[python-package] fix mypy errors about early stopping rounds (#5795)

python-package/lightgbm/engine.py

@@ -217,7 +217,7 @@ def train(
     if "early_stopping_round" in params:
         callbacks_set.add(
             callback.early_stopping(
-                stopping_rounds=params["early_stopping_round"],
+                stopping_rounds=params["early_stopping_round"],  # type: ignore[arg-type]
                 first_metric_only=first_metric_only,
                 verbose=_choose_param_value(
                     main_param_name="verbosity",
@@ -708,7 +708,7 @@ def cv(
     if "early_stopping_round" in params:
         callbacks_set.add(
             callback.early_stopping(
-                stopping_rounds=params["early_stopping_round"],
+                stopping_rounds=params["early_stopping_round"],  # type: ignore[arg-type]
                 first_metric_only=first_metric_only,
                 verbose=_choose_param_value(
                     main_param_name="verbosity",

tests/python_package_test/test_engine.py

@@ -4015,3 +4015,11 @@ def test_validate_features():
 
     # check that disabling the check doesn't raise the error
     bst.refit(df2, y, validate_features=False)
+
+
+def test_train_raises_informative_error_for_params_of_wrong_type():
+    X, y = make_synthetic_regression()
+    params = {"early_stopping_round": "too-many"}
+    dtrain = lgb.Dataset(X, label=y)
+    with pytest.raises(lgb.basic.LightGBMError, match="Parameter early_stopping_round should be of type int, got \"too-many\""):
+        lgb.train(params, dtrain)