如何在传递到任务队列之前测试是否保存了模型？

Question

In the following method the correct behaviour would to to save the news model before passing its id to the asynchronous Task queue for further processing. Otherwise the task queue has the older version.

As you can see I have made a mistake and am saving the model after sending its id to the task queue.

def save_scraped_body_into_model(url_string):
        news = ndb.Key(urlsafe=url_string).get()
        ...
        news.body = result
        news.stage = 1
        taskqueue.Task(url='/api/v1.0/worker/bbc-stage-2', headers=header,
                           payload=json.dumps({'news_url_string': news.key.urlsafe()})).add(queue_name='newstasks')
        news.put()

How do I possibly test for that? My test below passes as long as the model was saved. But thats wrong, the order matters, and this test doesn't capture it !!! Is there a way to achieve this with mock?

def test_news_instance_saved_before_next_stage(self, get_head):
        BBCSpider.save_scraped_body_into_model(self.news.key.urlsafe())
        context = ndb.get_context()
        context.clear_cache()
        news = ndb.Key(urlsafe=self.news.key.urlsafe()).get()
        self.assertEqual(news.stage, 1)

Answer 1

I'm doing these kind of tests by using patch from unittest.mock framework .

When I need to do test like this the first thing I do is looking for a point where I can put a hook. Then I patch the hook and use side_effect callback to do the test.

In your case the hook will be BBCSpider.taskqueue.Task and its side_effect something like this:

lambda *args,**kwargs: self.assertEqual(1, ndb.Key(urlsafe=self.news.key.urlsafe()).get().stage)

So your test become:

@patch('BBCSpider.taskqueue.Task', autospec=True)
def test_news_instance_saved_before_next_stage(self, get_head, mock_task):
    def check_if_model_saved(*args,**kwargs):
        news = ndb.Key(urlsafe=self.news.key.urlsafe()).get()
        self.assertEqual(news.stage,1)
    mock_task.side_effect = check_if_model_saved
    BBCSpider.save_scraped_body_into_model(self.news.key.urlsafe())
    self.assertTrue(mock_task.called)

Note autospec=True is not mandatory but I like to use it every time I do a patch to avoid silly errors.

I'm apologize if the code contains some mistake (I cannot test it without considerable effort) but I hope the idea is clear.