more tests

python-package/lightgbm/basic.py

@@ -3245,14 +3245,14 @@ def __init__(
             params = self._get_loaded_param()
         elif model_str is not None:
             self.model_from_string(model_str)
+            # ensure params are updated on the C++ side
+            self.params = params
+            self.reset_parameter(params)
         else:
             raise TypeError('Need at least one training dataset or model file or model string '
                             'to create Booster instance')
         self.params = params
 
-        # ensure params are updated on the C++ side
-        self.reset_parameter(params)
-
     def __del__(self) -> None:
         try:
             if self._network:

tests/python_package_test/test_basic.py

@@ -826,26 +826,42 @@ def test_feature_names_are_set_correctly_when_no_feature_names_passed_into_Datas
 
 
 def test_booster_deepcopy_preserves_parameters():
+    orig_params = {
+        'num_leaves': 5,
+        'verbosity': -1
+    }
     bst = lgb.train(
-        params={'num_leaves': 5, 'verbosity': -1},
+        params=orig_params,
         num_boost_round=2,
         train_set=lgb.Dataset(np.random.rand(100, 2))
     )
-    bst2 = copy.deepcopy(bst)
+    bst2 = deepcopy(bst)
     assert bst2.params == bst.params
     assert bst.params["num_leaves"] == 5
     assert bst.params["verbosity"] == -1
 
+    # passed-in params shouldn't have been modified outside of lightgbm
+    assert orig_params == {'num_leaves': 5, 'verbosity': -1}
+
 
 def test_booster_params_kwarg_overrides_params_from_model_string():
+    orig_params = {
+        'num_leaves': 5,
+        'verbosity': -1
+    }
     bst = lgb.train(
-        params={'num_leaves': 5, 'learning_rate': 0.708, 'verbosity': -1},
+        params=orig_params,
         num_boost_round=2,
         train_set=lgb.Dataset(np.random.rand(100, 2))
     )
     bst2 = lgb.Booster(
         params={'num_leaves': 7},
         model_str=bst.model_to_string()
     )
+
+    # params should have been updated on the Python object and the C++ side
     assert bst2.params["num_leaves"] == 7
-    assert bst2.params["learning_rate"] == 0.708
+    assert "[num_leaves: 7]" in bst2.model_to_string()
+
+    # passed-in params shouldn't have been modified outside of lightgbm
+    assert orig_params == {'num_leaves': 5, 'verbosity': -1}