not inited:
             _init(env)
         for data_name, eval_name, result, _ in env.evaluation_result_list:
             eval_result[data_name][eval_name].append(result)
+        if env.iteration == env.end_iteration - 1:
+            inited = False
     _callback.order = 20  # type: ignore
     return _callback
 

python-package/lightgbm/engine.py

@@ -34,7 +34,6 @@ def train(
     feature_name: Union[List[str], str] = 'auto',
     categorical_feature: Union[List[str], List[int], str] = 'auto',
     early_stopping_rounds: Optional[int] = None,
-    evals_result: Optional[Dict[str, Any]] = None,
     keep_training_booster: bool = False,
     callbacks: Optional[List[Callable]] = None
 ) -> Booster:
@@ -119,19 +118,6 @@ def train(
         To check only the first metric, set the ``first_metric_only`` parameter to ``True`` in ``params``.
         The index of iteration that has the best performance will be saved in the ``best_iteration`` field
         if early stopping logic is enabled by setting ``early_stopping_rounds``.
-    evals_result : dict or None, optional (default=None)
-        Dictionary used to store all evaluation results of all the items in ``valid_sets``.
-        This should be initialized outside of your call to ``train()`` and should be empty.
-        Any initial contents of the dictionary will be deleted.
-
-        .. rubric:: Example
-
-        With a ``valid_sets`` = [valid_set, train_set],
-        ``valid_names`` = ['eval', 'train']
-        and a ``params`` = {'metric': 'logloss'}
-        returns {'train': {'logloss': ['0.48253', '0.35953', ...]},
-        'eval': {'logloss': ['0.480385', '0.357756', ...]}}.
-
     keep_training_booster : bool, optional (default=False)
         Whether the returned Booster will be used to keep training.
         If False, the returned value will be converted into _InnerPredictor before returning.
@@ -221,11 +207,6 @@ def train(
     if early_stopping_rounds is not None and early_stopping_rounds > 0:
         callbacks_set.add(callback.early_stopping(early_stopping_rounds, first_metric_only))
 
-    if evals_result is not None:
-        _log_warning("'evals_result' argument is deprecated and will be removed in a future release of LightGBM. "
-                     "Pass 'record_evaluation()' callback via 'callbacks' argument instead.")
-        callbacks_set.add(callback.record_evaluation(evals_result))
-
     callbacks_before_iter_set = {cb for cb in callbacks_set if getattr(cb, 'before_iteration', False)}
     callbacks_after_iter_set = callbacks_set - callbacks_before_iter_set
     callbacks_before_iter = sorted(callbacks_before_iter_set, key=attrgetter('order'))