[python] make reset_parameter callback pickleable (#5109)

python-package/lightgbm/callback.py

@@ -130,7 +130,8 @@ def __call__(self, env: CallbackEnv) -> None:
                 self.eval_result[data_name][eval_name].append(result)
             else:
                 data_name, eval_name = item[1].split()
-                res_mean, res_stdv = item[2], item[4]
+                res_mean = item[2]
+                res_stdv = item[4]
                 self.eval_result[data_name][f'{eval_name}-mean'].append(res_mean)
                 self.eval_result[data_name][f'{eval_name}-stdv'].append(res_stdv)
 
@@ -171,6 +172,34 @@ def record_evaluation(eval_result: Dict[str, Dict[str, List[Any]]]) -> Callable:
     return _RecordEvaluationCallback(eval_result=eval_result)
 
 
+class _ResetParameterCallback:
+    """Internal reset parameter callable class."""
+
+    def __init__(self, **kwargs: Union[list, Callable]) -> None:
+        self.order = 10
+        self.before_iteration = True
+
+        self.kwargs = kwargs
+
+    def __call__(self, env: CallbackEnv) -> None:
+        new_parameters = {}
+        for key, value in self.kwargs.items():
+            if isinstance(value, list):
+                if len(value) != env.end_iteration - env.begin_iteration:
+                    raise ValueError(f"Length of list {key!r} has to be equal to 'num_boost_round'.")
+                new_param = value[env.iteration - env.begin_iteration]
+            elif callable(value):
+                new_param = value(env.iteration - env.begin_iteration)
+            else:
+                raise ValueError("Only list and callable values are supported "
+                                 "as a mapping from boosting round index to new parameter value.")
+            if new_param != env.params.get(key, None):
+                new_parameters[key] = new_param
+        if new_parameters:
+            env.model.reset_parameter(new_parameters)
+            env.params.update(new_parameters)
+
+
 def reset_parameter(**kwargs: Union[list, Callable]) -> Callable:
     """Create a callback that resets the parameter after the first iteration.
 
@@ -189,26 +218,10 @@ def reset_parameter(**kwargs: Union[list, Callable]) -> Callable:
 
     Returns
     -------
-    callback : callable
+    callback : _ResetParameterCallback
         The callback that resets the parameter after the first iteration.
     """
-    def _callback(env: CallbackEnv) -> None:
-        new_parameters = {}
-        for key, value in kwargs.items():
-            if isinstance(value, list):
-                if len(value) != env.end_iteration - env.begin_iteration:
-                    raise ValueError(f"Length of list {key!r} has to equal to 'num_boost_round'.")
-                new_param = value[env.iteration - env.begin_iteration]
-            else:
-                new_param = value(env.iteration - env.begin_iteration)
-            if new_param != env.params.get(key, None):
-                new_parameters[key] = new_param
-        if new_parameters:
-            env.model.reset_parameter(new_parameters)
-            env.params.update(new_parameters)
-    _callback.before_iteration = True  # type: ignore
-    _callback.order = 10  # type: ignore
-    return _callback
+    return _ResetParameterCallback(**kwargs)
 
 
 class _EarlyStoppingCallback:

tests/python_package_test/test_callback.py

@@ -22,6 +22,10 @@ def pickle_and_unpickle_object(obj, serializer):
     return obj_from_disk
 
 
+def reset_feature_fraction(boosting_round):
+    return 0.6 if boosting_round < 15 else 0.8
+
+
 @pytest.mark.parametrize('serializer', SERIALIZERS)
 def test_early_stopping_callback_is_picklable(serializer):
     rounds = 5
@@ -53,3 +57,17 @@ def test_record_evaluation_callback_is_picklable(serializer):
     assert callback_from_disk.before_iteration is False
     assert callback.eval_result == callback_from_disk.eval_result
     assert callback.eval_result is results
+
+
+@pytest.mark.parametrize('serializer', SERIALIZERS)
+def test_reset_parameter_callback_is_picklable(serializer):
+    params = {
+        'bagging_fraction': [0.7] * 5 + [0.6] * 5,
+        'feature_fraction': reset_feature_fraction
+    }
+    callback = lgb.reset_parameter(**params)
+    callback_from_disk = pickle_and_unpickle_object(obj=callback, serializer=serializer)
+    assert callback_from_disk.order == 10
+    assert callback_from_disk.before_iteration is True
+    assert callback.kwargs == callback_from_disk.kwargs
+    assert callback.kwargs == params