code: def test_append_callback(self):
      self.assertEqual(len(self.callback_list.callbacks), 1)
      a_callback = MagicMock(spec=Callback)
      self.callback_list.append(a_callback)
  
      self.assertEqual(len(self.callback_list.callbacks), 2)
  
imports: from unittest import TestCase
  from unittest.mock import MagicMock
  from poutyne import Callback, CallbackList
label_desc: Use image node_id 2 for calling the CallbackListTest obj's underlying member method code with example usage: obj.test_append_callback() without return types