Mock the Failure

29.05.2025 22:13 python django test pytest

I’m not a big fan of mocking, especially when working with databases. For me, when we create tests involving a database, we can’t just mock the result because I want to make sure my code actually stores the correct data. I think it is important to verify that the data is stored correctly during testing. Even if we have a nice algorithm in our code, if the stored data is incorrect, it becomes useless.

But that doesn’t mean I’m not using mock at all. Sometimes I do use mock, for example, in this case, to mock a failure.

Lets look at this snippet.

# app/service.py
def insert_retry(data) -> int:
    count = 1
    while count <= 5:
        try:
            Play.objects.create(**data)
            break
        except Exception as e:
            print(e)
            count += 1
    return count

It already uses the ORM, so I think we don’t need to test simple CRUD operations. But for this snippet, we still need to check the failure case, does it still retry up to 5 times if something goes wrong? To test that behavior, I used mock.

class InsertRetryTest(TestCase):

    @patch('app.service.Play.objects.create')
    def test_retry_always_fail(self, mock_create):

        before = Play.objects.count()
        mock_create.side_effect = Exception("Always Error")

        data = {
            "title": "test",
            "description": "desc"
        }
        result = insert_retry(data)

        after = Play.objects.count()

        # count start from 1 so will stop at 6
        self.assertEqual(result, 6)
        # make sure we call the ORM 5 times
        self.assertEqual(mock_create.call_count, 5)
        # before and after equal, no insert
        self.assertEqual(after, before)

Then run the test. By the way, this example I uses Django.

./manage.py test

Creating test database for alias 'default'...
System check identified no issues (0 silenced).
.
----------------------------------------------------------------------
Ran 1 tests in 0.001s

OK

And because we used mock for the failure, we can try another scenario, let’s update our snippet.

def insert_retry(data) -> int:
    count = 1
    while count <= 5:
        try:
            Play.objects.create(**data)
            break
        except (ValueError, DatabaseError):
            # no need to retry
            return count
        except Exception as e:
            count += 1
    return count

In this updated snippet, we separate the types of failures. Sometimes, we return early, while some errors still need to be retried. So we can update the test like this.

class InsertRetryTest(TestCase):

    @patch('app.service.Play.objects.create')
    def test_retry_always_fail(self, mock_create):

        before = Play.objects.count()
        mock_create.side_effect = Exception("Always Error")

        data = {
            "title": "test",
            "description": "desc"
        }
        result = insert_retry(data)

        after = Play.objects.count()

        # count start from 1 so will stop at 6
        self.assertEqual(result, 6)
        # make sure we call the ORM 5 times
        self.assertEqual(mock_create.call_count, 5)
        # before and after equal, no insert
        self.assertEqual(after, before)

    @patch('app.service.Play.objects.create')
    def test_no_retry_fail(self, mock_create):
        before = Play.objects.count()
        mock_create.side_effect = [ValueError, DatabaseError]

        data = {
            "title": "test",
            "description": "desc"
        }
        result = insert_retry(data)

        after = Play.objects.count()

        # count start from 1 so will stop at 1
        self.assertEqual(result, 1)
        # make sure we call the ORM 1 time
        self.assertEqual(mock_create.call_count, 1)
        # before and after equal, no insert
        self.assertEqual(after, before)

And Run again the test

Creating test database for alias 'default'...
System check identified no issues (0 silenced).
..
----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK