django/docs/intro/tutorial05.txt

=====================================
Writing your first Django app, part 5
=====================================

This tutorial begins where :doc:`Tutorial 4 </intro/tutorial04>` 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 :doc:`Tutorial 1 </intro/tutorial01>`, 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.

.. _test-driven development: http://en.wikipedia.org/wiki/Test-driven_development/

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 ``Poll.was_published_recently()`` method returns ``True`` if
the ``Poll`` was published within the last day (which is correct) but also if
the ``Poll``'s ``pub_date`` field is in the future (which certainly isn't).

You can see this in the Admin; create a poll whose date lies in the future;
you'll see that the ``Poll`` change list claims it was published recently.

You can also see this using the shell::

    >>> import datetime
    >>> from django.utils import timezone
    >>> from polls.models import Poll
    >>> # create a Poll instance with pub_date 30 days in the future
    >>> future_poll = Poll(pub_date=timezone.now() + datetime.timedelta(days=30))
    >>> # was it published recently?
    >>> future_poll.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.

The best place for an application's tests is in the application's ``tests.py``
file - the testing system will look there for tests automatically.

Put the following in the ``tests.py`` file in the ``polls`` application (you'll
notice  ``tests.py`` contains some dummy tests, you can remove those)::

    import datetime

    from django.utils import timezone
    from django.test import TestCase

    from polls.models import Poll

    class PollMethodTests(TestCase):

        def test_was_published_recently_with_future_poll(self):
            """
            was_published_recently() should return False for polls whose
            pub_date is in the future
            """
            future_poll = Poll(pub_date=timezone.now() + datetime.timedelta(days=30))
            self.assertEqual(future_poll.was_published_recently(), False)

What we have done here is created a :class:`django.test.TestCase` subclass
with a method that creates a ``Poll`` instance with a ``pub_date`` in the
future. We then check the output of ``was_published_recently()`` - which
*ought* to be False.

Running tests
-------------

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'...
    F
    ======================================================================
    FAIL: test_was_published_recently_with_future_poll (polls.tests.PollMethodTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/path/to/mysite/polls/tests.py", line 16, in test_was_published_recently_with_future_poll
        self.assertEqual(future_poll.was_published_recently(), False)
    AssertionError: True != False

    ----------------------------------------------------------------------
    Ran 1 test in 0.001s

    FAILED (failures=1)
    Destroying test database for alias 'default'...

What happened is this:

* ``python manage.py test polls`` looked for tests in the ``polls`` application

* it found a subclass of the :class:`django.test.TestCase` class

* it created a special database for the purpose of testing

* it looked for test methods - ones whose names begin with ``test``

* in ``test_was_published_recently_with_future_poll`` it created a ``Poll``
  instance whose ``pub_date`` field is 30 days in the future

* ... and using the ``assertEqual()`` method, it discovered that its
  ``was_published_recently()`` returns ``True``, though we wanted it to return
  ``False``

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: ``Poll.was_published_recently()`` should
return ``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
past::

    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'...
    .
    ----------------------------------------------------------------------
    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 ``was_published_recently()``
method; in fact, it would be positively embarrassing if in fixing one bug we had
introduced another.

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_poll(self):
        """
        was_published_recently() should return False for polls whose pub_date
        is older than 1 day
        """
        old_poll = Poll(pub_date=timezone.now() - datetime.timedelta(days=30))
        self.assertEqual(old_poll.was_published_recently(), False)

    def test_was_published_recently_with_recent_poll(self):
        """
        was_published_recently() should return True for polls whose pub_date
        is within the last day
        """
        recent_poll = Poll(pub_date=timezone.now() - datetime.timedelta(hours=1))
        self.assertEqual(recent_poll.was_published_recently(), True)

And now we have three tests that confirm that ``Poll.was_published_recently()``
returns sensible values for past, recent, and future polls.

Again, ``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 poll,
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 Poll 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
----------------------

Django provides a test :class:`~django.test.client.Client` to simulate a user
interacting with the code at the view level.  We can use it in ``tests.py``
or even in the shell.

