Async Tests with httpx and pytest

When tests need to await things (async database fixtures, async services), use httpx.AsyncClient with ASGITransport(app=app) and an async test runner (pytest-asyncio or anyio's pytest plugin). Note that ASGITransport doesn't run lifespan events; use asgi-lifespan's LifespanManager or set up resources in fixtures.

import pytest
from httpx import ASGITransport, AsyncClient
from asgi_lifespan import LifespanManager
from app.main import app


@pytest.fixture
async def client():
    async with LifespanManager(app):
        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as c:
            yield c


@pytest.mark.anyio
async def test_create_and_fetch_book(client):
    created = await client.post("/books", json={"title": "Dune", "price_cents": 1299})
    assert created.status_code == 201
    fetched = await client.get(f"/books/{created.json()['id']}")
    assert fetched.json()["title"] == "Dune"

Common pitfalls

• Mixing sync TestClient with async fixtures on different event loops.
• Forgetting the async marker/config so tests never await.
• Assuming ASGITransport runs lifespan.

Key points

• AsyncClient + ASGITransport
• anyio or pytest-asyncio
• Handle lifespan explicitly

Further reading: FastAPI: Async tests