def test_pre_update_validate_accepts_a_change_in_skill_difficulty_only(
            self):
        change = {
            'cmd': question_domain.CMD_CREATE_NEW_FULLY_SPECIFIED_QUESTION,
            'question_dict': {
                'question_state_data': self._create_valid_question_data(
                    'default_state').to_dict(),
                'language_code': 'en',
                'question_state_data_schema_version': (
                    feconf.CURRENT_STATE_SCHEMA_VERSION)
            },
            'skill_id': 'skill_1',
            'skill_difficulty': 0.3
        }

        suggestion = suggestion_registry.SuggestionAddQuestion(
            'exploration.exp1.thread1', 'exp1', 1,
            suggestion_models.STATUS_ACCEPTED, self.author_id,
            self.reviewer_id, change,
            'question.topic_1', 'en', self.fake_date)

        new_change = {
            'cmd': question_domain.CMD_CREATE_NEW_FULLY_SPECIFIED_QUESTION,
            'question_dict': {
                'question_state_data': self._create_valid_question_data(
                    'default_state').to_dict(),
                'language_code': 'en',
                'question_state_data_schema_version': (
                    feconf.CURRENT_STATE_SCHEMA_VERSION)
            },
            'skill_id': 'skill_1',
            'skill_difficulty': 0.6
        }

        self.assertEqual(
            suggestion.pre_update_validate(
                question_domain.QuestionSuggestionChange(new_change)), None)