[python] preserve None in `_choose_param_value()` (#5289)

* [python] preserve None in _choose_param_value()

* Update python-package/lightgbm/basic.py
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

python-package/lightgbm/basic.py

@@ -408,21 +408,27 @@ def _choose_param_value(main_param_name: str, params: Dict[str, Any], default_va
     # avoid side effects on passed-in parameters
     params = deepcopy(params)
 
-    # find a value, and remove other aliases with .pop()
-    # prefer the value of 'main_param_name' if it exists, otherwise search the aliases
-    found_value = None
+    aliases = _ConfigAliases.get(main_param_name) - {main_param_name}
+
+    # if main_param_name was provided, keep that value and remove all aliases
     if main_param_name in params.keys():
-        found_value = params[main_param_name]
+        for param in aliases:
+            params.pop(param, None)
+        return params
 
-    for param in _ConfigAliases.get(main_param_name):
-        val = params.pop(param, None)
-        if found_value is None and val is not None:
-            found_value = val
+    # if main param name was not found, search for an alias
+    for param in aliases:
+        if param in params.keys():
+            params[main_param_name] = params[param]
+            break
 
-    if found_value is not None:
-        params[main_param_name] = found_value
-    else:
-        params[main_param_name] = default_value
+    if main_param_name in params.keys():
+        for param in aliases:
+            params.pop(param, None)
+        return params
+
+    # neither of main_param_name, aliases were found
+    params[main_param_name] = default_value
 
     return params
 

tests/python_package_test/test_basic.py

@@ -513,6 +513,42 @@ def test_choose_param_value():
     assert original_params == expected_params
 
 
+def test_choose_param_value_preserves_nones():
+
+    # preserves None found for main param and still removes aliases
+    params = lgb.basic._choose_param_value(
+        main_param_name="num_threads",
+        params={
+            "num_threads": None,
+            "n_jobs": 4,
+            "objective": "regression"
+        },
+        default_value=2
+    )
+    assert params == {"num_threads": None, "objective": "regression"}
+
+    # correctly chooses value when only an alias is provided
+    params = lgb.basic._choose_param_value(
+        main_param_name="num_threads",
+        params={
+            "n_jobs": None,
+            "objective": "regression"
+        },
+        default_value=2
+    )
+    assert params == {"num_threads": None, "objective": "regression"}
+
+    # adds None if that's given as the default and param not found
+    params = lgb.basic._choose_param_value(
+        main_param_name="min_data_in_leaf",
+        params={
+            "objective": "regression"
+        },
+        default_value=None
+    )
+    assert params == {"objective": "regression", "min_data_in_leaf": None}
+
+
 @pytest.mark.parametrize("objective_alias", lgb.basic._ConfigAliases.get("objective"))
 def test_choose_param_value_objective(objective_alias):
     # If callable is found in objective