more changes

python-package/lightgbm/basic.py

@@ -3246,8 +3246,13 @@ def __init__(
         elif model_str is not None:
             self.model_from_string(model_str)
             # ensure params are updated on the C++ side
+            # NOTE: models loaded from file are initially set to "boosting: GBDT", so "boosting"
+            #       shouldn't be passed through here
             self.params = params
+            boosting_type = params.pop("boosting", None)
             self.reset_parameter(params)
+            if boosting_type is not None:
+                params["boosting"] = boosting_type
         else:
             raise TypeError('Need at least one training dataset or model file or model string '
                             'to create Booster instance')

tests/python_package_test/test_basic.py

@@ -15,7 +15,7 @@
 import lightgbm as lgb
 from lightgbm.compat import PANDAS_INSTALLED, pd_DataFrame, pd_Series
 
-from .utils import dummy_obj, load_breast_cancer, mse_obj
+from .utils import BOOSTING_TYPES, dummy_obj, load_breast_cancer, mse_obj
 
 
 def test_basic(tmp_path):
@@ -825,8 +825,11 @@ def test_feature_names_are_set_correctly_when_no_feature_names_passed_into_Datas
     assert ds.construct().feature_name == ["Column_0", "Column_1", "Column_2"]
 
 
-def test_booster_deepcopy_preserves_parameters():
+@pytest.mark.parametrize('boosting_type', BOOSTING_TYPES)
+def test_booster_deepcopy_preserves_parameters(boosting_type):
     orig_params = {
+        'boosting': boosting_type,
+        'feature_fraction': 0.708,
         'num_leaves': 5,
         'verbosity': -1
     }
@@ -841,11 +844,19 @@ def test_booster_deepcopy_preserves_parameters():
     assert bst.params["verbosity"] == -1
 
     # passed-in params shouldn't have been modified outside of lightgbm
-    assert orig_params == {'num_leaves': 5, 'verbosity': -1}
+    assert orig_params == {
+        'boosting': boosting_type,
+        'feature_fraction': 0.708,
+        'num_leaves': 5,
+        'verbosity': -1
+    }
 
 
-def test_booster_params_kwarg_overrides_params_from_model_string():
+@pytest.mark.parametrize('boosting_type', BOOSTING_TYPES)
+def test_booster_params_kwarg_overrides_params_from_model_string(boosting_type):
     orig_params = {
+        'boosting': boosting_type,
+        'feature_fraction': 0.708,
         'num_leaves': 5,
         'verbosity': -1
     }
@@ -863,5 +874,14 @@ def test_booster_params_kwarg_overrides_params_from_model_string():
     assert bst2.params["num_leaves"] == 7
     assert "[num_leaves: 7]" in bst2.model_to_string()
 
+    # boosting type should have been preserved in the new model
+    if boosting_type != "gbdt":
+        raise RuntimeError
+
     # passed-in params shouldn't have been modified outside of lightgbm
-    assert orig_params == {'num_leaves': 5, 'verbosity': -1}
+    assert orig_params == {
+        'boosting': boosting_type,
+        'feature_fraction': 0.708,
+        'num_leaves': 5,
+        'verbosity': -1
+    }

tests/python_package_test/utils.py

@@ -10,6 +10,7 @@
 
 import lightgbm as lgb
 
+BOOSTING_TYPES = ['gbdt', 'dart', 'goss', 'rf']
 SERIALIZERS = ["pickle", "joblib", "cloudpickle"]