We will start again with the shell, where we need to do a couple of things that
won't be necessary in ``tests.py``. The first is to set up the test environment
in the shell::

    >>> from django.test.utils import setup_test_environment
    >>> setup_test_environment()

Next we need to import the test client class (later in ``tests.py`` we will use
the :class:`django.test.TestCase` class, which comes with its own client, so
this won't be required)::

    >>> from django.test.client 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('/')
    >>> # we should expect a 404 from that address
    >>> response.status_code
    404
    >>> # on the other hand we should expect to find something at '/polls/'
    >>> # we'll use 'reverse()' rather than a harcoded URL
    >>> from django.core.urlresolvers import reverse
    >>> response = client.get(reverse('polls:index'))
    >>> response.status_code
    200
    >>> response.content
    '\n\n\n    <p>No polls are available.</p>\n\n'
    >>> # note - you might get unexpected results if your ``TIME_ZONE``
    >>> # in ``settings.py`` is not correct. If you need to change it,
    >>> # you will also need to restart your shell session
    >>> from polls.models import Poll
    >>> from django.utils import timezone
    >>> # create a Poll and save it
    >>> p = Poll(question="Who is your favorite Beatle?", pub_date=timezone.now())
    >>> p.save()
    >>> # check the response once again
    >>> response = client.get('/polls/')
    >>> response.content
    '\n\n\n    <ul>\n    \n        <li><a href="/polls/1/">Who is your favorite Beatle?</a></li>\n    \n    </ul>\n\n'
    >>> response.context['latest_poll_list']
    [<Poll: Who is your favorite Beatle?>]

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.

In :doc:`Tutorial 4 </intro/tutorial04>` we deleted the view functions from
``views.py`` in favor of a :class:`~django.views.generic.list.ListView` in
``urls.py``::

    url(r'^$',
        ListView.as_view(
            queryset=Poll.objects.order_by('-pub_date')[:5],
            context_object_name='latest_poll_list',
            template_name='polls/index.html'),
        name='index'),

``response.context_data['latest_poll_list']`` extracts the data this view
places into the context.

We need to amend the line that gives us the ``queryset``::

    queryset=Poll.objects.order_by('-pub_date')[:5],

Let's change the queryset so that it also checks the date by comparing it with
``timezone.now()``. First we need to add an import::

    from django.utils import timezone

and then we must amend the existing ``url`` function to::

    url(r'^$',
        ListView.as_view(
            queryset=Poll.objects.filter(pub_date__lte=timezone.now) \
                .order_by('-pub_date')[:5],
            context_object_name='latest_poll_list',
            template_name='polls/index.html'),
        name='index'),

``Poll.objects.filter(pub_date__lte=timezone.now)`` returns a queryset
containing Polls whose ``pub_date`` is less than or equal to - that is, earlier
than or equal to - ``timezone.now``. Notice that we use a callable queryset
argument, ``timezone.now``, which will be evaluated at request time. If we had
included the parentheses, ``timezone.now()`` would be evaluated just once when
the web server is started.

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 ``Polls`` 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 ``polls/tests.py``::

    from django.core.urlresolvers import reverse

and we'll create a factory method to create polls as well as a new test class::

    def create_poll(question, days):
        """
        Creates a poll with the given `question` published the given number of
        `days` offset to now (negative for polls published in the past,
        positive for polls that have yet to be published).
        """
        return Poll.objects.create(question=question,
            pub_date=timezone.now() + datetime.timedelta(days=days))

    class PollViewTests(TestCase):
        def test_index_view_with_no_polls(self):
            """
            If no polls exist, an appropriate message should be 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_poll_list'], [])

        def test_index_view_with_a_past_poll(self):
            """
            Polls with a pub_date in the past should be displayed on the index page.
            """
            create_poll(question="Past poll.", days=-30)
            response = self.client.get(reverse('polls:index'))
            self.assertQuerysetEqual(
                response.context['latest_poll_list'],
                ['<Poll: Past poll.>']
            )

        def test_index_view_with_a_future_poll(self):
            """
            Polls with a pub_date in the future should not be displayed on the
            index page.
            """
            create_poll(question="Future poll.", days=30)
            response = self.client.get(reverse('polls:index'))
            self.assertContains(response, "No polls are available.", status_code=200)
            self.assertQuerysetEqual(response.context['latest_poll_list'], [])

        def test_index_view_with_future_poll_and_past_poll(self):
            """
            Even if both past and future polls exist, only past polls should be
            displayed.
            """
            create_poll(question="Past poll.", days=-30)
            create_poll(question="Future poll.", days=30)
            response = self.client.get(reverse('polls:index'))
            self.assertQuerysetEqual(
                response.context['latest_poll_list'],
                ['<Poll: Past poll.>']
            )

        def test_index_view_with_two_past_polls(self):
            """
            The polls index page may display multiple polls.
            """
            create_poll(question="Past poll 1.", days=-30)
            create_poll(question="Past poll 2.", days=-5)
            response = self.client.get(reverse('polls:index'))
            self.assertQuerysetEqual(
                response.context['latest_poll_list'],
                 ['<Poll: Past poll 2.>', '<Poll: Past poll 1.>']
            )

