python-api-design

Python API Design (FastAPI)

Overview

Keep the FastAPI app small and testable. Build with create_app() so pytest can use TestClient without starting a real server on a fixed port.

Process

1. Register routes on the app from create_app() in app/main.py.
2. Use Pydantic models for request/response bodies when shapes matter.
3. Return correct status codes; use HTTPException for client errors.
4. Add a pytest case for each new route or behavior change.
5. Run pytest before claiming completion.

Project Layout

• app/main.py — create_app() factory and route definitions
• tests/ — pytest tests using fastapi.testclient.TestClient
• requirements.txt — pinned runtime + test dependencies

Testing

from fastapi.testclient import TestClient
from app.main import create_app

def test_health():
    client = TestClient(create_app())
    assert client.get("/api/health").json() == {"status": "ok"}

Guidelines

• Type hints on route functions and shared helpers.
• Secrets from environment variables (os.environ), never hardcoded.
• Keep route handlers thin; extract logic to pure functions when non-trivial.
• Prefer dependency injection for DB sessions and auth when the app grows.

Anti-patterns

• Business logic embedded directly in route decorators without tests.
• Returning raw exception strings to clients in production.
• Skipping tests because "FastAPI validates for us" — test behavior, not just schemas.