Chapter 3. Testing a Simple Home Page with Unit Tests

We finished the last chapter with a functional test failing, telling us that it wanted the home page for our site to have “To-Do” in its title. It’s time to start working on our application.

Warning: Things Are About to Get Real

The first two chapters were intentionally nice and light. From now on, we get into some more meaty coding. Here’s a prediction: at some point, things are going to go wrong. You’re going to see different results from what I say you should see. This is a Good Thing, because it will be a genuine character-building Learning Experience™.

One possibility is that I’ve given some ambiguous explanations, and you’ve done something different from what I intended. Step back and have a think about what we’re trying to achieve at this point in the book. Which file are we editing, what do we want the user to be able to do, what are we testing and why? It may be that you’ve edited the wrong file or function, or are running the wrong tests. I reckon you’ll learn more about TDD from these stop and think moments than you do from all the bits where the following instructions and copy-pasting goes smoothly.

Or it may be a real bug. Be tenacious, read the error message carefully (see my aside on reading tracebacks a little later on in the chapter), and you’ll get to the bottom of it. It’s probably just a missing comma, or trailing-slash, or maybe a missing “s” in one of the Selenium find methods. But, as Zed Shaw put it so well, this kind of debugging is also an absolutely vital part of learning, so do stick it out!

You can always drop me an email (or try the Google Group) if you get really stuck. Happy debugging!

Our First Django App, and Our First Unit Test

Django encourages you to structure your code into apps: the theory is that one project can have many apps, you can use third-party apps developed by other people, and you might even reuse one of your own apps in a different project…although I admit I’ve never actually managed it myself! Still, apps are a good way to keep your code organised.

Let’s start an app for our to-do lists:

$ python3 manage.py startapp lists

That will create a folder at superlists/lists, next to superlists/superlists, and within it a number of placeholder files for things like models, views, and, of immediate interest to us, tests:

superlists/
├── db.sqlite3
├── functional_tests.py
├── lists
│   ├── admin.py
│   ├── __init__.py
│   ├── migrations
│   │   └── __init__.py
│   ├── models.py
│   ├── tests.py
│   └── views.py
├── manage.py
└── superlists
    ├── __init__.py
    ├── __pycache__
    ├── settings.py
    ├── urls.py
    └── wsgi.py

Unit Tests, and How They Differ from Functional Tests

As with so many of the labels we put on things, the line between unit tests and functional tests can become a little blurry at times. The basic distinction, though, is that functional tests test the application from the outside, from the point of view of the user. Unit tests test the application from the inside, from the point of view of the programmer.

The TDD approach I’m following wants our application to be covered by both types of test. Our workflow will look a bit like this:

  1. We start by writing a functional test, describing the new functionality from the user’s point of view.
  2. Once we have a functional test that fails, we start to think about how to write code that can get it to pass (or at least to get past its current failure). We now use one or more unit tests to define how we want our code to behave—the idea is that each line of production code we write should be tested by (at least) one of our unit tests.
  3. Once we have a failing unit test, we write the smallest amount of application code we can, just enough to get the unit test to pass. We may iterate between steps 2 and 3 a few times, until we think the functional test will get a little further.
  4. Now we can rerun our functional tests and see if they pass, or get a little further. That may prompt us to write some new unit tests, and some new code, and so on.

You can see that, all the way through, the functional tests are driving what development we do from a high level, while the unit tests drive what we do at a low level.

Does that seem slightly redundant? Sometimes it can feel that way, but functional tests and unit tests do really have very different objectives, and they will usually end up looking quite different.

Note

Functional tests should help you build an application with the right functionality, and guarantee you never accidentally break it. Unit tests should help you to write code that’s clean and bug free.

Enough theory for now, let’s see how it looks in practice.

Unit Testing in Django

Let’s see how to write a unit test for our home page view. Open up the new file at lists/tests.py, and you’ll see something like this:

lists/tests.py.  

from django.test import TestCase

# Create your tests here.

Django has helpfully suggested we use a special version of TestCase, which it provides. It’s an augmented version of the standard unittest.TestCase, with some additional Django-specific features, which we’ll discover over the next few chapters.

