[python-package] add more type hints on Booster (#5309)

[python-package] add more hints on Booster

python-package/lightgbm/basic.py

@@ -2781,7 +2781,7 @@ class Booster:
         self.network = False
         return self
 
-    def trees_to_dataframe(self):
+    def trees_to_dataframe(self) -> pd_DataFrame:
         """Parse the fitted model and return in an easy-to-read pandas DataFrame.
 
         The returned DataFrame has the following columns.
@@ -2917,7 +2917,7 @@ class Booster:
 
         return pd_DataFrame(model_list, columns=model_list[0].keys())
 
-    def set_train_data_name(self, name):
+    def set_train_data_name(self, name: str) -> "Booster":
         """Set the name to the training Dataset.
 
         Parameters
@@ -2933,7 +2933,7 @@ class Booster:
         self._train_data_name = name
         return self
 
-    def add_valid(self, data, name):
+    def add_valid(self, data: Dataset, name: str) -> "Booster":
         """Add validation data.
 
         Parameters
@@ -2963,7 +2963,7 @@ class Booster:
         self.__is_predicted_cur_iter.append(False)
         return self
 
-    def reset_parameter(self, params):
+    def reset_parameter(self, params: Dict[str, Any]) -> "Booster":
         """Reset parameters of Booster.
 
         Parameters
@@ -3100,7 +3100,7 @@ class Booster:
         self.__is_predicted_cur_iter = [False for _ in range(self.__num_dataset)]
         return is_finished.value == 1
 
-    def rollback_one_iter(self):
+    def rollback_one_iter(self) -> "Booster":
         """Rollback one iteration.
 
         Returns
@@ -3113,7 +3113,7 @@ class Booster:
         self.__is_predicted_cur_iter = [False for _ in range(self.__num_dataset)]
         return self
 
-    def current_iteration(self):
+    def current_iteration(self) -> int:
         """Get the index of the current iteration.
 
         Returns
@@ -3127,7 +3127,7 @@ class Booster:
             ctypes.byref(out_cur_iter)))
         return out_cur_iter.value
 
-    def num_model_per_iteration(self):
+    def num_model_per_iteration(self) -> int:
         """Get number of models per iteration.
 
         Returns
@@ -3141,7 +3141,7 @@ class Booster:
             ctypes.byref(model_per_iter)))
         return model_per_iter.value
 
-    def num_trees(self):
+    def num_trees(self) -> int:
         """Get number of weak sub-models.
 
         Returns
@@ -3155,7 +3155,7 @@ class Booster:
             ctypes.byref(num_trees)))
         return num_trees.value
 
-    def upper_bound(self):
+    def upper_bound(self) -> float:
         """Get upper bound value of a model.
 
         Returns
@@ -3169,7 +3169,7 @@ class Booster:
             ctypes.byref(ret)))
         return ret.value
 
-    def lower_bound(self):
+    def lower_bound(self) -> float:
         """Get lower bound value of a model.
 
         Returns
@@ -3353,7 +3353,7 @@ class Booster:
             ctypes.c_int(end_iteration)))
         return self
 
-    def model_from_string(self, model_str):
+    def model_from_string(self, model_str: str) -> "Booster":
         """Load Booster from a string.
 
         Parameters
@@ -3665,7 +3665,7 @@ class Booster:
         new_booster.network = self.network
         return new_booster
 
-    def get_leaf_output(self, tree_id, leaf_id):
+    def get_leaf_output(self, tree_id: int, leaf_id: int) -> float:
         """Get the output of a leaf.
 
         Parameters