A Django Unit Testing Primer

One thing I really slacked on while developing GreaterDebater was testing. Maybe that's not the right way to put it. It's not like I didn't test. In fact, I did a lot of testing. A lot of painstaking manual testing. For every change. So, in a way it was really the opposite of slacking. It was working a whole lot harder than I really needed to. It was also dumb. What I did slack on, was figuring out how Django's unit testing framework worked. Well, no more! In fact, it's really easy to set up. Writing tests can also be somewhat painstaking when it comes to ensuring you've covered every possible input, but at least that pain is an investment that repays itself every time you make a change to the program and you can know right away if you've broken something that used to work.

As usual the Django documentation is excellent. I highly recommend it for learning everything there is to know about testing with Django. What I hope to do here is give a brief overview of what you need to do some basic testing. Partly as a reminder for myself on future projects, and hopefully to help out anyone in the same position I was before I started.

Getting Started

If your Django app is relatively new, the startapp command already created a tests.py file when you ran it to set up your app. This file includes some helpful examples, like so:

"""
This file demonstrates two different styles of tests (one doctest and one
unittest). These will both pass when you run "manage.py test".

Replace these with more appropriate tests for your application.
"""

from django.test import TestCase

class SimpleTest(TestCase):
    def test_basic_addition(self):
        """
        Tests that 1 + 1 always equals 2.
        """
        self.failUnlessEqual(1 + 1, 2)

__test__ = {"doctest": """
Another way to test that 1 + 1 is equal to 2.

>>> 1 + 1 == 2
True
"""}

If you don't already have a tests.py file, the important things to note are that you have to import the TestCase class from django.test and create a new class which subclasses it. Each method defined in this class will correspond to one test, usually the test of a corresponding function in your views.py file. To test a particular function from views.py, define a method with the name test_functionname.

Test data

Now, I'm guessing your application probably manipulates some sort of data. The Django test runner is kind enough to setup and destroy a complete test database for the purposes of running your tests. There's two important consequences to this. First, the database user specified in your settings.py file has to have the necessary permissions to create and destroy databases. So, make sure that's the case. Second, if you already have a test database set up on your development machine (or wherever), with data you've inserted during your manual tests, the test runner won't have access to any of it.

To get some data for your tests, you can try exporting the data already in your test database with Django's dumpdata command. The test runner can then load it as a fixture. Honestly, I just couldn't get this to work. Some googling about the errors I was getting seemed to indicate that I wasn't the only one having problems with it. The update to natural keys announced for Django 1.2 seemed like it would help, but I've still been unsuccessful, so far. In any event, I plan on keeping an eye on future developments to see if there's any improvements in this area.

In the meantime, I went ahead and did it the hard way. I created a separate file called testsetup.py. I imported all the models for my project and defined a function also called testsetup. All this function does is create a few instances for each model with all their various possible configurations. By defining a method called setUp in the new class I created in tests.py and calling this function there, all of the necessary data will be added to the test database before the tests are run.

class ViewTest(TestCase):

    def setUp(self):
        testsetup()

GET Requests

The easiest of your views to test are the ones that only handle GET requests. All you have to do is retrieve the correct item from the database and display the information. Let's use the canonical book/author/publisher example. Suppose we're testing a view that displays the details for a particular book.

def test_book_detail(self):
    testbook = Book.objects.all()[0]
    url = '/books/id/' + str(testbook.id)
    response = self.client.get(url)
    self.assertContains(response, testbook.title)

The first thing I want to stress, is that I think it's important that no specific test data is used in the tests. Always try to select objects you're going to use for your tests in a generic way. This way it won't matter how your test data gets moved around or changed over time. In this example I just picked the first book returned in the queryset for all books. What you don't want to do is something like this:

testbook = Book.objects.get(title="The Test Book")

That assumes your test database has a book with that title in it, which it may or may not. Over time it's bound to get mighty confusing trying to keep the data in your test database synchronized with the data in your tests, so it's best to save yourself the headache.

