[python-package] make _InnerPredictor construction stricter

python-package/lightgbm/basic.py

@@ -33,6 +33,7 @@
     'Sequence',
 ]
 
+_BoosterHandle = ctypes.c_void_p
 _DatasetHandle = ctypes.c_void_p
 _ctypes_int_ptr = Union[
     "ctypes._Pointer[ctypes.c_int32]",
@@ -837,52 +838,98 @@ class _InnerPredictor:
 
     def __init__(
         self,
-        model_file: Optional[Union[str, Path]] = None,
-        booster_handle: Optional[ctypes.c_void_p] = None,
-        pred_parameter: Optional[Dict[str, Any]] = None
+        booster_handle: _BoosterHandle,
+        pandas_categorical: Optional[List[List]],
+        pred_parameter: Dict[str, Any],
+        manage_handle: bool
     ):
         """Initialize the _InnerPredictor.
 
         Parameters
         ----------
-        model_file : str, pathlib.Path or None, optional (default=None)
-            Path to the model file.
-        booster_handle : object or None, optional (default=None)
+        booster_handle : object
             Handle of Booster.
-        pred_parameter: dict or None, optional (default=None)
+        pandas_categorical : list of list, or None
+            If provided, list of categories for ``pandas`` categorical columns.
+            Where the ``i``th element of the list contains the categories for the ``i``th categorical feature.
+        pred_parameter : dict
             Other parameters for the prediction.
+        manage_handle : bool
+            If ``True``, free the corresponding Booster on the C++ side when this Python object is deleted.
         """
-        self._handle = ctypes.c_void_p()
-        self.__is_manage_handle = True
-        if model_file is not None:
-            """Prediction task"""
-            out_num_iterations = ctypes.c_int(0)
-            _safe_call(_LIB.LGBM_BoosterCreateFromModelfile(
-                _c_str(str(model_file)),
-                ctypes.byref(out_num_iterations),
-                ctypes.byref(self._handle)))
-            out_num_class = ctypes.c_int(0)
-            _safe_call(_LIB.LGBM_BoosterGetNumClasses(
-                self._handle,
-                ctypes.byref(out_num_class)))
-            self.num_class = out_num_class.value
-            self.num_total_iteration = out_num_iterations.value
-            self.pandas_categorical = _load_pandas_categorical(file_name=model_file)
-        elif booster_handle is not None:
-            self.__is_manage_handle = False
-            self._handle = booster_handle
-            out_num_class = ctypes.c_int(0)
-            _safe_call(_LIB.LGBM_BoosterGetNumClasses(
+        self._handle = booster_handle
+        self.__is_manage_handle = manage_handle
+        self.pandas_categorical = pandas_categorical
+        self.pred_parameter = _param_dict_to_str(pred_parameter)
+
+        out_num_class = ctypes.c_int(0)
+        _safe_call(
+            _LIB.LGBM_BoosterGetNumClasses(
                 self._handle,
-                ctypes.byref(out_num_class)))
-            self.num_class = out_num_class.value
-            self.num_total_iteration = self.current_iteration()
-            self.pandas_categorical = None
-        else:
-            raise TypeError('Need model_file or booster_handle to create a predictor')
+                ctypes.byref(out_num_class)
+            )
+        )
+        self.num_class = out_num_class.value
 
-        pred_parameter = {} if pred_parameter is None else pred_parameter
-        self.pred_parameter = _param_dict_to_str(pred_parameter)
+    @classmethod
+    def from_booster(
+        cls,
+        booster: "Booster",
+        pred_parameter: Dict[str, Any]
+    ) -> "_InnerPredictor":
+        """Initialize an ``_InnerPredictor`` from a ``Booster``.
+
+        Parameters
+        ----------
+        booster : Booster
+            Booster.
+        pred_parameter : dict
+            Other parameters for the prediction.
+        """
+        out_cur_iter = ctypes.c_int(0)
+        _safe_call(
+            _LIB.LGBM_BoosterGetCurrentIteration(
+                booster._handle,
+                ctypes.byref(out_cur_iter)
+            )
+        )
+        return cls(
+            booster_handle=booster._handle,
+            pandas_categorical=booster.pandas_categorical,
+            pred_parameter=pred_parameter,
+            manage_handle=False
+        )
+
+    @classmethod
+    def from_model_file(
+        cls,
+        model_file: Union[str, Path],
+        pred_parameter: Dict[str, Any]
+    ) -> "_InnerPredictor":
+        """Initialize an ``_InnerPredictor`` from a text file containing a LightGBM model.
+
+        Parameters
+        ----------
+        model_file : str or pathlib.Path
+            Path to the model file.
+        pred_parameter : dict
+            Other parameters for the prediction.
+        """
+        booster_handle = ctypes.c_void_p()
+        out_num_iterations = ctypes.c_int(0)
+        _safe_call(
+            _LIB.LGBM_BoosterCreateFromModelfile(
+                _c_str(str(model_file)),
+                ctypes.byref(out_num_iterations),
+                ctypes.byref(booster_handle)
+            )
+        )
+        return cls(
+            booster_handle=booster_handle,
+            pandas_categorical=_load_pandas_categorical(file_name=model_file),
+            pred_parameter=pred_parameter,
+            manage_handle=True
+        )
 
