pytest-patterns

Pytest Patterns

Quality testing patterns with pytest for FastAPI projects. TDD-first approach.

Triggers

Use this skill when the user:

• Wants to write tests
• Asks about pytest, fixtures, mocks
• Wants to set up testing for FastAPI
• Uses TDD approach

Main Principle: TDD

1. Red — write a failing test
2. Green — write minimal code
3. Refactor — improve while keeping tests green

What Makes a Good Test

1. Clear Name

# ✅ Good — describes what, when, and expectation
async def test_create_user_with_valid_data_returns_user_with_id():
async def test_create_user_with_duplicate_email_raises_conflict_error():
async def test_get_user_by_id_when_not_exists_raises_not_found():

# ❌ Bad
async def test_create_user():
async def test_user():
async def test_1():

2. Single Responsibility

# ✅ Good — one test = one scenario
async def test_create_user_returns_user_with_id(service):
    result = await service.create(user_data)
    assert result.id is not None

async def test_create_user_saves_email(service):
    result = await service.create(user_data)
    assert result.email == user_data["email"]

# ❌ Bad — tests multiple things
async def test_create_user(service):
    result = await service.create(user_data)
    assert result.id is not None
    assert result.email == user_data["email"]
    assert result.created_at is not None
    assert await service.get(result.id) == result

3. Arrange-Act-Assert

async def test_create_order_calculates_total(service):
    # Arrange — prepare data
    items = [
        {"product_id": 1, "quantity": 2, "price": 100},
        {"product_id": 2, "quantity": 1, "price": 50},
    ]

    # Act — execute action
    order = await service.create(items=items)

    # Assert — verify result
    assert order.total == 250

4. Isolation

Tests should not depend on each other:

# ✅ Good — each test is independent
@pytest.fixture
async def session(engine):
    async with engine.connect() as conn:
        await conn.begin()
        async with AsyncSession(bind=conn) as session:
            yield session
        await conn.rollback()  # Rollback after each test

# ❌ Bad — shared state
_created_user = None

async def test_create_user(service):
    global _created_user
    _created_user = await service.create(data)

async def test_get_user(service):
    user = await service.get(_created_user.id)  # Depends on previous test!