Writing your first Django app, part 5¶
This tutorial begins where Tutorial 4 left off. We’ve built a Web-poll application, and we’ll now create some automated tests for it.
Introducing automated testing¶
What are automated tests?¶
Tests are simple routines that check the operation of your code.
Testing operates at different levels. Some tests might apply to a tiny detail
(does a particular model method return values as expected?) while others
examine the overall operation of the software (does a sequence of user inputs
on the site produce the desired result?). That’s no different from the kind of
testing you did earlier in Tutorial 2, using the
shell to examine the behavior of a method, or running the
application and entering data to check how it behaves.
What’s different in automated tests is that the testing work is done for you by the system. You create a set of tests once, and then as you make changes to your app, you can check that your code still works as you originally intended, without having to perform time consuming manual testing.
Why you need to create tests¶
So why create tests, and why now?
You may feel that you have quite enough on your plate just learning Python/Django, and having yet another thing to learn and do may seem overwhelming and perhaps unnecessary. After all, our polls application is working quite happily now; going through the trouble of creating automated tests is not going to make it work any better. If creating the polls application is the last bit of Django programming you will ever do, then true, you don’t need to know how to create automated tests. But, if that’s not the case, now is an excellent time to learn.
Tests will save you time¶
Up to a certain point, ‘checking that it seems to work’ will be a satisfactory test. In a more sophisticated application, you might have dozens of complex interactions between components.
A change in any of those components could have unexpected consequences on the application’s behavior. Checking that it still ‘seems to work’ could mean running through your code’s functionality with twenty different variations of your test data just to make sure you haven’t broken something - not a good use of your time.
That’s especially true when automated tests could do this for you in seconds. If something’s gone wrong, tests will also assist in identifying the code that’s causing the unexpected behavior.
Sometimes it may seem a chore to tear yourself away from your productive, creative programming work to face the unglamorous and unexciting business of writing tests, particularly when you know your code is working properly.
However, the task of writing tests is a lot more fulfilling than spending hours testing your application manually or trying to identify the cause of a newly-introduced problem.
Tests don’t just identify problems, they prevent them¶
It’s a mistake to think of tests merely as a negative aspect of development.
Without tests, the purpose or intended behavior of an application might be rather opaque. Even when it’s your own code, you will sometimes find yourself poking around in it trying to find out what exactly it’s doing.
Tests change that; they light up your code from the inside, and when something goes wrong, they focus light on the part that has gone wrong - even if you hadn’t even realized it had gone wrong.
Tests make your code more attractive¶
You might have created a brilliant piece of software, but you will find that many other developers will simply refuse to look at it because it lacks tests; without tests, they won’t trust it. Jacob Kaplan-Moss, one of Django’s original developers, says “Code without tests is broken by design.”
That other developers want to see tests in your software before they take it seriously is yet another reason for you to start writing tests.
Tests help teams work together¶
The previous points are written from the point of view of a single developer maintaining an application. Complex applications will be maintained by teams. Tests guarantee that colleagues don’t inadvertently break your code (and that you don’t break theirs without knowing). If you want to make a living as a Django programmer, you must be good at writing tests!
Basic testing strategies¶
There are many ways to approach writing tests.
Some programmers follow a discipline called “test-driven development”; they actually write their tests before they write their code. This might seem counter-intuitive, but in fact it’s similar to what most people will often do anyway: they describe a problem, then create some code to solve it. Test-driven development simply formalizes the problem in a Python test case.
More often, a newcomer to testing will create some code and later decide that it should have some tests. Perhaps it would have been better to write some tests earlier, but it’s never too late to get started.
Sometimes it’s difficult to figure out where to get started with writing tests. If you have written several thousand lines of Python, choosing something to test might not be easy. In such a case, it’s fruitful to write your first test the next time you make a change, either when you add a new feature or fix a bug.
So let’s do that right away.
Writing our first test¶
We identify a bug¶
Fortunately, there’s a little bug in the
polls application for us to fix
right away: the
Question.was_published_recently() method returns
Question was published within the last day (which is correct) but also if
pub_date field is in the future (which certainly isn’t).
To check if the bug really exists, using the Admin create a question whose date
lies in the future and check the method using the
>>> import datetime >>> from django.utils import timezone >>> from polls.models import Question >>> # create a Question instance with pub_date 30 days in the future >>> future_question = Question(pub_date=timezone.now() + datetime.timedelta(days=30)) >>> # was it published recently? >>> future_question.was_published_recently() True
Since things in the future are not ‘recent’, this is clearly wrong.
