diff --git a/python-package/lightgbm/basic.py b/python-package/lightgbm/basic.py
index 3a07cc1cb8f3..0e04539a69d2 100644
--- a/python-package/lightgbm/basic.py
+++ b/python-package/lightgbm/basic.py
@@ -1784,7 +1784,7 @@ def __sample(self, seqs: List[Sequence], total_nrow: int) -> Tuple[List[np.ndarr
     def __init_from_seqs(
         self,
         seqs: List[Sequence],
-        ref_dataset: Optional[_DatasetHandle] = None
+        ref_dataset: Optional[_DatasetHandle]
     ) -> "Dataset":
         """
         Initialize data from list of Sequence objects.
@@ -4275,7 +4275,7 @@ def __inner_eval(
         self,
         data_name: str,
         data_idx: int,
-        feval: Optional[Union[_LGBM_CustomEvalFunction, List[_LGBM_CustomEvalFunction]]] = None
+        feval: Optional[Union[_LGBM_CustomEvalFunction, List[_LGBM_CustomEvalFunction]]]
     ) -> List[_LGBM_BoosterEvalMethodResultType]:
         """Evaluate training or validation data."""
         if data_idx >= self.__num_dataset:
diff --git a/python-package/lightgbm/callback.py b/python-package/lightgbm/callback.py
index 3cd77a831d8f..330bfd5c76a7 100644
--- a/python-package/lightgbm/callback.py
+++ b/python-package/lightgbm/callback.py
@@ -49,7 +49,7 @@ def __init__(self, best_iteration: int, best_score: _EvalResultTuple) -> None:
      "evaluation_result_list"])
 
 
-def _format_eval_result(value: _EvalResultTuple, show_stdv: bool = True) -> str:
+def _format_eval_result(value: _EvalResultTuple, show_stdv: bool) -> str:
     """Format metric string."""
     if len(value) == 4:
         return f"{value[0]}'s {value[1]}: {value[2]:g}"
@@ -338,7 +338,7 @@ def _init(self, env: CallbackEnv) -> None:
     def _final_iteration_check(self, env: CallbackEnv, eval_name_splitted: List[str], i: int) -> None:
         if env.iteration == env.end_iteration - 1:
             if self.verbose:
-                best_score_str = '\t'.join([_format_eval_result(x) for x in self.best_score_list[i]])
+                best_score_str = '\t'.join([_format_eval_result(x, show_stdv=True) for x in self.best_score_list[i]])
                 _log_info('Did not meet early stopping. '
                           f'Best iteration is:\n[{self.best_iter[i] + 1}]\t{best_score_str}')
                 if self.first_metric_only:
@@ -364,7 +364,7 @@ def __call__(self, env: CallbackEnv) -> None:
                 continue  # train data for lgb.cv or sklearn wrapper (underlying lgb.train)
             elif env.iteration - self.best_iter[i] >= self.stopping_rounds:
                 if self.verbose:
-                    eval_result_str = '\t'.join([_format_eval_result(x) for x in self.best_score_list[i]])
+                    eval_result_str = '\t'.join([_format_eval_result(x, show_stdv=True) for x in self.best_score_list[i]])
                     _log_info(f"Early stopping, best iteration is:\n[{self.best_iter[i] + 1}]\t{eval_result_str}")
                     if self.first_metric_only:
                         _log_info(f"Evaluated only: {eval_name_splitted[-1]}")
diff --git a/python-package/lightgbm/engine.py b/python-package/lightgbm/engine.py
index 93e7953fc6a6..c112f762e946 100644
--- a/python-package/lightgbm/engine.py
+++ b/python-package/lightgbm/engine.py
@@ -434,10 +434,10 @@ def _make_n_folds(
     nfold: int,
     params: Dict[str, Any],
     seed: int,
-    fpreproc: Optional[_LGBM_PreprocFunction] = None,
-    stratified: bool = True,
-    shuffle: bool = True,
-    eval_train_metric: bool = False
+    fpreproc: Optional[_LGBM_PreprocFunction],
+    stratified: bool,
+    shuffle: bool,
+    eval_train_metric: bool
 ) -> CVBooster:
     """Make a n-fold list of Booster from random indices."""
     full_data = full_data.construct()
@@ -685,7 +685,7 @@ def cv(
              .set_categorical_feature(categorical_feature)
 
     results = collections.defaultdict(list)
-    cvfolds = _make_n_folds(train_set, folds=folds, nfold=nfold,
+    cvfolds = _make_n_folds(full_data=train_set, folds=folds, nfold=nfold,
                             params=params, seed=seed, fpreproc=fpreproc,
                             stratified=stratified, shuffle=shuffle,
                             eval_train_metric=eval_train_metric)
diff --git a/python-package/lightgbm/plotting.py b/python-package/lightgbm/plotting.py
index 483dcddbc96a..58b26b0f5f3f 100644
--- a/python-package/lightgbm/plotting.py
+++ b/python-package/lightgbm/plotting.py
@@ -20,13 +20,13 @@
 ]
 
 
-def _check_not_tuple_of_2_elements(obj: Any, obj_name: str = 'obj') -> None:
+def _check_not_tuple_of_2_elements(obj: Any, obj_name: str) -> None:
     """Check object is not tuple or does not have 2 elements."""
     if not isinstance(obj, tuple) or len(obj) != 2:
         raise TypeError(f"{obj_name} must be a tuple of 2 elements.")
 
 
-def _float2str(value: float, precision: Optional[int] = None) -> str:
+def _float2str(value: float, precision: Optional[int]) -> str:
     return (f"{value:.{precision}f}"
             if precision is not None and not isinstance(value, str)
             else str(value))