Django TDD Patterns — A Visual Field Guide

Kuldeep Pisda — Django & growth consultant · creator of django-rls · DjangoCon US tutorials 2022–24 · kdpisda.in

01The Factory Pattern Taxonomy

SITUATION	REACH FOR	WHY
Test never touches the DB	build() / build_batch()	In-memory, no INSERT — the fast default for pure logic.
Model needs a parent FK	SubFactory	The dependency must exist before the child row.
Parent should spawn children	RelatedFactory	Runs after creation — reverse relations without loops.
Field derived from siblings	LazyAttribute	lambda o: f"{o.username}@corp.dev"
Circular FKs	string ref + SelfAttribute	Break one direction — never two eager SubFactories.

FAKER RULE  Match the provider to the domain — faker.company(), not "test1". Failure messages should read like your data.

02Permission Testing, Without Drowning in Fixtures

Django apps stack three permission layers — model (Meta.permissions), view (permission_classes), object (django-guardian / RLS). Test each layer once in isolation, then test the role × action matrix at the view with one parameterized test instead of thirty functions:

Roles come from factory traits — one factory, four personas, zero JSON fixtures.

03The Mock Boundary

• MOCK THE VIEW/SERVICE — brittle: you assert your own implementation; refactors break green tests.
• MOCK NOTHING — slow & flaky: real network, rate limits, CI red on someone else's outage.
• AT THE ADAPTER — responses / unittest.mock on the client you wrote around code you didn't.

“Mock at the boundary where your code meets code you don’t own.”

04The Rogues’ Gallery — Five Anti-Patterns

ANTI-PATTERN	THE SMELL	THE FIX
The Leaky Test	on_commit hooks silently never fire under TestCase.	Use TransactionTestCase when the code manages transactions.
Over-Mocking	assert_called_once_with everywhere — tests pin implementation.	Assert behavior: assert order.total == Decimal("21.60").
Fixture Graveyard	JSON fixture files nobody dares touch; every migration breaks them.	Factories declare only what the test needs: OrgFactory(plan="pro").
Order-Dependent Tests	Module-level state; test_b passes only after test_a.	State lives in fixtures; enforce with pytest-randomly in CI.
The God Factory	Default factory creates org, plan, team, invoice… half the schema per test.	Opt in with traits: UserFactory(with_org=True).