You’ve already seen that the TDD cycle involves starting with a test that fails, then writing code to get it to pass. Well, before we can even get that far, we want to know that the unit test we’re writing will definitely be run by our automated test runner, whatever it is. In the case of functional_tests.py, we’re running it directly, but this file made by Django is a bit more like magic. So, just to make sure, let’s make a deliberately silly failing test:

lists/tests.py.  

from django.test import TestCase

class SmokeTest(TestCase):

    def test_bad_maths(self):
        self.assertEqual(1 + 1, 3)

Now let’s invoke this mysterious Django test runner. As usual, it’s a manage.py command:

$ python3 manage.py test
Creating test database for alias 'default'...
F
======================================================================
FAIL: test_bad_maths (lists.tests.SmokeTest)
 ---------------------------------------------------------------------
Traceback (most recent call last):
  File "/workspace/superlists/lists/tests.py", line 6, in test_bad_maths
    self.assertEqual(1 + 1, 3)
AssertionError: 2 != 3

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

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

Excellent. The machinery seems to be working. This is a good point for a commit:

$ git status  # should show you lists/ is untracked
$ git add lists
$ git diff --staged  # will show you the diff that you're about to commit
$ git commit -m "Add app for lists, with deliberately failing unit test"

As you’ve no doubt guessed, the -m flag lets you pass in a commit message at the command line, so you don’t need to use an editor. It’s up to you to pick the way you like to use the Git command line, I’ll just show you the main ones I’ve seen used. The main rule is: make sure you always review what you’re about to commit before you do it.

Django’s MVC, URLs, and View Functions

Django is broadly structured along a classic Model-View-Controller (MVC) pattern. Well, broadly. It definitely does have models, but its views are more like a controller, and it’s the templates that are actually the view part, but the general idea is there. If you’re interested, you can look up the finer points of the discussion in the Django FAQs.

Irrespective of any of that, like any web server, Django’s main job is to decide what to do when a user asks for a particular URL on our site. Django’s workflow goes something like this:

  1. An HTTP request comes in for a particular URL.
  2. Django uses some rules to decide which view function should deal with the request (this is referred to as resolving the URL).
  3. The view function processes the request and returns an HTTP response.

So we want to test two things:

  • Can we resolve the URL for the root of the site (“/”) to a particular view function we’ve made?
  • Can we make this view function return some HTML which will get the functional test to pass?

Let’s start with the first. Open up lists/tests.py, and change our silly test to something like this:

lists/tests.py.  

from django.core.urlresolvers import resolve
from django.test import TestCase
from lists.views import home_page #1

class HomePageTest(TestCase):

    def test_root_url_resolves_to_home_page_view(self):
        found = resolve('/')  #2
        self.assertEqual(found.func, home_page)  #3

What’s going on here?

2 3

resolve is the function Django uses internally to resolve URLs, and find what view function they should map to. We’re checking that resolve, when called with “/”, the root of the site, finds a function called home_page.

1

What function is that? It’s the view function we’re going to write next, which will actually return the HTML we want. You can see from the import that we’re planning to store it in lists/views.py.

So, what do you think will happen when we run the tests?

$ python3 manage.py test
ImportError: cannot import name 'home_page'

It’s a very predictable and uninteresting error: we tried to import something we haven’t even written yet. But it’s still good news—for the purposes of TDD, an exception which was predicted counts as an expected failure. Since we have both a failing functional test and a failing unit test, we have the Testing Goat’s full blessing to code away.

At Last! We Actually Write Some Application Code!

It is exciting isn’t it? Be warned, TDD means that long periods of anticipation are only defused very gradually, and by tiny increments. Especially since we’re learning and only just starting out, we only allow ourselves to change (or add) one line of code at a time—and each time, we make just the minimal change required to address the current test failure.

I’m being deliberately extreme here, but what’s our current test failure? We can’t import home_page from lists.views? OK, let’s fix that—and only that. In lists/views.py:

lists/views.py.  

from django.shortcuts import render

# Create your views here.
home_page = None

"You must be joking!" I can hear you say.