Create a test to expose the bug¶
What we’ve just done in the
shell to test for the problem is exactly
what we can do in an automated test, so let’s turn that into an automated test.
A conventional place for an application’s tests is in the application’s
tests.py file; the testing system will automatically find tests in any file
whose name begins with
Put the following in the
tests.py file in the
import datetime from django.utils import timezone from django.test import TestCase from .models import Question class QuestionModelTests(TestCase): def test_was_published_recently_with_future_question(self): """ was_published_recently() returns False for questions whose pub_date is in the future. """ time = timezone.now() + datetime.timedelta(days=30) future_question = Question(pub_date=time) self.assertIs(future_question.was_published_recently(), False)
What we have done here is created a
with a method that creates a
Question instance with a
pub_date in the
future. We then check the output of
was_published_recently() - which
ought to be False.
In the terminal, we can run our test:
$ python manage.py test polls
and you’ll see something like:
Creating test database for alias 'default'... System check identified no issues (0 silenced). F ====================================================================== FAIL: test_was_published_recently_with_future_question (polls.tests.QuestionModelTests) ---------------------------------------------------------------------- Traceback (most recent call last): File "/path/to/mysite/polls/tests.py", line 16, in test_was_published_recently_with_future_question self.assertIs(future_question.was_published_recently(), False) AssertionError: True is not False ---------------------------------------------------------------------- Ran 1 test in 0.001s FAILED (failures=1) Destroying test database for alias 'default'...
What happened is this:
python manage.py test pollslooked for tests in the
- it found a subclass of the
- it created a special database for the purpose of testing
- it looked for test methods - ones whose names begin with
test_was_published_recently_with_future_questionit created a
pub_datefield is 30 days in the future
- … and using the
assertIs()method, it discovered that its
True, though we wanted it to return
The test informs us which test failed and even the line on which the failure occurred.
Fixing the bug¶
We already know what the problem is:
False if its
pub_date is in the future. Amend the method in
models.py, so that it will only return
True if the date is also in the
def was_published_recently(self): now = timezone.now() return now - datetime.timedelta(days=1) <= self.pub_date <= now
and run the test again:
Creating test database for alias 'default'... System check identified no issues (0 silenced). . ---------------------------------------------------------------------- Ran 1 test in 0.001s OK Destroying test database for alias 'default'...
After identifying a bug, we wrote a test that exposes it and corrected the bug in the code so our test passes.
Many other things might go wrong with our application in the future, but we can be sure that we won’t inadvertently reintroduce this bug, because simply running the test will warn us immediately. We can consider this little portion of the application pinned down safely forever.
More comprehensive tests¶
While we’re here, we can further pin down the
method; in fact, it would be positively embarrassing if in fixing one bug we had
Add two more test methods to the same class, to test the behavior of the method more comprehensively:
def test_was_published_recently_with_old_question(self): """ was_published_recently() returns False for questions whose pub_date is older than 1 day. """ time = timezone.now() - datetime.timedelta(days=1, seconds=1) old_question = Question(pub_date=time) self.assertIs(old_question.was_published_recently(), False) def test_was_published_recently_with_recent_question(self): """ was_published_recently() returns True for questions whose pub_date is within the last day. """ time = timezone.now() - datetime.timedelta(hours=23, minutes=59, seconds=59) recent_question = Question(pub_date=time) self.assertIs(recent_question.was_published_recently(), True)
And now we have three tests that confirm that
returns sensible values for past, recent, and future questions.
polls is a simple application, but however complex it grows in the
future and whatever other code it interacts with, we now have some guarantee
that the method we have written tests for will behave in expected ways.
Test a view¶
The polls application is fairly undiscriminating: it will publish any question,
including ones whose
pub_date field lies in the future. We should improve
this. Setting a
pub_date in the future should mean that the Question is
published at that moment, but invisible until then.
A test for a view¶
When we fixed the bug above, we wrote the test first and then the code to fix it. In fact that was a simple example of test-driven development, but it doesn’t really matter in which order we do the work.
In our first test, we focused closely on the internal behavior of the code. For this test, we want to check its behavior as it would be experienced by a user through a web browser.
Before we try to fix anything, let’s have a look at the tools at our disposal.
