[python-package] allow use of early_stopping_round<=0 to turn off early stopping (fixes #6401)

python-package/lightgbm/callback.py

@@ -280,8 +280,7 @@ def __init__(
         verbose: bool = True,
         min_delta: Union[float, List[float]] = 0.0,
     ) -> None:
-        if not isinstance(stopping_rounds, int) or stopping_rounds <= 0:
-            raise ValueError(f"stopping_rounds should be an integer and greater than 0. got: {stopping_rounds}")
+        self.enabled = _should_enable_early_stopping(stopping_rounds)
 
         self.order = 30
         self.before_iteration = False
@@ -291,7 +290,6 @@ def __init__(
         self.verbose = verbose
         self.min_delta = min_delta
 
-        self.enabled = True
         self._reset_storages()
 
     def _reset_storages(self) -> None:
@@ -438,6 +436,18 @@ def __call__(self, env: CallbackEnv) -> None:
             self._final_iteration_check(env, eval_name_splitted, i)
 
 
+def _should_enable_early_stopping(stopping_rounds: Any) -> bool:
+    """Check if early stopping should be activated.
+
+    This function will evaluate to True if the early stopping callback should be
+    activated (i.e. stopping_rounds > 0).  It also provides an informative error if the
+    type is not int.
+    """
+    if not isinstance(stopping_rounds, int):
+        raise TypeError(f"early_stopping_round should be an integer. Got '{type(stopping_rounds).__name__}'")
+    return stopping_rounds > 0
+
+
 def early_stopping(
     stopping_rounds: int,
     first_metric_only: bool = False,

python-package/lightgbm/engine.py

@@ -236,7 +236,7 @@ def train(
             cb.__dict__.setdefault("order", i - len(callbacks))
         callbacks_set = set(callbacks)
 
-    if "early_stopping_round" in params:
+    if callback._should_enable_early_stopping(params.get("early_stopping_round", 0)):
         callbacks_set.add(
             callback.early_stopping(
                 stopping_rounds=params["early_stopping_round"],  # type: ignore[arg-type]
@@ -760,7 +760,7 @@ def cv(
             cb.__dict__.setdefault("order", i - len(callbacks))
         callbacks_set = set(callbacks)
 
-    if "early_stopping_round" in params:
+    if callback._should_enable_early_stopping(params.get("early_stopping_round", 0)):
         callbacks_set.add(
             callback.early_stopping(
                 stopping_rounds=params["early_stopping_round"],  # type: ignore[arg-type]

tests/python_package_test/test_callback.py

@@ -22,14 +22,14 @@ def test_early_stopping_callback_is_picklable(serializer):
 
 
 def test_early_stopping_callback_rejects_invalid_stopping_rounds_with_informative_errors():
-    with pytest.raises(ValueError, match="stopping_rounds should be an integer and greater than 0. got: 0"):
-        lgb.early_stopping(stopping_rounds=0)
+    with pytest.raises(TypeError, match="early_stopping_round should be an integer. Got 'str'"):
+        lgb.early_stopping(stopping_rounds="neverrrr")
 
-    with pytest.raises(ValueError, match="stopping_rounds should be an integer and greater than 0. got: -1"):
-        lgb.early_stopping(stopping_rounds=-1)
 
-    with pytest.raises(ValueError, match="stopping_rounds should be an integer and greater than 0. got: neverrrr"):
-        lgb.early_stopping(stopping_rounds="neverrrr")
+@pytest.mark.parametrize("stopping_rounds", [-10, -1, 0])
+def test_early_stopping_callback_accepts_non_positive_stopping_rounds(stopping_rounds):
+    cb = lgb.early_stopping(stopping_rounds=stopping_rounds)
+    assert cb.enabled is False
 
 
 @pytest.mark.parametrize("serializer", SERIALIZERS)

tests/python_package_test/test_engine.py

@@ -938,6 +938,54 @@ def test_early_stopping_via_global_params(first_metric_only):
     assert "error" in gbm.best_score[valid_set_name]
 
 
+@pytest.mark.parametrize("early_stopping_round", [-10, -1, 0, None, "None"])
+def test_early_stopping_is_not_enabled_for_non_positive_stopping_rounds(early_stopping_round):
+    X, y = load_breast_cancer(return_X_y=True)
+    num_trees = 5
+    params = {
+        "num_trees": num_trees,
+        "objective": "binary",
+        "metric": "None",
+        "verbose": -1,
+        "early_stopping_round": early_stopping_round,
+        "first_metric_only": True,
+    }
+    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.1, random_state=42)
+    lgb_train = lgb.Dataset(X_train, y_train)
+    lgb_eval = lgb.Dataset(X_test, y_test, reference=lgb_train)
+    valid_set_name = "valid_set"
+
+    if early_stopping_round is None:
+        gbm = lgb.train(
+            params,
+            lgb_train,
+            feval=[constant_metric],
+            valid_sets=lgb_eval,
+            valid_names=valid_set_name,
+        )
+        assert "early_stopping_round" not in gbm.params
+        assert gbm.num_trees() == num_trees
+    elif early_stopping_round == "None":
+        with pytest.raises(TypeError, match="early_stopping_round should be an integer. Got str"):
+            gbm = lgb.train(
+                params,
+                lgb_train,
+                feval=[constant_metric],
+                valid_sets=lgb_eval,
+                valid_names=valid_set_name,
+            )
+    elif early_stopping_round <= 0:
+        gbm = lgb.train(
+            params,
+            lgb_train,
+            feval=[constant_metric],
+            valid_sets=lgb_eval,
+            valid_names=valid_set_name,
+        )
+        assert gbm.params["early_stopping_round"] == early_stopping_round
+        assert gbm.num_trees() == num_trees
+
+
 @pytest.mark.parametrize("first_only", [True, False])
 @pytest.mark.parametrize("single_metric", [True, False])
 @pytest.mark.parametrize("greater_is_better", [True, False])