I'm Matthew J. Morrison.

A Passionate, professional software developer & hobbyist; Language nerd & regular user of Unix, Python, Ruby & JavaScript.

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Mocking Django

February 20, 2011

This is going to be a fairly long article, so I'll try not to ramble. I've been using Django for about a year now, an I've been doing TDD with Django for a while as well. Out of the box Django includes some pretty awesome testing tools. They've got a test runner that will bootstrap your database and will take care of cleaning up the database between tests. Django also includes a super easy way to use fixtures in tests, and also has a lot of useful assertions. All of these things are great tools and make test driving Django applications very easy. The downside is that when you are relying on the database and fixtures in your unit test suite inevitably it will end up taking a long time to run all tests, which is where mocking comes in to save the day.

When it comes to mocking in Django, I reccomend the mock library. Another advantage, besides faster tests, of using mocking instead of Django's default testing behavior is that you don't need django to setup the database to run your tests, and you can subclass unittest's TestCase instead of django TestCase (which saves you the database cleanup between tests). With the examples provided here I can run these with manage.py test django_testing or with python -m unittest discover -s django_testing (using Python2.7). Let's jump right into some code examples.

from django.db import models
 
class SampleManager(models.Manager):
    def get_by_user(self, user):
        self.filter(user=user)
 
class Sample(models.Model):
    pass


import unittest
import mock
from django_testing import models
 
class SampleTests(unittest.TestCase):
    def test_filters_by_user(self):
        user = mock.Mock()
        manager = mock.Mock(spec=models.SampleManager)
        models.SampleManager.get_by_user(manager, user)
        manager.filter.assert_called_with(user=user)


Let's walk though the test line by line. First, create a mock user. user = mock.Mock() Second, create a mock manager. manager = mock.Mock(spec=models.SampleManager) Next, call the method that you want to have do something. You'll notice something a bit of trickery here, I'm using the actual SampleManager class and passing my manager mock object in as the self argument. This allows us to capture what our implementation code does with our manager inside the get_by_user method. models.SampleManager.get_by_user(manager, user) Finally, assert that your desired result occured. Here, we are asserting that the filter method of our manager mock object was called with the keyword user argument with the value of our user mock. manager.filter.assert_called_with(user=user)

Let's take a look at a different way to write that same test.

    @mock.patch('django_testing.models.SampleManager.filter', mock.Mock())
    def test_filters_by_user_with_patch(self):
        user = mock.Mock()
        models.Sample.objects.get_by_user(user)
        models.Sample.objects.filter.assert_called_with(user=user)

Here, I used the mock library's patch decorator to mock the filter method on the SampleManager class instead of using the 'mock as self' trickery. Let's look at one more way to write this test.

    @mock.patch('django_testing.models.SampleManager.filter')
    def test_filters_by_user_with_patch_and_filter_passed_in(self, filter_method):
        user = mock.Mock()
        models.Sample.objects.get_by_user(user)
        filter_method.assert_called_with(user=user)

In this example, I'm using the patch decorator a little bit differently. By omitting the second argument, the patch decorator will pass the mock into your test method which will then allow you to do assertions directly against it. Now, say we want to check for specific return values, consider this test.

    @mock.patch('django_testing.models.SampleManager.get_last')
    @mock.patch('django_testing.models.SampleManager.get_first')
    def test_result_of_one_query_in_args_of_another(self, get_first, get_last):
        result = models.Sample.objects.get_first_and_last()
        self.assertEqual((get_first.return_value, get_last.return_value), result)

We want to make sure that the result of get_first_and_last returns a tuple of the result of get_first and get_last. Our implementation code would look like this.

from django.db import models
 
class SampleManager(models.Manager):
    def get_first(self):
        pass
    def get_last(self):
        pass
    def get_first_and_last(self):
        return self.get_first(), self.get_last()
 
class Sample(models.Model):
    objects = SampleManager()

That is really all there is to getting started with mocking django. There are a few more advanced things that I will save for a follow up post, so stay tuned for that. I hope this has been helpful, thanks for reading.