I can hear you because it’s what I used to say (with feeling) when my colleagues first demonstrated TDD to me. Well, bear with me, we’ll talk about whether or not this is all taking it too far in a little while. For now, let yourself follow along, even if it’s with some exasperation, and see where it takes us.

Let’s run the tests again:

$ python3 manage.py test
Creating test database for alias 'default'...
E
======================================================================
ERROR: test_root_url_resolves_to_home_page_view (lists.tests.HomePageTest)
 ---------------------------------------------------------------------
Traceback (most recent call last):
  File "/workspace/superlists/lists/tests.py", line 8, in
test_root_url_resolves_to_home_page_view
    found = resolve('/')
  File "/usr/local/lib/python3.4/dist-packages/django/core/urlresolvers.py",
line 522, in resolve
    return get_resolver(urlconf).resolve(path)
  File "/usr/local/lib/python3.4/dist-packages/django/core/urlresolvers.py",
line 388, in resolve
    raise Resolver404({'tried': tried, 'path': new_path})
django.core.urlresolvers.Resolver404: {'tried': [[<RegexURLResolver
<RegexURLPattern list> (admin:admin) ^admin/>]], 'path': ''}

 ---------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (errors=1)
Destroying test database for alias 'default'...
Reading Tracebacks

Let’s spend a moment talking about how to read tracebacks, since it’s something we have to do a lot in TDD. You soon learn to scan through them and pick up relevant clues:

======================================================================
ERROR: test_root_url_resolves_to_home_page_view (lists.tests.HomePageTest)1
 ---------------------------------------------------------------------
Traceback (most recent call last):
  File "/workspace/superlists/lists/tests.py", line 8, in
test_root_url_resolves_to_home_page_view
    found = resolve('/')2
  File "/usr/local/lib/python3.4/dist-packages/django/core/urlresolvers.py",
line 522, in resolve
    return get_resolver(urlconf).resolve(path)
  File "/usr/local/lib/python3.4/dist-packages/django/core/urlresolvers.py",
line 388, in resolve
    raise Resolver404({'tried': tried, 'path': new_path})
django.core.urlresolvers.Resolver404: {'tried': [[<RegexURLResolver3
<RegexURLPattern list> (admin:admin) ^admin/>]], 'path': ''}4
 ---------------------------------------------------------------------
[...]
3 4

The first place you look is usually the error itself. Sometimes that’s all you need to see, and it will let you identify the problem immediately. But sometimes, like in this case, it’s not quite self-evident.

1

The next thing to double-check is: which test is failing? Is it definitely the one we expected, i.e., the one we just wrote? In this case, the answer is yes.

2

Then we look for the place in our test code that kicked off the failure. We work our way down from the top of the traceback, looking for the filename of the tests file, to check which test function, and what line of code, the failure is coming from. In this case it’s the line where we call the resolve function for the “/” URL.

There is ordinarily a fourth step, where we look further down for any of our own application code which was involved with the problem. In this case it’s all Django code, but we’ll see plenty of examples of this fourth step later in the book.

Pulling it all together, we interpret the traceback as telling us that, when trying to resolve “/”, Django raised a 404 error—in other words, Django can’t find a URL mapping for “/”. Let’s help it out.

urls.py

Django uses a file called urls.py to define how URLs map to view functions. There’s a main urls.py for the whole site in the superlists/superlists folder. Let’s go take a look:

superlists/urls.py.  

"""superlists URL Configuration

The `urlpatterns` list routes URLs to views. For more information please see:
    https://docs.djangoproject.com/en/1.8/topics/http/urls/
Examples:
Function views
    1. Add an import:  from my_app import views
    2. Add a URL to urlpatterns:  url(r'^$', views.home, name='home')
Class-based views
    1. Add an import:  from other_app.views import Home
    2. Add a URL to urlpatterns:  url(r'^$', Home.as_view(), name='home')
Including another URLconf
    1. Add an import:  from blog import urls as blog_urls
    2. Add a URL to urlpatterns:  url(r'^blog/', include(blog_urls))
"""
from django.conf.urls import include, url
from django.contrib import admin

urlpatterns = [
    url(r'^admin/', include(admin.site.urls)),
]

As usual, lots of helpful comments and default suggestions from Django.

