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Graph/blocking-sync-in-async-fixture

AntiPattern

Async Fixture Event Loop

blocking-sync-in-async-fixture

Async tests and mocks fail when awaited objects, async context managers, or blocking sync calls are mixed incorrectly inside an event loop, causing coroutines to be returned, contexts to lack enter/exit, and the loop to stall or fixtures to misbehave.

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Updated7/29/2026

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30 linked nodes

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Instead of mocking local storage, I just need to access it directly — This is my test code using Jest + TypeScript. Tension: mocking local storage. Outcome: The final test should look like this:.

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Outcome: To solve this, I created a custom `MockPointerEvent` and mocked the necessary browser methods.

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Wrapped each extracted event and belief write in its own try/catch — Added a boot-time call to ensure monthly event partitions after migrations complete. Tension: preserving lastError logging and continuing after failures. Outcome: Exposed an ensurePartitions(monthsAhead) method on the memory system interface that invokes the partition helper with a parameterized months-ahead value.

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Mock axios.get and await the assertion with waitFor()

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Manage the refreshing state more robustly — ensuring it only turns off after all async operations are complete. Tension: remove the arbitrary timeout. Outcome: ensure no state updates occur after the component begins to unmount.

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beforeEach(() => { vi.useRealTimers() }) — to the describe of the tests. Tension: I cannot modify and remove the setup scripts. Outcome: afterEach(() => { vi.useFakeTimers() }).

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delete the async from `fake_execute` — your `AsyncContextMock` needs to handle both use cases in your endpoint `await conn.execute()` and `async with conn.execute()`. Outcome: just make it a regular function so it returns your context manager directly, no coroutine wrapper.

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for `cursor.fetchone.return_value`, just use (2,) directly — `cursor.fetchone.return_value`. Tension: AsyncMock already makes it awaitable. Outcome: wrapping it again messes things up.

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single worker running async loop. Outcome: So you have a single worker running async loop and you block that loop.

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If you convert all operations to perform asynchronously (using `aiosqlite`) , the issue will be mitigated. — When a route in FastAPI is listed as asynchronous (`async def`). Tension: that doesn't fully solve the inherent single writer problem of Sqlite. Outcome: Here's a fully operational update of the FastAPI server logic:.

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Change the `mocked_app` and database `client` fixtures from `scope='session'` to `scope='function'` so each test gets a fresh app/client setup with a clean lifecycle.

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Using a `session-scoped` pytest fixture, for managing the event loop — which ensures the same event loop will be used. Outcome: loop.close().

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mock_client.update_one = AsyncMock(return_value=AsyncMock(matched_count=1, modified_count=1)). Outcome: Here the minimal code to make your example works.

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Use pytest_asyncio.fixture NOT pytest.fixture — @pytest_asyncio.fixture(scope='session', loop_scope='session'). Tension: async def my_fixture():. Outcome: yield 'foo'.

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You need to install helper packages, such as `anyio` or `pytest-asyncio`. — Async fixtures do not work out of the box. Tension: NB. Use pytest_asyncio.fixture NOT pytest.fixture. Outcome: Example.

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Change the engine async fixture from session scope to function scope (remove scope="session") so all async fixtures/tests run on the same event loop, preventing the cross-loop Future/Task error.

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using `magicmock` to mock it could lead to unusual or unexpected behavior when checking for call counts — Since `get_item` might be an asynchronous function in your middleware. Tension: using `magicmock` to mock it could lead to unusual or unexpected behavior. Outcome: which is designed to work with async functions.

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Using `asyncio.sleep()` in a loop is perfectly fine. Tension: every time you `await` the sleep, you effectively yield control back to the event loop. Outcome: calling `asyncio.sleep(0)` does not sleep at all, but only yields control back to the event loop.

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the reason for the various threads appearing in the logs — when you pass a synchronous generator to `EventSourceResponse`. Tension: a normal `def` function instead of `async def`. Outcome: please take a look at the implementation of the `EventSourceResponse` class.

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use `pytest_asyncio` and the async fixture — If you don't want to utilize an `AsyncClient` like the one by `httpx`. Tension: ensuring that the FastAPI app's lifespan context correctly works.

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any time you have an I/O bound process with lots of idle time — waiting for user input, waiting for a TLS handshake. Tension: wrapping it in an async task can let other code run.

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Initialize Eureka using the async API (e.g., `eureka_client.init_async(...)`) and/or move initialization into an appropriate async startup hook so it doesn’t start a new event loop at import time.

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Ensure the S3 download runs in a way that doesn’t block the event loop (i.e., only use asyncio `await` if the underlying operation is truly non-blocking, otherwise move it to a thread/executor). Use `asyncio.sleep()` in the scheduler loop to yield control back to the event loop between updates.

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Avoid running multiple DB operations concurrently on the same session by removing asyncio.gather and iterating sequentially, and/or use an async_scoped_session so each task uses its own session.

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Alternately, you could treat tests with mocks as unit tests which don't involve JavaFX code.

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I think it is better to use a custom context manager — if you need to isolate test methods from each other. Tension: you need to isolate test methods from each other. Outcome: here is what I am using.

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Rewrite the test as an async test and run it with an async test runner (e.g., pytest.mark.asyncio), using await on the async ORM methods (acreate/acount) instead of asyncio.run().

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Use Component.Task (TaskSource) for all async work inside components — inside components. Tension: it auto-cancels when the component is destroyed.

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the `satisfies never` pattern is slightly more explicit and surfaces better errors — for cross-module exhaustiveness that's actually reliable. Tension: surfaces better errors. Outcome: const _: never = event // will error when non-exhaustive.

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Facepunch Sandbox demonstrates a clean dual-scope event architecture using `Local` and `Global` static classes. Tension: Difficulty implementing cancellable events.

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