Fix a bug for `CallbackHandler.callback_list` (#8052)

* Fix callback_list

* Add test
Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

* Fix test
Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

src/transformers/trainer_callback.py

@@ -325,7 +325,7 @@ class CallbackHandler(TrainerCallback):
 
     @property
     def callback_list(self):
-        return "\n".join(self.callbacks)
+        return "\n".join(cb.__class__.__name__ for cb in self.callbacks)
 
     def on_init_end(self, args: TrainingArguments, state: TrainerState, control: TrainerControl):
         return self.call_event("on_init_end", args, state, control)

tests/test_trainer_callback.py

@@ -221,3 +221,10 @@ class TrainerCallbackTest(unittest.TestCase):
         trainer.train()
         events = trainer.callback_handler.callbacks[-2].events
         self.assertEqual(events, self.get_expected_events(trainer))
+
+        # warning should be emitted for duplicated callbacks
+        with unittest.mock.patch("transformers.trainer_callback.logger.warn") as warn_mock:
+            trainer = self.get_trainer(
+                callbacks=[MyTestTrainerCallback, MyTestTrainerCallback],
+            )
+            assert str(MyTestTrainerCallback) in warn_mock.call_args[0][0]