code: def test_iterator(self):
      a_callback = MagicMock(spec=Callback)
      self.callback_list.append(a_callback)
      self.assertEqual(len(list(self.callback_list)), 2)
  
imports: from unittest import TestCase
  from unittest.mock import MagicMock
  from poutyne import Callback, CallbackList
label_desc: Use image node_id 3 for calling the CallbackListTest obj's underlying member method code with example usage: obj.test_iterator() without return types