Fix contextvars not propagated from fixture to test

[linw1995]

I manage to use a global value current_context, that stores the current context after entering the async generator fixture. Apply it before running fixtures set up and test functions. Clean it up after the test functions finished.

Fix #127

[linw1995]

There is no one who maintains this repository? If my code has something wrong, please tell me.

[Tinche]

I'm just very busy, thank you for your patience :)

[jpic]

Is this an alternative to #153 ?

[linw1995]

Is this an alternative to #153 ?

Well yes but actually no. It uses copy_context() to store the context after the yield fixture setup, and then applies this context to run the coroutine tests. It will work fine in some cases.

I use it for a long time, and I got a case that will cause an exception.

var_db = ContextVar("db")


@pytest.fixture
@pytest.mark.asyncio
def ensure_db():
    db = ...
    token = var_db.set(db)
    try:
        yield
    finally:
        var_db.reset(token)

var_db.reset(token) will raise an exception due to applying a new context on its finalizer.

To avoid applying a new context instead of its original context object, the new solution needs to implement two extra functions.

1. able to access and write the original context object, instead of duplicating it via copy_context.
2. able to manage all original context objects, apply them on their right coroutines.

There may are two ways to implement the first function. But I haven't enough time to do that.

1. Change the CPython code to expose APIs that allow us to modify the current context variables.
2. Set the asyncio.tasks._PyTask as the default task factory, so we can modify the context by accessing the _PyTask._context attribute.

Update

This is a general problem not causing by this pull request but by the pytest-asyncio.
See this link for more details about why it is a general problem. https://gist.github.com/linw1995/a50928ed90d5e8c8c1b9575d7d7fe70c
pytest-asyncio runs the __anext__ method of the same async_generator in different coroutines.

so, we can say this pull request resolves this #153?

[jpic]

Confirmed this is even better than #153 because it shares contextvars from fixtures to test indeed.

Thank you so much ❤️

🎩

[zbentley]

What are the chances of this or #153 making it into the next release of pytest-asyncio?

No problem either way; if it's unlikely we'll publish a wheel of one of those branches temporarily to our internal pypi mirror, but if it's going to ship soon there's no need to spend that time. Just curious!

[joeblackwaslike]

Hey guys, first off this is really excellent work in this PR! I noticed however that it's a bit out of date (2020). I was able to apply the same approach to the current branch on master with a little bit of finess (https://github.com/joeblackwaslike/pytest-asyncio/blob/master/pytest_asyncio/plugin.py).

Since this PR is kind of stale and contains some nontrivial conflicts preventing it from being approved/merged, I'm curious what would best help get the ball rolling? I don't believe I can push to the linw1995 repo but could maybe open my own PR from mine if that helps?

Let me know the best approach forward here. I'd personally love to see this merged because without it, it requires a lot of app/request context boilerplate to write integration tests for quart apps. For that reason we're currently vendoring this dependency, which is less than ideal.

[seifertm]

It's really unfortunate that this PR has been lying around for such a long time, especially since it seems to be very well tested. It's very nice to hear about a specific use case (Quart tests), though.

At the moment, I'm not a huge supporter of the PR. Pytest-asyncio already tends to break in unexpected ways between releases. I fear that introducing functionality for transplanting the contextvars will open a can of worms. I'd much prefer an approach that reduces complexity, instead.

Python 3.11 introduces asyncio.Runner, a context manager which allows multiple functions to run in the same asyncio.Context. [0][1] There is a proposal to use this in pytest-asyncio, because it would make various things much easier. The condition for asyncio.Runner to be usable is that there's a reasonable way to backport the functionality to earlier Python versions.

@joeblackwaslike To get the ball rolling from my POV: We should look into how asyncio.Runner can be used to run tests and their fixtures in the same asyncio context. Any feedback is appreciated! This is probably not the answer you wanted to hear, sorry :)

[0] https://bugs.python.org/issue47062
[1] python/cpython#31799

[joeblackwaslike]

Python 3.11 introduces asyncio.Runner, a context manager which allows multiple functions to run in the same asyncio.Context. [0][1] There is a proposal to use this in pytest-asyncio, because it would make various things much easier. The condition for asyncio.Runner to be usable is that there's a reasonable way to backport the functionality to earlier Python versions.

@joeblackwaslike To get the ball rolling from my POV: We should look into how asyncio.Runner can be used to run tests and their fixtures in the same asyncio context. Any feedback is appreciated! This is probably not the answer you wanted to hear, sorry :)

[0] https://bugs.python.org/issue47062 [1] python/cpython#31799

I'm currently unaware of any attempts to backport asyncio functionality from newer versions of python to older versions so I'm really not sure where to start. If you do that would be useful information. Can you link me to the current proposal to use asyncio.Runner?

When I have some additional free time, I may try to backport the asyncio.Runner class to python 3.7 to see if it's even compatible with an older event loop.