The Django test client¶
>>> from django.test.utils import setup_test_environment >>> setup_test_environment()
setup_test_environment() installs a template renderer
which will allow us to examine some additional attributes on responses such as
response.context that otherwise wouldn’t be available. Note that this
method does not setup a test database, so the following will be run against
the existing database and the output may differ slightly depending on what
questions you already created. You might get unexpected results if your
settings.py isn’t correct. If you don’t remember setting
it earlier, check it before continuing.
Next we need to import the test client class (later in
tests.py we will use
django.test.TestCase class, which comes with its own client, so
this won’t be required):
>>> from django.test import Client >>> # create an instance of the client for our use >>> client = Client()
With that ready, we can ask the client to do some work for us:
>>> # get a response from '/' >>> response = client.get('/') Not Found: / >>> # we should expect a 404 from that address; if you instead see an >>> # "Invalid HTTP_HOST header" error and a 400 response, you probably >>> # omitted the setup_test_environment() call described earlier. >>> response.status_code 404 >>> # on the other hand we should expect to find something at '/polls/' >>> # we'll use 'reverse()' rather than a hardcoded URL >>> from django.urls import reverse >>> response = client.get(reverse('polls:index')) >>> response.status_code 200 >>> response.content b'\n <ul>\n \n <li><a href="/polls/1/">What's up?</a></li>\n \n </ul>\n\n' >>> response.context['latest_question_list'] <QuerySet [<Question: What's up?>]>
Improving our view¶
The list of polls shows polls that aren’t published yet (i.e. those that have a
pub_date in the future). Let’s fix that.
class IndexView(generic.ListView): template_name = 'polls/index.html' context_object_name = 'latest_question_list' def get_queryset(self): """Return the last five published questions.""" return Question.objects.order_by('-pub_date')[:5]
We need to amend the
get_queryset() method and change it so that it also
checks the date by comparing it with
timezone.now(). First we need to add
from django.utils import timezone
and then we must amend the
get_queryset method like so:
def get_queryset(self): """ Return the last five published questions (not including those set to be published in the future). """ return Question.objects.filter( pub_date__lte=timezone.now() ).order_by('-pub_date')[:5]
Question.objects.filter(pub_date__lte=timezone.now()) returns a queryset
pub_date is less than or equal to - that
is, earlier than or equal to -
Testing our new view¶
Now you can satisfy yourself that this behaves as expected by firing up the
runserver, loading the site in your browser, creating
Questions with dates
in the past and future, and checking that only those that have been published
are listed. You don’t want to have to do that every single time you make any
change that might affect this - so let’s also create a test, based on our
shell session above.
Add the following to
from django.urls import reverse
and we’ll create a shortcut function to create questions as well as a new test class:
def create_question(question_text, days): """ Create a question with the given `question_text` and published the given number of `days` offset to now (negative for questions published in the past, positive for questions that have yet to be published). """ time = timezone.now() + datetime.timedelta(days=days) return Question.objects.create(question_text=question_text, pub_date=time) class QuestionIndexViewTests(TestCase): def test_no_questions(self): """ If no questions exist, an appropriate message is displayed. """ response = self.client.get(reverse('polls:index')) self.assertEqual(response.status_code, 200) self.assertContains(response, "No polls are available.") self.assertQuerysetEqual(response.context['latest_question_list'], ) def test_past_question(self): """ Questions with a pub_date in the past are displayed on the index page. """ create_question(question_text="Past question.", days=-30) response = self.client.get(reverse('polls:index')) self.assertQuerysetEqual( response.context['latest_question_list'], ['<Question: Past question.>'] ) def test_future_question(self): """ Questions with a pub_date in the future aren't displayed on the index page. """ create_question(question_text="Future question.", days=30) response = self.client.get(reverse('polls:index')) self.assertContains(response, "No polls are available.") self.assertQuerysetEqual(response.context['latest_question_list'], ) def test_future_question_and_past_question(self): """ Even if both past and future questions exist, only past questions are displayed. """ create_question(question_text="Past question.", days=-30) create_question(question_text="Future question.", days=30) response = self.client.get(reverse('polls:index')) self.assertQuerysetEqual( response.context['latest_question_list'], ['<Question: Past question.>'] ) def test_two_past_questions(self): """ The questions index page may display multiple questions. """ create_question(question_text="Past question 1.", days=-30) create_question(question_text="Past question 2.", days=-5) response = self.client.get(reverse('polls:index')) self.assertQuerysetEqual( response.context['latest_question_list'], ['<Question: Past question 2.>', '<Question: Past question 1.>'] )
Let’s look at some of these more closely.
