Backend Tests

#python #cicd #testing #fastapi

Part of The Coercion Saga — making AI write quality code.

The Behavior Gate

Quality gates catch syntax. Tests catch behavior. A function that type-checks but returns garbage is worse than one that fails to compile.

The Async Trap

FastAPI is async. Get testing wrong and tests hang or miss real bugs.

# conftest.py
import asyncio
import pytest

@pytest.fixture(scope="session")
def event_loop():
    loop = asyncio.new_event_loop()
    loop.set_debug(True)  # Catches blocking calls
    yield loop
    loop.close()

set_debug(True) catches time.sleep() instead of asyncio.sleep(), synchronous I/O, libraries that claim async but aren't. One line, whole category of bugs.

Test Client

@pytest.fixture
async def client(session: AsyncSession) -> AsyncGenerator[AsyncClient, None]:
    app.dependency_overrides[get_session] = lambda: session
    async with AsyncClient(app=app, base_url="http://test") as client:
        yield client
    app.dependency_overrides.clear()

Endpoint code doesn't know it's being tested. Runs exactly like production.

What to Test

Service layer:

async def test_create_user(session: AsyncSession):
    user = await create_user(session, email="test@example.com", password="secret")
    assert user.password != "secret"  # Should be hashed

Error cases:

async def test_register_duplicate(client: AsyncClient, session: AsyncSession):
    await create_user(session, email="taken@example.com", password="secret")
    await session.commit()

    response = await client.post("/auth/register", json={
        "email": "taken@example.com", "password": "different"
    })
    assert response.status_code == 409  # Not 500

409 Conflict means you caught it intentionally. 500 means you caught it in a crash.

The Gate

test:backend:
  stage: test
  image: python:3.12-slim
  services:
    - postgres:16
  variables:
    DATABASE_URL: postgresql+asyncpg://postgres:postgres@db:5432/test
  before_script:
    - pip install uv && cd backend && uv sync --frozen
    - uv run alembic upgrade head
  script:
    - uv run pytest -v --cov=app --cov-report=term-missing --cov-report=html
  allow_failure: false

Copy, paste, adapt. It works.

The Point

Tests are the safety net. Refactor with confidence. Ship knowing the code works.

That's the deal.

Next up: [Frontend Tests] -coming soon- — Now prove the UI works too.

DEV Community