I’ve recently developed some Django sites where I was able to ensure all features were thoroughly tested. I learnt a lot about writing maintainable tests, and I’d like to share with you.
A basic view
When you’re starting out developing your site, your views will be simple. Your index view probably looks something like this:
from django.shortcuts import render
def index(request):
return render(request, "index.html")Now, what properties do we want this view to have? We want it to render without errors. Here’s a test:
from django.test import TestCase
from django.core.urlresolvers import reverse
class IndexTests(TestCase):
def test_index_renders(self):
response = self.client.get(reverse('index'))
self.assertEqual(response.status_code, 200)Hey presto, your index view is tested!
When thinking about testing, you should ask yourself: ‘what properties do I want this view to have?’ In this case, we just want to check the view renders. This helps us catch any silly errors (such as incorrect template names) that may occur.
What’s nice about this test is that no legitimate refactoring will
cause the test to fail. Django provides many specific assertion
methods, such as assertTemplateUsed and
assertHTMLEqual. These have their uses, but they limit you. If
you use assertTemplateUsed and refactor your templates, you will
have to update your tests. Grrr.
Testing a form
Suppose we’re writing a form for users to sign up. Here’s what our view looks like:
from django.shortcuts import render
from .forms import UserRegistrationForm
def register_user(request):
if request.POST:
form = UserRegistrationForm(request.POST)
if form.is_valid():
form.save()
return redirect('completed-signup')
else:
form = UserRegistrationForm()
return render(request, "register_user.html",
{'form': form})What are we interested in here? We want the page to load, we want users to be able to register, and we want form validation. I’ve written django-test-mixins to provide these assertions.
from django.core.urlresolvers import reverse
from django_test_mixins import HttpCodeTestCase, FormValidationTestCase
class RegistrationTests(HttpCodeTestCase, FormValidationTestCase):
def form_params(self):
return {'username': 'foobar',
'password': 'helloworld1'}
def test_view_renders(self):
response = self.client.get(reverse('user_registration'))
self.assertHttpOK(response)
def test_user_created(self):
params = self.form_params()
expected_username = params['username']
response = self.client.post(
reverse('user_registration'), self.form_params())
self.assertTrue(
User.objects.filter(username=expected_username).exists(),
"User was not created.")
def test_password_required(self):
params = self.form_params()
params.pop('password')
response = self.client.post(
reverse('user_registration'), params)
self.assertFormInvalid(response)We don’t want tests to become a burden in later development. There are two things we’ve done to ensure our tests are future-friendly.
Firstly, we’re careful to only make an assertion about the user we’re interested in. We don’t want to assume how many users exist in the system. Websites often start off with a single admin user, but we shouldn’t bake that assumption into our tests. This assertion:
# I have been guilty of this.
self.assertEqual(User.objects.count(), 2)is worse, because we are making assumptions about the state of our database.
Secondly, we’ve factored out the form parameters. The exact number of form parameters often changes as features are added. It’s a pain to change lots of tests because you’ve added one field to form, so keep it DRY.
Testing protected views
Finally, let’s look at a more complex view.
from django.contrib.admin.views.decorators import staff_member_required
from django.shortcuts import get_object_or_404, redirect, render
from .models import BlogPost
from .forms import BlogPostForm
@staff_member_required
def edit_post(request, post_id):
blog_post = get_object_or_404(BlogPostForm, id=post_id)
if request.POST:
form = BlogPostForm(request.POST, instance=blog_post)
if form.is_valid():
form.save()
return redirect('view_post', blog_post.id)
else:
form = BlogPostForm(instance=blog_post)
return render(request, "pages/page_edit.html"
{'form': form, 'blog_post': blog_post})We have authentication here and we need some existing models to test the view.
We need a blog post to test this view, and we could explicitly create one. This is tedious, and milkman can do this for us.
Finally, we want an easy way of creating users to test that our view applies authentication correctly.
from django.test import TestCase
from django.contrib.auth.models import User
from milkman.dairy import milkman
class UserTestCase(TestCase):
TEST_PASSWORD = "password"
def create_user(self, **kwargs):
return milkman(User, password=self.TEST_PASSWORD, **kwargs)
def create_staff_user(self, **kwargs):
return self.create_user(is_staff=True, **kwargs)
def log_in(self, user):
self.client.login(username=user.username,
password=self.TEST_PASSWORD)We factor out user creation so we have a single place in our tests that provides reusable methods for creating users according to th eneeds of our site.
We want to test that our view validates users correctly and that it modifies blog posts correctly. Since each test should only verify one thing, we will write three tests.
from django_test_mixins import HttpCodeTestCase
from milkman.dairy import milkman
from .models import BlogPost
class EditBlogPostTest(HttpCodeTestCase, UserTestCase):
def test_view_renders(self):
user = self.create_staff_user()
self.log_in(user)
post = milkman.deliver(BlogPost)
response = self.client.get(reverse('edit_post', args=[post.id]))
self.assertHttpOK(response)
def test_view_requires_staff_user(self):
user = self.create_user()
self.log_in(user)
post = milkman.deliver(BlogPost)
response = self.client.get(reverse('edit_post', args=[post.id]))
self.assertHttpForbidden(response)
def form_params(self):
return {'title': 'a test title',
'content': 'some test content'}
def test_blog_post_edited(self):
user = self.create_staff_user()
self.log_in(user)
post = milkman.deliver(BlogPost)
params = self.form_params()
response = self.client.post(
reverse('edit_post', args=[post.id]), params)
# Reload the post from the DB.
post = BlogPost.objects.get(id=post.id)
self.assertEqual(post.title, params['title'])
self.assertEqual(post.content, params['content'])This is how I write tests with Django. Don’t worry about writing perfect tests, having tests at all is a huge boon. Once you have them, don’t be afraid to keep iterating on them. Always seek out ways to make testing easier next time.