FastAPI testing with TestClient — sync and async patterns

FastAPI apps need integration tests that exercise the full request lifecycle including dependencies, middleware, and exception handlers — not just unit tests of individual functions.

Use TestClient (sync) for most tests. For async test helpers, use httpx.AsyncClient with ASGITransport. Override dependencies with app.dependency_overrides.

from fastapi.testclient import TestClient
from myapp.main import app
from myapp.database import get_db

# Override DB dependency to use test DB
def override_get_db():
    db = TestSessionLocal()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides[get_db] = override_get_db
client = TestClient(app)

def test_create_item():
    response = client.post('/items/', json={'name': 'Foo', 'price': 1.5})
    assert response.status_code == 201
    assert response.json()['name'] == 'Foo'

# Async test with httpx
import pytest
import httpx

@pytest.mark.anyio
async def test_async_endpoint():
    async with httpx.AsyncClient(transport=httpx.ASGITransport(app=app), base_url='http://test') as ac:
        resp = await ac.get('/items/')
    assert resp.status_code == 200

TestClient wraps the ASGI app with requests under the hood, giving a synchronous interface that runs the full ASGI stack. dependency_overrides is a dict mapping original dependency callables to replacement callables — the cleanest way to inject test doubles.

Writing automated tests for FastAPI applications