A url entry starts with a regular expression that defines which URLs it applies to, and goes on to say where it should send those requests—either to a view function you’ve imported, or maybe to another urls.py file somewhere else using include.

You can see there’s one entry in there by default there for the admin site. We’re not using that yet, so let’s comment it out for now:

superlists/urls.py.  

from django.conf.urls import include, url
from django.contrib import admin

urlpatterns = [
    # url(r'^admin/', include(admin.site.urls)),
]

The first example entry has the regular expression ^$, which means an empty string—could this be the same as the root of our site, which we’ve been testing with “/”? Let’s find out—what happens if we include it?

Note

If you’ve never come across regular expressions, you can get away with just taking my word for it, for now—but you should make a mental note to go learn about them.

superlists/urls.py.  

from django.conf.urls import url

from lists import views

urlpatterns = [
    url(r'^$', views.home_page, name='home'),
    # url(r'^admin/', include(admin.site.urls)),
]

Run the unit tests again, with python3 manage.py test:

[...]
AttributeError: 'NoneType' object has no attribute 'rindex'

That’s progress! We’re no longer getting a 404.

The message is slightly cryptic, but the unit tests have actually made the link between the URL / and the home_page = None in lists/views.py, and are now complaining that home_page is a NoneType. And that gives us a justification for changing it from being None to being an actual function. Every single code change is driven by the tests!

Back in lists/views.py:

lists/views.py.  

from django.shortcuts import render

# Create your views here.
def home_page():
    pass

And now?

$ python3 manage.py test
Creating test database for alias 'default'...
.
 ---------------------------------------------------------------------
Ran 1 test in 0.003s

OK
Destroying test database for alias 'default'...

Hooray! Our first ever unit test pass! That’s so momentous that I think it’s worthy of a commit:

$ git diff  # should show changes to urls.py, tests.py, and views.py
$ git commit -am "First unit test and url mapping, dummy view"

That was the last variation on git commit I’ll show, the a and m flags together, which adds all changes to tracked files and uses the commit message from the command line.

Warning

git commit -am is the quickest formulation, but also gives you the least feedback about what’s being committed, so make sure you’ve done a git status and a git diff beforehand, and are clear on what changes are about to go in.

Unit Testing a View

On to writing a test for our view, so that it can be something more than a do-nothing function, and instead be a function that returns a real response with HTML to the browser. Open up lists/tests.py, and add a new test method. I’ll explain each bit:

lists/tests.py.  

from django.core.urlresolvers import resolve
from django.test import TestCase
from django.http import HttpRequest

from lists.views import home_page


class HomePageTest(TestCase):

    def test_root_url_resolves_to_home_page_view(self):
        found = resolve('/')
        self.assertEqual(found.func, home_page)


    def test_home_page_returns_correct_html(self):
        request = HttpRequest()  #1
        response = home_page(request)  #2
        self.assertTrue(response.content.startswith(b'<html>'))  #3
        self.assertIn(b'<title>To-Do lists</title>', response.content)  #4
        self.assertTrue(response.content.endswith(b'</html>'))  #5

What’s going on in this new test?

1

We create an HttpRequest object, which is what Django will see when a user’s browser asks for a page.

2

We pass it to our home_page view, which gives us a response. You won’t be surprised to hear that this object is an instance of a class called HttpResponse. Then, we assert that the .content of the response—which is the HTML that we send to the user—has certain properties.

3 5

We want it to start with an <html> tag which gets closed at the end. Notice that response.content is raw bytes, not a Python string, so we have to use the b'' syntax to compare them. More info is available in Django’s Porting to Python 3 docs.

4

And we want a <title> tag somewhere in the middle, with the words “To-Do lists” in it—because that’s what we specified in our functional test.