First is a question shortcut function,
create_question, to take some
repetition out of the process of creating questions.
test_no_questions doesn’t create any questions, but checks the message:
“No polls are available.” and verifies the
latest_question_list is empty.
Note that the
django.test.TestCase class provides some additional
assertion methods. In these examples, we use
test_past_question, we create a question and verify that it appears in
test_future_question, we create a question with a
pub_date in the
future. The database is reset for each test method, so the first question is no
longer there, and so again the index shouldn’t have any questions in it.
And so on. In effect, we are using the tests to tell a story of admin input and user experience on the site, and checking that at every state and for every new change in the state of the system, the expected results are published.
What we have works well; however, even though future questions don’t appear in
the index, users can still reach them if they know or guess the right URL. So
we need to add a similar constraint to
class DetailView(generic.DetailView): ... def get_queryset(self): """ Excludes any questions that aren't published yet. """ return Question.objects.filter(pub_date__lte=timezone.now())
And of course, we will add some tests, to check that a
pub_date is in the past can be displayed, and that one with a
in the future is not:
class QuestionDetailViewTests(TestCase): def test_future_question(self): """ The detail view of a question with a pub_date in the future returns a 404 not found. """ future_question = create_question(question_text='Future question.', days=5) url = reverse('polls:detail', args=(future_question.id,)) response = self.client.get(url) self.assertEqual(response.status_code, 404) def test_past_question(self): """ The detail view of a question with a pub_date in the past displays the question's text. """ past_question = create_question(question_text='Past Question.', days=-5) url = reverse('polls:detail', args=(past_question.id,)) response = self.client.get(url) self.assertContains(response, past_question.question_text)
Ideas for more tests¶
We ought to add a similar
get_queryset method to
create a new test class for that view. It’ll be very similar to what we have
just created; in fact there will be a lot of repetition.
We could also improve our application in other ways, adding tests along the
way. For example, it’s silly that
Questions can be published on the site
that have no
Choices. So, our views could check for this, and exclude such
Questions. Our tests would create a
then test that it’s not published, as well as create a similar
Choices, and test that it is published.
Perhaps logged-in admin users should be allowed to see unpublished
Questions, but not ordinary visitors. Again: whatever needs to be added to
the software to accomplish this should be accompanied by a test, whether you
write the test first and then make the code pass the test, or work out the
logic in your code first and then write a test to prove it.
At a certain point you are bound to look at your tests and wonder whether your code is suffering from test bloat, which brings us to:
When testing, more is better¶
It might seem that our tests are growing out of control. At this rate there will soon be more code in our tests than in our application, and the repetition is unaesthetic, compared to the elegant conciseness of the rest of our code.
It doesn’t matter. Let them grow. For the most part, you can write a test once and then forget about it. It will continue performing its useful function as you continue to develop your program.
Sometimes tests will need to be updated. Suppose that we amend our views so that
Choices are published. In that case, many of our
existing tests will fail - telling us exactly which tests need to be amended to
bring them up to date, so to that extent tests help look after themselves.
At worst, as you continue developing, you might find that you have some tests that are now redundant. Even that’s not a problem; in testing redundancy is a good thing.
As long as your tests are sensibly arranged, they won’t become unmanageable. Good rules-of-thumb include having:
- a separate
TestClassfor each model or view
- a separate test method for each set of conditions you want to test
- test method names that describe their function
This tutorial only introduces some of the basics of testing. There’s a great deal more you can do, and a number of very useful tools at your disposal to achieve some very clever things.
For example, while our tests here have covered some of the internal logic of a
model and the way our views publish information, you can use an “in-browser”
framework such as Selenium to test the way your HTML actually renders in a
browser. These tools allow you to check not just the behavior of your Django
the tests launch a browser, and start interacting with your site, as if a human
being were driving it! Django includes
to facilitate integration with tools like Selenium.
If you have a complex application, you may want to run tests automatically with every commit for the purposes of continuous integration, so that quality control is itself - at least partially - automated.
A good way to spot untested parts of your application is to check code coverage. This also helps identify fragile or even dead code. If you can’t test a piece of code, it usually means that code should be refactored or removed. Coverage will help to identify dead code. See Integration with coverage.py for details.
Testing in Django has comprehensive information about testing.