api-testing

API Testing Patterns Skill

Expertise: API integration tests (supertest/httpx), contract testing (Pact), auth flows, error path coverage.

Integration Test Structure (FastAPI/httpx)

import pytest
import pytest_asyncio
from httpx import AsyncClient

# conftest.py — reusable fixtures
@pytest_asyncio.fixture
async def client(app, db_session) -> AsyncClient:
    async with AsyncClient(app=app, base_url="http://test") as c:
        yield c

@pytest_asyncio.fixture
async def auth_client(client, create_user) -> AsyncClient:
    user = await create_user(role="viewer")
    response = await client.post("/auth/token",
        data={"username": user.email, "password": "test_password"})
    token = response.json()["access_token"]
    client.headers["Authorization"] = f"Bearer {token}"
    return client

# Test — covers happy path + all error cases
class TestCreateOrder:
    async def test_creates_order_for_authenticated_user(self, auth_client, product_factory):
        product = await product_factory(price="29.99", stock=10)

        response = await auth_client.post("/api/v1/orders", json={
            "items": [{"product_id": product.id, "quantity": 2}]
        })

        assert response.status_code == 201
        body = response.json()
        assert body["status"] == "pending"
        assert body["total_amount"] == "59.98"
        assert body["id"].startswith("ord_")

    async def test_returns_401_without_auth(self, client):
        response = await client.post("/api/v1/orders", json={"items": []})
        assert response.status_code == 401

    async def test_returns_400_when_product_out_of_stock(self, auth_client, product_factory):
        product = await product_factory(stock=0)
        response = await auth_client.post("/api/v1/orders", json={
            "items": [{"product_id": product.id, "quantity": 1}]
        })
        assert response.status_code == 400
        assert response.json()["error"]["code"] == "PRODUCT_OUT_OF_STOCK"

    async def test_returns_404_for_nonexistent_product(self, auth_client):
        response = await auth_client.post("/api/v1/orders", json={
            "items": [{"product_id": "prod_nonexistent", "quantity": 1}]
        })
        assert response.status_code == 404