asyncio testing

docs/skills/pytest-scaffolding/references/asyncio-testing.md

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+# AsyncIO Testing Patterns (Pytest, FastAPI, AnyIO)
+
+!!! info "Primary sources"
+    - [pytest-asyncio configuration](https://pytest-asyncio.readthedocs.io/en/stable/reference/configuration.html)
+    - [pytest-asyncio concepts](https://pytest-asyncio.readthedocs.io/en/stable/concepts.html)
+    - [pytest-asyncio fixture loop scope how-to](https://pytest-asyncio.readthedocs.io/en/stable/how-to-guides/change_fixture_loop.html)
+    - [pytest-asyncio default fixture loop scope how-to](https://pytest-asyncio.readthedocs.io/en/stable/how-to-guides/change_default_fixture_loop.html)
+    - [AnyIO cancellation and cancel-scope safety](https://anyio.readthedocs.io/en/stable/cancellation.html)
+    - [FastAPI async tests](https://fastapi.tiangolo.com/advanced/async-tests/)
+
+## Agent Quick Path
+Use this reference when tests involve asynchronous fixtures, HTTP clients, task groups, or teardown failures.
+
+1. Confirm async plugin mode in pytest config (`asyncio_mode`).
+2. Keep async fixture loop scope predictable, defaulting to `function` unless there is a measured need to broaden it.
+3. Prefer one async testing model per lane (pytest-asyncio or AnyIO-style markers), and keep it consistent.
+4. Keep async fixtures small and isolate stateful resources to the narrowest useful scope.
+5. If teardown errors mention cancel scopes or task groups, validate that setup and teardown run in the same task context.
+
+## Baseline Configuration
+
+Recommended defaults for most projects using `pytest-asyncio`:
+
+```toml
+[tool.pytest.ini_options]
+asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "function"
+```
+
+Why:
+- [Strict mode](https://pytest-asyncio.readthedocs.io/en/stable/concepts.html#test-discovery-modes) is safer for multi-plugin environments, but [auto mode](https://pytest-asyncio.readthedocs.io/en/stable/concepts.html#test-discovery-modes) is often simpler when the suite is primarily asyncio-based.
+- [Function loop scope](https://pytest-asyncio.readthedocs.io/en/stable/reference/configuration.html#asyncio-default-fixture-loop-scope) minimizes cross-test coupling and avoids many lifecycle surprises.
+
+If a fixture or test needs broader loop sharing, make it explicit instead of changing suite-wide defaults:
+
+```python
+import pytest
+import pytest_asyncio
+
+
+@pytest_asyncio.fixture(loop_scope="module")
+async def shared_resource():
+    ...
+
+
+@pytest.mark.asyncio(loop_scope="module")
+async def test_uses_shared_loop(shared_resource):
+    ...
+```
+
+## FastAPI Endpoint Test Patterns
+
+Use [FastAPI's async testing guidance](https://fastapi.tiangolo.com/advanced/async-tests/) as the default:
+
+1. Use `httpx.AsyncClient` with `ASGITransport` for async endpoint tests.
+2. Mark async tests with one consistent marker style for the suite.
+3. If app lifespan hooks matter, add [LifespanManager](https://fastapi.tiangolo.com/advanced/async-tests/#httpx) support because `AsyncClient` alone does not trigger lifespan events.
+
+Example:
+
+```python
+import pytest
+from httpx import ASGITransport, AsyncClient
+
+
+@pytest.mark.asyncio
+async def test_healthz(app):
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+        response = await client.get("/healthz")
+
+    assert response.status_code == 200
+```
+
+## Fixture Design For Async Reliability
+
+Apply these patterns first:
+
+1. Keep async fixtures narrow (`function` scope by default).
+2. Keep one responsibility per fixture when possible.
+3. Prefer yield fixtures and pair each setup step with teardown in the same fixture.
+4. Avoid mixing many independent event-loop lifecycles in one fixture chain.
+
+When using transports that manage internal task groups (for example, streaming clients), avoid patterns that risk splitting lifecycle across different task contexts.
+
+## Troubleshooting Cancel-Scope Teardown Failures
+
+When you see errors like `Attempted to exit cancel scope in a different task than it was entered in`, treat it as an async lifecycle-ownership issue first.
+
+Checklist:
+
+1. Verify fixture and test loop scopes are compatible and explicit.
+2. Confirm async resource setup and teardown are owned by the same fixture context.
+3. Reduce fixture scope (`module` or `session` -> `function`) to test for loop/task ownership drift.
+4. Ensure the suite uses one primary async plugin model for the failing lane.
+5. Re-run with focused selection and skip reasons to isolate first failing fixture:
+   - `uv run --group test python -m pytest -m smoke tests/web -q -rs`
+
+Relevant references:
+- [Avoiding cancel scope stack corruption](https://anyio.readthedocs.io/en/stable/cancellation.html#avoiding-cancel-scope-stack-corruption)
+- [pytest-asyncio configuration](https://pytest-asyncio.readthedocs.io/en/stable/reference/configuration.html)
+- [pytest fixture teardown behavior](https://docs.pytest.org/en/stable/how-to/fixtures.html#teardown-cleanup-aka-fixture-finalization)
+
+## Commands Worth Remembering
+
+- `uv run --group test python -m pytest --collect-only -q`
+- `uv run --group test python -m pytest -m smoke tests/web -q -rs`
+- `uv run --group test python -m pytest -m integration -q`
+- `uv run --group test python -m pytest -q`