diff --git a/khaosz/trainer/callback.py b/khaosz/trainer/callback.py
index 4a6ef96..4e55311 100644
--- a/khaosz/trainer/callback.py
+++ b/khaosz/trainer/callback.py
@@ -6,32 +6,66 @@ from typing import cast
 
 
 class TrainerCallback:
+    """ 
+    Callback interface for trainer.
+    and we use '_' to ignore unused parameters.
+    """
+
     def on_train_begin(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the beginning of training.
+        """
+        _ = trainer, kwargs
     
     def on_train_end(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the end of training.
+        """
+        _ = trainer, kwargs
 
     def on_epoch_begin(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the beginning of each epoch.
+        """
+        _ = trainer, kwargs
 
     def on_epoch_end(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the end of each epoch.
+        """
+        _ = trainer, kwargs
 
     def on_batch_begin(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the beginning of each batch.
+        """
+        _ = trainer, kwargs
 
     def on_batch_end(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the end of each batch.
+        """
+        _ = trainer, kwargs
 
     def on_step_begin(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the beginning of each step.
+        """
+
+        _ = trainer, kwargs
 
     def on_step_end(self, trainer: 'Trainer', **kwargs):
-        pass
+        """ 
+        Called at the end of each step.
+        """
+
+        _ = trainer, kwargs
 
 
 class ProgressBarCallback(TrainerCallback):
+    """ 
+    Progress bar callback for trainer.
+    """
     def __init__(self):
         self.progress_bar: tqdm = None
     
@@ -53,16 +87,21 @@ class ProgressBarCallback(TrainerCallback):
         self.progress_bar.update(1)
     
     def on_epoch_end(self, trainer: 'Trainer', **kwargs):
+        _ = trainer, kwargs
         if self.progress_bar:
             self.progress_bar.close()
 
 
 class CheckpointCallback(TrainerCallback):
+    """ 
+    Checkpoint callback for trainer.
+    """
     def __init__(self, checkpoint_interval: int):
         self.checkpoint_interval = checkpoint_interval
         self.last_ckpt_iter = 0
     
     def on_train_begin(self, trainer: 'Trainer', **kwargs):
+        _ = trainer
         checkpoint = cast(Checkpoint, kwargs.get('checkpoint'))
         self.last_ckpt_iter = len(checkpoint.loss_list)
     
@@ -80,8 +119,11 @@ class CheckpointCallback(TrainerCallback):
 
 
 class GradientClippingCallback(TrainerCallback):
-    
+    """ 
+    Gradient clipping callback for trainer.
+    """
     def on_step_begin(self, trainer: 'Trainer', **kwargs):
+        _ = kwargs
         clip_grad_norm_(
             trainer.checkpoint.model.parameters(),
             trainer.train_config.max_grad_norm