[Feature] Change study to a property (#5)

Change `optuna.Study` to property

mqboost/engine.py

@@ -207,13 +207,13 @@ def _study_func(trial: optuna.Trial) -> float:
                 get_params_func=get_params_func,
             )
 
-        study = optuna.create_study(
+        self._study = optuna.create_study(
             study_name=f"MQBoost_{self._model}",
             direction="minimize",
             load_if_exists=True,
         )
-        study.optimize(_study_func, n_trials=n_trials)
-        return study.best_params
+        self._study.optimize(_study_func, n_trials=n_trials)
+        return self._study.best_params
 
     def __optuna_objective(
         self,
@@ -282,3 +282,7 @@ def __is_xgb(self) -> bool:
     def __is_fitted(self) -> None:
         if not getattr(self, "_fitted", False):
             raise FittingException("train must be executed before predict")
+
+    @property
+    def study(self) -> optuna.Study:
+        return getattr(self, "_study", None)