How do you test async FastAPI endpoints and async dependencies?

Use httpx.AsyncClient with ASGITransport for async tests, and pytest-asyncio to run async test functions. Combine with app.dependency_overrides to mock async dependencies.

# pip install pytest pytest-asyncio httpx
# conftest.py
import pytest
from httpx import AsyncClient, ASGITransport
from app.main import app, get_db
from app.database import AsyncSessionLocal, Base, engine

@pytest.fixture(scope="function")
async def async_client():
    # Use a test database
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)

    async def override_db():
        async with AsyncSessionLocal() as session:
            yield session

    app.dependency_overrides[get_db] = override_db

    async with AsyncClient(
        transport=ASGITransport(app=app),
        base_url="http://test",
    ) as client:
        yield client

    app.dependency_overrides.clear()
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.drop_all)

# tests/test_users.py
import pytest

@pytest.mark.asyncio
async def test_create_user(async_client):
    response = await async_client.post(
        "/users",
        json={"username": "alice", "email": "alice@example.com"},
    )
    assert response.status_code == 201
    data = response.json()
    assert data["username"] == "alice"
    assert "password" not in data  # response_model excludes it

@pytest.mark.asyncio
async def test_list_users_empty(async_client):
    response = await async_client.get("/users")
    assert response.status_code == 200
    assert response.json() == []

@pytest.mark.asyncio
async def test_validation_error(async_client):
    response = await async_client.post("/users", json={"username": "ab"})  # too short
    assert response.status_code == 422
    errors = response.json()["detail"]
    assert any("username" in str(e) for e in errors)

# pytest.ini
[pytest]
asyncio_mode = auto  # or use @pytest.mark.asyncio on each test

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