The Django TestCase class comes with a built in client to make all your requests. For a GET request all you have to do is pass it a URL and it will return the response generated by that request. The TestCase class also defines some new assertions in addition to the standard ones Python provides. assertContains, used in this example, takes a response object and verifies that it has a particular piece of text in it; in this case, the book's title. Optionally, you can specify a particular response code, and number of repititions for the piece of text. If you're expecting a particular url to not be found you might test:

badurl = '/books/id/notabookid'
response = self.client.get(badurl)
self.assertContains(response, "Book not found", status_code=404)

POST requests

Of course all of this data has to get into your database somehow. You know you don't want to put it all there yourself, so you're probably going to have your users do it. This likely means you'll have some forms on your site that you want them to submit. For our example, let's pretend authors are submitting their own books to the site. Let's assume a few things about how this function works, so we know what we're testing.

  • The user must be logged in to submit a book
  • A book requires a title, an author, and an ISBN field
  • Upon successfully submitting a book, we want to redirect the user to the detail page for that book.
  • The ID field for the Book model is auto-incremented (Django's default)

Simple enough, right? Django provides all the tools you need to test every aspect of this function.

def test_addbook(self):
    url = '/books/add/'

    author = Author.objects.all()[0]    
    user = author.user

    # User not logged in
    response = self.client.get(url)
    self.assertEqual(response.status_code, 403)

    self.client.login(username=user.username, password='password')

    # Valid user
    response = self.client.get(url)
    self.assertEqual(response.status_code, 200)

    # Invalid Form
    response = self.client.post(url, {'title': "Book Title",
                                      'author_id': author.id,
                                      'ISBN': "invalid ISBN"})
    self.assertFormError(response, 
                         "BookSubmitForm", 
                         'ISBN',
                         "ISBN field must contain a number")

    # Valid submission
    newid = Book.objects.aggregate(Max('id'))['id__max'] + 1
    redirect = '/books/id/%d' % newid
    response = self.client.post(url, {'title': "Test book",
                                      'author_id': author.id,
                                      'ISBN': 123456},
                                      follow=True)

    self.assertRedirects(response, redirect)
    self.assertContains(response, "Test book")

First you'll want to test a GET request for the url, to make sure the user can see the form properly to fill it in. Also check that the form is not visible to users who aren't logged in. In this case, that means simply making sure the response returns a 403 Forbidden status code. To log in as a user, simply use the self.client.login function with the appropriate name and password. It's easy enough to pull a random user from the ones you set up in the testsetup function, but I've violated my own rule a little by specifying a value for the user's password. In fact, I'm not sure there's any other way to do this. For the sake of simplicity, I'm just giving all of the users created in testsetup the same password. For POST requests, simply add a dictionary with all the required form fields and values. You can make sure the form validation is catching errors properly by submitting an invalid form and using the assertFormError assertion to verify that the appropriate form validation error occurs. This assertion takes 4 arguments: the response, the name the template uses for the form, the form field in question, and the error text (or a list of strings for multiple errors) that is expected. Finally, since the id for new books is just autoincremented, we can assume the id for the book we create will be one more than highest one in the database yet. By plugging this into the book detail url, we have the address of the page we'll be redirected to when the book is successfully submitted. If you want to use assertContains or anything else to check the contents of the final page you get redirected to, be sure to add follow=True in the post function. This way it will return the final response, not the intermediate redirect response.

I suppose now is a good time to mention that the Django test runner will return the database to its original state (after populated by the setUp method) after each test. That means that any books you add during the test__addbook method will be deleted when that test is over. You won't have access to them in, for example, test_deletebook that you might write afterwards, so keep that in mind.

I hope this has been a helpful introduction to Django's testing facilities. I've only scratched the surface of what it can do, but I think it's a good place to start. Like I said, the documentation is up to Django's usual high standards, so definitely make the most of it.

Happy Testing!

Originally published 2010-04-05 06:50:17 on That's Debatable