Once again, the unit test is driven by the functional test, but it’s also much closer to the actual code—we’re thinking like programmers now.

Let’s run the unit tests now and see how we get on:

TypeError: home_page() takes 0 positional arguments but 1 was given

The Unit-Test/Code Cycle

We can start to settle into the TDD unit-test/code cycle now:

  1. In the terminal, run the unit tests and see how they fail.
  2. In the editor, make a minimal code change to address the current test failure.

And repeat!

The more nervous we are about getting our code right, the smaller and more minimal we make each code change—the idea is to be absolutely sure that each bit of code is justified by a test. It may seem laborious, but once you get into the swing of things, it really moves quite fast—so much so that, at work, we usually keep our code changes microscopic even when we’re confident we could skip ahead.

Let’s see how fast we can get this cycle going:

  • Minimal code change:

lists/views.py.  

def home_page(request):
    pass

  • Tests: 
    self.assertTrue(response.content.startswith(b'<html>'))
AttributeError: 'NoneType' object has no attribute 'content'
  • Code—we use django.http.HttpResponse, as predicted:

lists/views.py.  

from django.http import HttpResponse

# Create your views here.
def home_page(request):
    return HttpResponse()

  • Tests again: 
    self.assertTrue(response.content.startswith(b'<html>'))
AssertionError: False is not true
  • Code again:

lists/views.py.  

def home_page(request):
    return HttpResponse('<html>')

  • Tests: 
AssertionError: b'<title>To-Do lists</title>' not found in b'<html>'
  • Code:

lists/views.py.  

def home_page(request):
    return HttpResponse('<html><title>To-Do lists</title>')

  • Tests—almost there? 
    self.assertTrue(response.content.endswith(b'</html>'))
AssertionError: False is not true
  • Come on, one last effort:

lists/views.py.  

def home_page(request):
    return HttpResponse('<html><title>To-Do lists</title></html>')

  • Surely? 
$ python3 manage.py test
Creating test database for alias 'default'...
..
 ---------------------------------------------------------------------
Ran 2 tests in 0.001s

OK
Destroying test database for alias 'default'...

Yes! Now, let’s run our functional tests. Don’t forget to spin up the dev server again, if it’s not still running. It feels like the final heat of the race here, surely this is it…could it be?

$ python3 functional_tests.py
F
======================================================================
FAIL: test_can_start_a_list_and_retrieve_it_later (__main__.NewVisitorTest)
 ---------------------------------------------------------------------
Traceback (most recent call last):
  File "functional_tests.py", line 20, in
test_can_start_a_list_and_retrieve_it_later
    self.fail('Finish the test!')
AssertionError: Finish the test!

 ---------------------------------------------------------------------
Ran 1 test in 1.609s

FAILED (failures=1)

Failed? What? Oh, it’s just our little reminder? Yes? Yes! We have a web page!

Ahem. Well, I thought it was a thrilling end to the chapter. You may still be a little baffled, perhaps keen to hear a justification for all these tests, and don’t worry, all that will come, but I hope you felt just a tinge of excitement near the end there.

Just a little commit to calm down, and reflect on what we’ve covered:

$ git diff  # should show our new test in tests.py, and the view in views.py
$ git commit -am "Basic view now returns minimal HTML"

That was quite a chapter! Why not try typing git log, possibly using the --oneline flag, for a reminder of what we got up to:

$ git log --oneline
a6e6cc9 Basic view now returns minimal HTML
450c0f3 First unit test and url mapping, dummy view
ea2b037 Add app for lists, with deliberately failing unit test
[...]

Not bad—we covered:

  • Starting a Django app
  • The Django unit test runner
  • The difference between FTs and unit tests
  • Django URL resolving and urls.py
  • Django view functions, request and response objects
  • And returning basic HTML
Useful Commands and Concepts
Running the Django dev server
python3 manage.py runserver
Running the functional tests
python3 functional_tests.py
Running the unit tests
python3 manage.py test
The unit-test/code cycle
  1. Run the unit tests in the terminal.
  2. Make a minimal code change in the editor.
  3. Repeat!

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