Unexpected ContextVar handling for lifespan context managers

[rmorshea]

Description

A ContextVar set within a lifespan context manager is not available inside request handlers. By contrast, doing the same thing via an application level dependency does appear set in the require handler.

MCVE

import asyncio
from collections.abc import AsyncIterator
from contextlib import asynccontextmanager
from contextvars import ContextVar

from litestar import Litestar
from litestar import get
from litestar.testing import AsyncTestClient

VAR = ContextVar[int]("VAR", default=0)


@asynccontextmanager
async def set_var() -> AsyncIterator[None]:
    token = VAR.set(1)
    try:
        yield
    finally:
        VAR.reset(token)


@get("/example")
async def example() -> int:
    return VAR.get()


app = Litestar([example], lifespan=[set_var()])


async def test() -> None:
    async with AsyncTestClient(app) as client:
        response = await client.get("/example")
        assert (value := response.json()) == 1, f"Expected 1, got {value}"


if __name__ == "__main__":
    asyncio.run(test())

Steps to reproduce

Run the example above. Either via python example.py or uvicorn example:app and check the /example route.

We should expect the response to be 1 since that's what's set during the lifespan context manager, but it's always 0.

If you instead swap the lifespan context manager for a dependency it works as expected:

import asyncio
from collections.abc import AsyncIterator
from contextvars import ContextVar

from litestar import Litestar
from litestar import get
from litestar.testing import AsyncTestClient

VAR = ContextVar[int]("VAR", default=0)


async def set_var() -> AsyncIterator[None]:
    token = VAR.set(1)
    try:
        yield
    finally:
        VAR.reset(token)


@get("/example")
async def example(set_var: None) -> int:
    return VAR.get()


app = Litestar([example], dependencies={"set_var": set_var})


async def test() -> None:
    async with AsyncTestClient(app) as client:
        response = await client.get("/example")
        assert (value := response.json()) == 1, f"Expected 1, got {value}"


if __name__ == "__main__":
    asyncio.run(test())

Litestar Version

2.12.1

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

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[euri10]

vey interesting, I played a litttle with it, changing lifespan to the old way with on_startup / on_sutdown and the result is even weirder.

I didn't go through all this thread but that seems related pytest-dev/pytest-asyncio#127

note on my "tests" I didnt use pytest-asyncio but anyio

[rmorshea]

It looks like this is expected behavior in Starlette/FastAPI. The response there is pretty turse so it's hard to see exactly why.

[rmorshea]

If this turns out to be a fundamental limitation it might be nice to support dependencies that have side effects but don't actually return a value. I could imagine something similar to Pytest's auto use fixtures:

VAR = ContextVar("VAR", default=0)


async def set_var() -> AsyncIterator[None]:
    token = VAR.set(1)
    try:
        yield
    finally:
        VAR.reset(token)


@get("/", dependencies={"set_var": Provide(set_var, always=True)})
async def get_thing() -> None:
    assert VAR.get() == 1

[euri10]

yep, pytest-asyncio might add another layer of complexity but this fails without regardless so the issue lies elsewhere,

now adding some goold old-style prints to this:

from collections.abc import AsyncIterator
from contextlib import asynccontextmanager
from contextvars import ContextVar

from litestar import Litestar
from litestar import get



VAR = ContextVar("VAR", default=0)
print(f"1: {VAR}")


@asynccontextmanager
async def set_var() -> AsyncIterator[None]:
    token = VAR.set(1)
    try:
        yield
    # except Exception as exc:
    #     print(exc)
    finally:
        print("Finally")
        VAR.reset(token)


@get("/example")
async def example() -> int:
    print(VAR)
    return VAR.get()


app = Litestar([example], lifespan=[set_var()], debug=True)

if __name__ == "__main__":
    # asyncio.run(test())
    print(f"1: {VAR}")
    import uvicorn
    uvicorn.run("cv:app", reload=True)

I get this log, note that there is something going on if I understand it clearly with the way the var is created / initialized as the handler seems to not use the same var that was created in lifespan

/home/lotso/PycharmProjects/abdul/.venv/bin/python /home/lotso/PycharmProjects/abdul/cv.py 
1: <ContextVar name='VAR' default=0 at 0x7fb24500be20>
1: <ContextVar name='VAR' default=0 at 0x7fb24500be20>
INFO:     Will watch for changes in these directories: ['/home/lotso/PycharmProjects/abdul']
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [436898] using WatchFiles
1: <ContextVar name='VAR' default=0 at 0x7fabc7fedfd0>
1: <ContextVar name='VAR' default=0 at 0x7fabc57972e0>
INFO:     Started server process [436904]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO - 2024-10-09 08:08:12,131 - watchfiles.main - main - 3 changes detected
<ContextVar name='VAR' default=0 at 0x7fabc57972e0>
INFO:     127.0.0.1:57262 - "GET /example HTTP/1.1" 200 OK

[rmorshea]

Here's a longer running issue: encode/uvicorn#1151

[rmorshea]

For additional context (😉) here's related Pytest issue.