python-tdd-book/textbook/chap3.md

Chapter 3. Testing a Simple Home Page with Unit Tests

Chap2 introduced writing a functional test using unittest module (expected failure), now create a working To-Do list app

3.1 Our First Django App, and Our First Unit Test

• Django encourage structure the code into apps. We can reuse app developed by us or others

Create an app lists (sub-directory) via python manage.py startapp lists in the directory which contains functional_tests.py & manage.py, this subdir should include tests.py be default

3.2 Unit Tests, and How They Differ from Functional Tests

Unit Tests vs Functional Tests:

• Unit Tests (UT): tests app from inside as programmer
• Functional Tests (FT): test app from outside as user

TDD approach will cover both, with following workflow:

1. Writing FT that describing new functionality
2. After expected failure of FT, design app that can get it pass FT. And write UTs that define how code behave. Benchmark, each production code we write should be tested by at least 1 of our unit tests.
3. After having a failure UT, write the smallest amount of app code to pass the UT.
4. Iterate step 2 & 3 to the point that we can test with FT
5. Rerun FT and it should pass.
6. Keep writing new UT & FT

Summary:

• FT evaluate app on high level
• UT drive development on low level

3.3 Unit Testing in Django

1. lists/tests.py provide place to write UT/FT. Adding following to import Django's TestCase which is inherited from unittest

from django.test import TestCase

# Create tests from here

2. Before writing any new FT/UT, make sure UT can be run by automated test runner, as lists/tests.py is created by unittest. while previously we directly call python functional_tests.py. Running Django's test framework via:

python manage.py test

3.4 Django's MVC, URLs, and View Functions

Django is structured along classic Model-View-Controller (MVC) pattern.

Django's main job is to decide what to do when a user asks for a particular URL on our site.

Django's workflow:

1. An HTTP request comes in for a particular URL
2. Resolving the URL: Django uses some rules to decide which view function should deal with the request.
3. The view function processes the request and returns an HTTP response

Let's design a tests:

1. Verify we can resolve the URL for the root of site ("/") to a particular view function we've made?
2. Verify view function return some HTML which will get the functional test to pass?

from django.test import TestCase
from django.urls import resolve
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)

Where:

• The resolve() function if django.url package can be used for resolving URL paths to the corresponding view functions. The function returns a ResolverMatch object that allows you to access various metadata about the resolved URL.
• In assertEqual(...), we try to test that found.func is home_page (a view function we haven't defined yet)

Result of running test:

ImportError: cannot import name 'home_page'

3.5 At Last! We Actually Write Some Application Code!

In 3.4, we created a test_root_url_resolves_to_home_page_view() test case that check "django can resolve request to root and return a view function"

Hence, add content in lists/views.py, we can pass the test

from django.shortcuts import render

home_page = None

Running the test generate following info

Creating test database for alias 'default'...
System check identified no issues (0 silenced).
E
======================================================================
ERROR: test_root_url_resolves_to_home_page_view (lists.tests.HomePageTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jason/HomeWorkstation/SynologyGiteaSpace/python-tdd-book-src/src/lists/tests.py", line 8, in test_root_url_resolves_to_home_page_view
    found = resolve('/')
  File "/home/jason/miniconda3/envs/python-tdd-book/lib/python3.6/site-packages/django/urls/base.py", line 27, in resolve
    return get_resolver(urlconf).resolve(path)
  File "/home/jason/miniconda3/envs/python-tdd-book/lib/python3.6/site-packages/django/urls/resolvers.py", line 394, in resolve
    raise Resolver404({'tried': tried, 'path': new_path})
django.urls.exceptions.Resolver404: {'tried': [[<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>]], 'path': ''}

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

• error due to django.urls.exceptions.Resolver404: {'tried': [[<RegexURLResolver <RegexURLPattern list> (admin:admin) ^admin/>]], 'path': ''}
• Happen in python-tdd-book-src/src/lists/tests.py", line 8, in test_root_url_resolves_to_home_page_view

Overall, the traceback can be interpreted as: "when trying to resolve /, Django raised a 404 error". i.e. Django can't find a URL mapping for "/"

3.6 urls.py

Django use urls.py in each app to map URLs to view functions.

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

Note:

• This book use Django v1.11. So path function is not utilized, it's used in v3.x

Explain:

• url start with a regex, which defines which URLs it looks for, and where (function) should these request to send
• ^$ means empty string

Modifying superlists.urls to

from django.conf.urls import url
from lists import views

urlpatterns = [
    url(r'^$', views.home_page, name='home'),
]

Generate error as shown

...
File "/home/jason/HomeWorkstation/SynologyGiteaSpace/python-tdd-book-src/src/superlists/urls.py", line 20, in <module>
    url(r'^$', views.home_page, name='home'),
  File "/home/jason/miniconda3/envs/python-tdd-book/lib/python3.6/site-packages/django/conf/urls/__init__.py", line 85, in url
    raise TypeError('view must be a callable or a list/tuple in the case of include().')
TypeError: view must be a callable or a list/tuple in the case of include().

Analyzing error message "TypeError: view must be a callable or a list/tuple in the case of include().": unit tests have actually made the link btw the URL "/" and home_page = None in lists/views.py, and are now complaining that home_page view is not callable.

Fix this problem via modifying lists/views.py

def home_page():
    pass

3.7 Unit Testing a View

After creating simple home_page empty function. We can create test function in lists/tests.py

    def test_home_page_returns_correct_html(self):
        """
        1. Create an HttpRequest object, which is what Django will see when a user's browser asks for a page
        2. Pass it to `home_page` view, which gives us a response.
        3. Extract `.content` of the response, which are byte value, and should be decoded into string (HTML format)
        4. Check HTML starts and end with <html> tag
        5. Want to find <title> tag in the middle
        """

        request = HttpRequest() #1
        response = home_page(request) #2
        html = response.content.decode('utf8') #3
        self.assertTrue(html.startswith('<html>')) #4
        self.assertIn('<title>To-Do lists</title>', html.strip()) #5
        self.assertTrue(html.strip().endswith('</html>')) #4

Result of python manager.py test is:

line 15, in test_home_page_returns_correct_html
    response = home_page(request)
TypeError: home_page() takes 0 positional arguments but 1 was given

3.7.1 The Unit-Test/Code Cycle

TDD unit-test/code cycle:

1. In the terminal, run unit tests and see how they fail
2. In the editor, create minimal code change to fix test failure.
3. Repeat

Fix the views.py to

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

Conclusion of covering:

• Starting a Django app
• The Django unit test runner
• The difference btw FTs and unit tests
• Django URL resolving and urls.py
• Django view functions, request and response objects
• And returning basic HTML