[backport][sklearn] Fix loading model attributes. (#9808) (#9880)

python-package/xgboost/dask.py

@@ -78,7 +78,6 @@ from .data import _is_cudf_ser, _is_cupy_array
 from .sklearn import (
     XGBClassifier,
     XGBClassifierBase,
-    XGBClassifierMixIn,
     XGBModel,
     XGBRanker,
     XGBRankerMixIn,
@@ -1854,7 +1853,7 @@ class DaskXGBRegressor(DaskScikitLearnBase, XGBRegressorBase):
     "Implementation of the scikit-learn API for XGBoost classification.",
     ["estimators", "model"],
 )
-class DaskXGBClassifier(DaskScikitLearnBase, XGBClassifierMixIn, XGBClassifierBase):
+class DaskXGBClassifier(DaskScikitLearnBase, XGBClassifierBase):
     # pylint: disable=missing-class-docstring
     async def _fit_async(
         self,
@@ -2036,10 +2035,6 @@ class DaskXGBClassifier(DaskScikitLearnBase, XGBClassifierMixIn, XGBClassifierBa
             preds = da.map_blocks(_argmax, pred_probs, drop_axis=1)
         return preds
 
-    def load_model(self, fname: ModelIn) -> None:
-        super().load_model(fname)
-        self._load_model_attributes(self.get_booster())
-
 
 @xgboost_model_doc(
     """Implementation of the Scikit-Learn API for XGBoost Ranking.

python-package/xgboost/sklearn.py

@@ -43,19 +43,6 @@ from .data import _is_cudf_df, _is_cudf_ser, _is_cupy_array, _is_pandas_df
 from .training import train
 
 
-class XGBClassifierMixIn:  # pylint: disable=too-few-public-methods
-    """MixIn for classification."""
-
-    def __init__(self, *args: Any, **kwargs: Any) -> None:
-        super().__init__(*args, **kwargs)
-
-    def _load_model_attributes(self, booster: Booster) -> None:
-        config = json.loads(booster.save_config())
-        self.n_classes_ = int(config["learner"]["learner_model_param"]["num_class"])
-        # binary classification is treated as regression in XGBoost.
-        self.n_classes_ = 2 if self.n_classes_ < 2 else self.n_classes_
-
-
 class XGBRankerMixIn:  # pylint: disable=too-few-public-methods
     """MixIn for ranking, defines the _estimator_type usually defined in scikit-learn
     base classes.
@@ -845,21 +832,38 @@ class XGBModel(XGBModelBase):
         self.get_booster().load_model(fname)
 
         meta_str = self.get_booster().attr("scikit_learn")
-        if meta_str is None:
-            return
+        if meta_str is not None:
+            meta = json.loads(meta_str)
+            t = meta.get("_estimator_type", None)
+            if t is not None and t != self._get_type():
+                raise TypeError(
+                    "Loading an estimator with different type. Expecting: "
+                    f"{self._get_type()}, got: {t}"
+                )
 
-        meta = json.loads(meta_str)
-        t = meta.get("_estimator_type", None)
-        if t is not None and t != self._get_type():
-            raise TypeError(
-                "Loading an estimator with different type. Expecting: "
-                f"{self._get_type()}, got: {t}"
-            )
         self.feature_types = self.get_booster().feature_types
         self.get_booster().set_attr(scikit_learn=None)
+        config = json.loads(self.get_booster().save_config())
+        self._load_model_attributes(config)
 
     load_model.__doc__ = f"""{Booster.load_model.__doc__}"""
 
+    def _load_model_attributes(self, config: dict) -> None:
+        """Load model attributes without hyper-parameters."""
+        from sklearn.base import is_classifier
+
+        booster = self.get_booster()
+
+        self.objective = config["learner"]["objective"]["name"]
+        self.booster = config["learner"]["gradient_booster"]["name"]
+        self.base_score = config["learner"]["learner_model_param"]["base_score"]
+        self.feature_types = booster.feature_types
+
+        if is_classifier(self):
+            self.n_classes_ = int(config["learner"]["learner_model_param"]["num_class"])
+            # binary classification is treated as regression in XGBoost.
+            self.n_classes_ = 2 if self.n_classes_ < 2 else self.n_classes_
+
     # pylint: disable=too-many-branches
     def _configure_fit(
         self,
@@ -1409,7 +1413,7 @@ def _cls_predict_proba(n_classes: int, prediction: PredtT, vstack: Callable) ->
         Number of boosting rounds.
 """,
 )
-class XGBClassifier(XGBModel, XGBClassifierMixIn, XGBClassifierBase):
+class XGBClassifier(XGBModel, XGBClassifierBase):
     # pylint: disable=missing-docstring,invalid-name,too-many-instance-attributes
     @_deprecate_positional_args
     def __init__(
@@ -1637,10 +1641,6 @@ class XGBClassifier(XGBModel, XGBClassifierMixIn, XGBClassifierBase):
     def classes_(self) -> np.ndarray:
         return np.arange(self.n_classes_)
 
-    def load_model(self, fname: ModelIn) -> None:
-        super().load_model(fname)
-        self._load_model_attributes(self.get_booster())
-
 
 @xgboost_model_doc(
     "scikit-learn API for XGBoost random forest classification.",