Automated Testing & TDD

Why tests are a hiring filter

A test is code that checks your other code does what you expect — automatically. Instead of clicking through the site by hand after every change, you run one command and the computer verifies everything still works. Backend teams treat tests as non-negotiable: the best-selling Django courses are built entirely around testing, and "do you write tests?" is a common interview gate. The good news: Django ships testing in the box.

Your first test with TestCase

Django’s TestCase is a class you subclass; each method whose name starts with test_ is one test. The magic word is assert — a statement that says "this MUST be true; fail the test if not." Django also gives every test a fresh, empty database so tests never touch your real data.

# blog/tests.py
from django.test import TestCase
from .models import Post

class PostModelTest(TestCase):
    def test_str_returns_title(self):
        post = Post.objects.create(title="Hello", body="Hi")
        # the model's __str__ should return its title:
        self.assertEqual(str(post), "Hello")

Reading it: PostModelTest groups related tests. Inside, test_str_returns_title creates a post in the throwaway test database, then self.assertEqual(str(post), "Hello") checks the two values match. If they differ, the test fails and tells you exactly where.

Testing a view with the test client

The test client is a fake browser built into TestCase as self.client. It lets you request your own pages in a test and inspect the response — no real server needed. You check the status code (200 means OK, 404 not found) and that the right content appears:

from django.test import TestCase
from django.urls import reverse
from .models import Post

class PostListViewTest(TestCase):
    def test_list_shows_posts(self):
        Post.objects.create(title="Hello", body="Hi")
        # ask for the page using its URL name:
        response = self.client.get(reverse("post_list"))
        self.assertEqual(response.status_code, 200)     # page loaded
        self.assertContains(response, "Hello")          # title is on the page

Step by step: we create a post, then self.client.get(...) requests the list page. assertEqual(response.status_code, 200) confirms it loaded without error, and assertContains(response, "Hello") confirms our post’s title actually shows up in the HTML.

Running the tests

One command finds every test_ method in your project and runs them all:

python manage.py test

TDD: write the test first

Test-Driven Development (TDD) flips the order: you write the test before the code. It sounds backwards but keeps you focused and guarantees coverage. The cycle is three steps, repeated:

1. Red — write a test for a feature that does not exist yet, and run it. It fails (red) because the code is not there.
2. Green — write the smallest amount of code to make that test pass (green). Nothing fancy.
3. Refactor — clean up the code now that the test protects you. Run the tests again; still green means you did not break anything.