How would you test asynchronous code written with asyncio?
Use pytest-asyncio or IsolatedAsyncioTestCase to run coroutines, fake slow dependencies, and control the clock
Async code needs an event loop to run, so a plain test function cannot just await. The modern options are pytest-asyncio with @pytest.mark.asyncio, which gives you an async test function and a loop per test, or unittest.IsolatedAsyncioTestCase if you are staying in the stdlib. Both give a fresh event loop and clean up pending tasks, which matters because leftover tasks across tests cause flakiness. For dependencies, use AsyncMock to fake coroutine collaborators and assert with assert_awaited_once_with. For timing, avoid real sleeps: patch asyncio.sleep or use a library like aioresponses or a faker clock, because real sleeps make the suite slow and nondeterministic. Test cancellation and timeouts explicitly, because those paths are the ones that break in production.
- 1
pytest-asyncio with @pytest.mark.asyncio or asyncio_mode = auto is the common setup.
- 2
unittest.IsolatedAsyncioTestCase is the stdlib option and works with the standard runner.
- 3
AsyncMock (3.8+) is required to mock coroutine functions. A regular Mock returns a coroutine that is never awaited and the test will warn or fail.
- 4
Use asyncio.wait_for to test timeouts, and assert on asyncio.TimeoutError or TimeoutError in 3.11+.
- 5
Test task cancellation by cancelling and asserting on the cleanup path.
- 6
Trade-off: loop-per-test is more isolated but slower than a shared loop. Loop-per-test is the safer default.
- 7
Common mistake: leaving background tasks running after the test, which leaks into other tests and causes flaky failures.
- 8
Common mistake: patching time.sleep when the code awaits asyncio.sleep. Patch the right function.
- 9
Version note: asyncio.TimeoutError was unified with built-in TimeoutError in 3.11, which simplifies exception assertions.
Scenario Questions
0-2 years experience
- 1Why can't a normal def test function await a coroutine?
- 2Which mock class do you use to replace an async function?
2-5 years experience
- 1Your async test sometimes fails because a background task keeps running after the test ends. How do you fix it?
- 2You want to test a timeout path without waiting the real timeout. How do you do it?
5-8 years experience
- 1You need to test a client that fans out to three services with per-call timeouts and partial failures. How do you structure the test and the fakes?
- 2Your tests pass locally and fail in CI due to event loop policy differences. How do you make the loop configuration consistent?
8+ years experience
- 1Design a test suite for an async service that covers cancellation, back-pressure, and graceful shutdown without flakiness or leaked tasks.
- 2Explain how pytest-asyncio modes, loop fixtures, and async generators interact, and how to migrate a large suite safely.
Follow-up Questions
- Why does a regular Mock fail when mocking a coroutine function?
- How would you test that a coroutine cancels its children when the parent is cancelled?