Let's look at some of these more closely.

First is a poll factory method, ``create_poll``, to take some repetition out
of the process of creating polls.

``test_index_view_with_no_polls`` doesn't create any polls, but checks the
message: "No polls are available." and verifies the ``latest_poll_list`` is
empty. Note that the :class:`django.test.TestCase` class provides some
additional assertion methods. In these examples, we use
:meth:`~django.test.TestCase.assertContains()` and
:meth:`~django.test.TestCase.assertQuerysetEqual()`.

In ``test_index_view_with_a_past_poll``, we create a poll and verify that it
appears in the list.

In ``test_index_view_with_a_future_poll``, we create a poll with a ``pub_date``
in the future. The database is reset for each test method, so the first poll is
no longer there, and so again the index shouldn't have any polls 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.

Testing the ``DetailView``
--------------------------

What we have works well; however, even though future polls don't appear in the
*index*, users can still reach them if they know or guess the right URL. So we
need similar constraints in the ``DetailViews``, by adding::

    queryset=Poll.objects.filter(pub_date__lte=timezone.now)

to them - for example::

    url(r'^(?P<pk>\d+)/$',
        DetailView.as_view(
            queryset=Poll.objects.filter(pub_date__lte=timezone.now),
            model=Poll,
            template_name='polls/detail.html'),
        name='detail'),

and of course, we will add some tests, to check that a ``Poll`` whose
``pub_date`` is in the past can be displayed, and that one with a ``pub_date``
in the future is not::

    class PollIndexDetailTests(TestCase):
        def test_detail_view_with_a_future_poll(self):
            """
            The detail view of a poll with a pub_date in the future should
            return a 404 not found.
            """
            future_poll = create_poll(question='Future poll.', days=5)
            response = self.client.get(reverse('polls:detail', args=(future_poll.id,)))
            self.assertEqual(response.status_code, 404)

        def test_detail_view_with_a_past_poll(self):
            """
            The detail view of a poll with a pub_date in the past should display
            the poll's question.
            """
            past_poll = create_poll(question='Past Poll.', days=-5)
            response = self.client.get(reverse('polls:detail', args=(past_poll.id,)))
            self.assertContains(response, past_poll.question, status_code=200)

Ideas for more tests
--------------------

We ought to add similar ``queryset`` arguments to the other ``DetailView``
URLs, and create a new test class for each view. They'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 ``Polls`` can be published on the site that
have no ``Choices``. So, our views could check for this, and exclude such
``Polls``. Our tests would create a ``Poll`` without ``Choices`` and then test
that it's not published, as well as create a similar ``Poll`` *with*
``Choices``, and test that it *is* published.

Perhaps logged-in admin users should be allowed to see unpublished ``Polls``,
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
only ``Polls`` with ``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 ``TestClass`` for each model or view
* a separate test method for each set of conditions you want to test
* test method names that describe their function

Further testing
===============

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
code, but also, for example, of your JavaScript. It's quite something to see
the tests launch a browser, and start interacting with your site, as if a human
being were driving it! Django includes :class:`~django.test.LiveServerTestCase`
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
:ref:`topics-testing-code-coverage` for details.

:doc:`Testing Django applications </topics/testing>` has comprehensive
information about testing.

.. _Selenium: http://seleniumhq.org/
.. _continuous integration: http://en.wikipedia.org/wiki/Continuous_integration

What's next?
============

The beginner tutorial ends here for the time being. In the meantime, you might
want to check out some pointers on :doc:`where to go from here
</intro/whatsnext>`.

If you are familiar with Python packaging and interested in learning how to
turn polls into a "reusable app", check out :doc:`Advanced tutorial: How to
write reusable apps</intro/reusable-apps>`.