     def __del__(self) -> None:
         try:
@@ -3046,7 +3093,7 @@ def __init__(
         model_str : str or None, optional (default=None)
             Model will be loaded from this string.
         """
-        self._handle = None
+        self._handle = ctypes.c_void_p()
         self._network = False
         self.__need_reload_eval_info = True
         self._train_data_name = "training"
@@ -3097,7 +3144,6 @@ def __init__(
             # copy the parameters from train_set
             params.update(train_set.get_params())
             params_str = _param_dict_to_str(params)
-            self._handle = ctypes.c_void_p()
             _safe_call(_LIB.LGBM_BoosterCreate(
                 train_set._handle,
                 _c_str(params_str),
@@ -3126,7 +3172,6 @@ def __init__(
         elif model_file is not None:
             # Prediction task
             out_num_iterations = ctypes.c_int(0)
-            self._handle = ctypes.c_void_p()
             _safe_call(_LIB.LGBM_BoosterCreateFromModelfile(
                 _c_str(str(model_file)),
                 ctypes.byref(out_num_iterations),
@@ -3905,8 +3950,9 @@ def model_from_string(self, model_str: str) -> "Booster":
         self : Booster
             Loaded Booster object.
         """
-        if self._handle is not None:
-            _safe_call(_LIB.LGBM_BoosterFree(self._handle))
+        # ensure that existing Booster is freed before replacing it
+        # with a new one createdfrom file
+        _safe_call(_LIB.LGBM_BoosterFree(self._handle))
         self._free_buffer()
         self._handle = ctypes.c_void_p()
         out_num_iterations = ctypes.c_int(0)
@@ -4106,7 +4152,10 @@ def predict(
             Prediction result.
             Can be sparse or a list of sparse objects (each element represents predictions for one class) for feature contributions (when ``pred_contrib=True``).
         """
-        predictor = self._to_predictor(pred_parameter=deepcopy(kwargs))
+        predictor = _InnerPredictor.from_booster(
+            booster=self,
+            pred_parameter=deepcopy(kwargs),
+        )
         if num_iteration is None:
             if start_iteration <= 0:
                 num_iteration = self.best_iteration
@@ -4223,7 +4272,10 @@ def refit(
             raise LightGBMError('Cannot refit due to null objective function.')
         if dataset_params is None:
             dataset_params = {}
-        predictor = self._to_predictor(pred_parameter=deepcopy(kwargs))
+        predictor = _InnerPredictor.from_booster(
+            booster=self,
+            pred_parameter=deepcopy(kwargs)
+        )
         leaf_preds: np.ndarray = predictor.predict(  # type: ignore[assignment]
             data=data,
             start_iteration=-1,
@@ -4327,15 +4379,6 @@ def set_leaf_output(
         )
         return self
 
-    def _to_predictor(
-        self,
-        pred_parameter: Dict[str, Any]
-    ) -> _InnerPredictor:
-        """Convert to predictor."""
-        predictor = _InnerPredictor(booster_handle=self._handle, pred_parameter=pred_parameter)
-        predictor.pandas_categorical = self.pandas_categorical
-        return predictor
-
     def num_feature(self) -> int:
         """Get number of features.
 

python-package/lightgbm/engine.py

@@ -183,10 +183,20 @@ def train(
 
     predictor: Optional[_InnerPredictor] = None
     if isinstance(init_model, (str, Path)):
-        predictor = _InnerPredictor(model_file=init_model, pred_parameter=params)
+        predictor = _InnerPredictor.from_model_file(
+            model_file=init_model,
+            pred_parameter=params
+        )
     elif isinstance(init_model, Booster):
-        predictor = init_model._to_predictor(pred_parameter=dict(init_model.params, **params))
-    init_iteration = predictor.num_total_iteration if predictor is not None else 0
+        predictor = _InnerPredictor.from_booster(
+            booster=init_model,
+            pred_parameter=dict(init_model.params, **params)
+        )
+
+    if predictor is not None:
+        init_iteration = predictor.current_iteration()
+    else:
+        init_iteration = 0
 
     train_set._update_params(params) \
              ._set_predictor(predictor) \
@@ -685,9 +695,15 @@ def cv(
     first_metric_only = params.get('first_metric_only', False)
 
     if isinstance(init_model, (str, Path)):
-        predictor = _InnerPredictor(model_file=init_model, pred_parameter=params)
+        predictor = _InnerPredictor.from_model_file(
+            model_file=init_model,
+            pred_parameter=params
+        )
     elif isinstance(init_model, Booster):
-        predictor = init_model._to_predictor(pred_parameter=dict(init_model.params, **params))
+        predictor = _InnerPredictor.from_booster(
+            booster=init_model,
+            pred_parameter=dict(init_model.params, **params)
+        )
     else:
         predictor = None