Async testing documentation is missing details on selecting anyio backend

[Insolites]

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Example Code

# This is just the async testing example!
@pytest.mark.anyio
async def test_root():
    async with AsyncClient(app=app, base_url="http://test") as ac:
        response = await ac.get("/")

Description

This is due to this change: https://fastapi.tiangolo.com/release-notes/?h=trio#0690

Pytest's AnyIO plugin will try to run tests on all supported backends: https://anyio.readthedocs.io/en/stable/testing.html#specifying-the-backends-to-run-on

Unfortunately if you install FastAPI the usual way and don't explicitly install trio, then you don't actually have all the supported backends.

So if you simply follow the instructions for running async tests then you will get a really huge stack trace with:

FAILED tests/test_main.py::test_hello[trio] - ModuleNotFoundError: No module named 'trio'

A possible solution as described here and in the anyio docs is to add a conftest.py to select just the asyncio backend:

import pytest
@pytest.fixture
def anyio_backend():
    return 'asyncio'

If there's any way this could be done helpfully/magically by FastAPI that would be great, if not it should be mentioned in the documentation.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.110.0

Pydantic Version

2.6.4

Python Version

Python 3.11.6

Additional Context

No response

[gioscarda]

@Insolites I fixed it by changing the decorator:

@pytest.mark.parametrize('anyio_backend', ['asyncio'])
async def test_root(anyio_backend):
    ...

but for sure the FastAPI docs should be updated.

[stephanschielke]

Also, if you want to set this for different scopes (other than on the function level via a decorator):

pytestmark = pytest.mark.parametrize("anyio_backend", ["asyncio"], scope="module")