FastAPI/Starlette tests fail with 'RuntimeError: Event loop is closed' or 'attached to a different loop' (pytest-asyncio, httpx AsyncClient): how do I fix it?

Cause: Async objects (database engines or pools, httpx clients, background tasks) are bound to the event loop they were created in. pytest-asyncio gives each test its own function-scoped event loop by default, and closes it afterwards. If a module-level or session-scoped fixture, or an object created at import time, is used from a test running in a different loop, or is cleaned up after its loop has been closed, you get "RuntimeError: Event loop is closed" or "Task attached to a different loop". Fix (best first): 1. Create loop-bound objects inside async code, not at import time: build the engine, client or pool in an async fixture or in the app's startup/lifespan code. 2. Make fixtures and tests share a loop. pytest-asyncio provides one loop per collector scope (function, class, module, package, session) and runs tests in the function-scope loop unless told otherwise. Give long-lived async fixtures and the tests that use them a matching loop_scope, for example @pytest.mark.asyncio(loop_scope="module") on the tests together with an async fixture declared for the same loop scope (see the fixture and concepts pages for the exact spelling in your version). Keep neighbouring tests on the same scope. 3. In strict mode, declare async fixtures with @pytest_asyncio.fixture, not @pytest.fixture. 4. Use httpx's AsyncClient inside an async with block so it is closed before the loop ends, with ASGITransport(app=app) and a placeholder base_url like http://test, as the FastAPI docs show. TestClient cannot be used inside async test functions. 5. AsyncClient does not run lifespan startup/shutdown events; if your app needs them, use asgi-lifespan's LifespanManager or the synchronous TestClient in a non-async test. 6. Do not try to override or re-create the event_loop fixture; current pytest-asyncio versions use loop scopes instead. Verify: run pytest -x -p no:randomly several times; the suite passes without "Event loop is closed" and without teardown warnings. Sources: https://pytest-asyncio.readthedocs.io/en/stable/concepts.html https://fastapi.tiangolo.com/advanced/async-tests/ https://www.python-httpx.org/async/