[python-package] add type hints on Dataset feature processing (#5745)

python-package/lightgbm/basic.py

@@ -33,6 +33,8 @@
 _LGBM_EvalFunctionResultType = Tuple[str, float, bool]
 _LGBM_BoosterBestScoreType = Dict[str, Dict[str, float]]
 _LGBM_BoosterEvalMethodResultType = Tuple[str, str, float, bool]
+_LGBM_CategoricalFeatureConfiguration = Union[List[str], List[int], str]
+_LGBM_FeatureNameConfiguration = Union[List[str], str]
 _LGBM_LabelType = Union[
     list,
     np.ndarray,
@@ -588,7 +590,12 @@ def _check_for_bad_pandas_dtypes(pandas_dtypes_series: pd_Series) -> None:
                          f'Fields with bad pandas dtypes: {", ".join(bad_pandas_dtypes)}')
 
 
-def _data_from_pandas(data, feature_name, categorical_feature, pandas_categorical):
+def _data_from_pandas(
+    data,
+    feature_name: Optional[_LGBM_FeatureNameConfiguration],
+    categorical_feature: Optional[_LGBM_CategoricalFeatureConfiguration],
+    pandas_categorical: Optional[List[List]]
+):
     if isinstance(data, pd_DataFrame):
         if len(data.shape) != 2 or data.shape[0] < 1:
             raise ValueError('Input data must be 2 dimensional and non empty.')
@@ -638,7 +645,10 @@ def _data_from_pandas(data, feature_name, categorical_feature, pandas_categorica
     return data, feature_name, categorical_feature, pandas_categorical
 
 
-def _dump_pandas_categorical(pandas_categorical, file_name=None):
+def _dump_pandas_categorical(
+    pandas_categorical: Optional[List[List]],
+    file_name: Optional[Union[str, Path]] = None
+) -> str:
     categorical_json = json.dumps(pandas_categorical, default=_json_default_with_numpy)
     pandas_str = f'\npandas_categorical:{categorical_json}\n'
     if file_name is not None:
@@ -650,7 +660,7 @@ def _dump_pandas_categorical(pandas_categorical, file_name=None):
 def _load_pandas_categorical(
     file_name: Optional[Union[str, Path]] = None,
     model_str: Optional[str] = None
-) -> Optional[str]:
+) -> Optional[List[List]]:
     pandas_key = 'pandas_categorical:'
     offset = -len(pandas_key)
     if file_name is not None:
@@ -1320,8 +1330,8 @@ def __init__(
         weight=None,
         group=None,
         init_score=None,
-        feature_name='auto',
-        categorical_feature='auto',
+        feature_name: _LGBM_FeatureNameConfiguration = 'auto',
+        categorical_feature: _LGBM_CategoricalFeatureConfiguration = 'auto',
         params: Optional[Dict[str, Any]] = None,
         free_raw_data: bool = True
     ):
@@ -1371,8 +1381,8 @@ def __init__(
         self.weight = weight
         self.group = group
         self.init_score = init_score
-        self.feature_name = feature_name
-        self.categorical_feature = categorical_feature
+        self.feature_name: _LGBM_FeatureNameConfiguration = feature_name
+        self.categorical_feature: _LGBM_CategoricalFeatureConfiguration = categorical_feature
         self.params = deepcopy(params)
         self.free_raw_data = free_raw_data
         self.used_indices: Optional[List[int]] = None
@@ -2294,13 +2304,13 @@ def get_field(self, field_name: str) -> Optional[np.ndarray]:
 
     def set_categorical_feature(
         self,
-        categorical_feature: Union[List[int], List[str], str]
+        categorical_feature: _LGBM_CategoricalFeatureConfiguration
     ) -> "Dataset":
         """Set categorical features.
 
         Parameters
         ----------
-        categorical_feature : list of int or str
+        categorical_feature : list of str or int, or 'auto'
             Names or indices of categorical features.
 
         Returns
@@ -3937,8 +3947,8 @@ def refit(
         weight=None,
         group=None,
         init_score=None,
-        feature_name: Union[str, List[str]] = 'auto',
-        categorical_feature: Union[str, List[str], List[int]] = 'auto',
+        feature_name: _LGBM_FeatureNameConfiguration = 'auto',
+        categorical_feature: _LGBM_CategoricalFeatureConfiguration = 'auto',
         dataset_params: Optional[Dict[str, Any]] = None,
         free_raw_data: bool = True,
         validate_features: bool = False,

python-package/lightgbm/engine.py

@@ -11,7 +11,8 @@
 
 from . import callback
 from .basic import (Booster, Dataset, LightGBMError, _choose_param_value, _ConfigAliases, _InnerPredictor,
-                    _LGBM_CustomObjectiveFunction, _log_warning)
+                    _LGBM_CategoricalFeatureConfiguration, _LGBM_CustomObjectiveFunction,
+                    _LGBM_FeatureNameConfiguration, _log_warning)
 from .compat import SKLEARN_INSTALLED, _LGBMBaseCrossValidator, _LGBMGroupKFold, _LGBMStratifiedKFold
 
 __all__ = [
@@ -40,8 +41,8 @@ def train(
     valid_names: Optional[List[str]] = None,
     feval: Optional[Union[_LGBM_CustomMetricFunction, List[_LGBM_CustomMetricFunction]]] = None,
     init_model: Optional[Union[str, Path, Booster]] = None,
-    feature_name: Union[List[str], str] = 'auto',
-    categorical_feature: Union[List[str], List[int], str] = 'auto',
+    feature_name: _LGBM_FeatureNameConfiguration = 'auto',
+    categorical_feature: _LGBM_CategoricalFeatureConfiguration = 'auto',
     keep_training_booster: bool = False,
     callbacks: Optional[List[Callable]] = None
 ) -> Booster:
@@ -523,8 +524,8 @@ def cv(
     metrics: Optional[Union[str, List[str]]] = None,
     feval: Optional[Union[_LGBM_CustomMetricFunction, List[_LGBM_CustomMetricFunction]]] = None,
     init_model: Optional[Union[str, Path, Booster]] = None,
-    feature_name: Union[str, List[str]] = 'auto',
-    categorical_feature: Union[str, List[str], List[int]] = 'auto',
+    feature_name: _LGBM_FeatureNameConfiguration = 'auto',
+    categorical_feature: _LGBM_CategoricalFeatureConfiguration = 'auto',
     fpreproc: Optional[_LGBM_PreprocFunction] = None,
     seed: int = 0,
     callbacks: Optional[List[Callable]] = None,