Add mechanism for explicit marking of fixtures which should be run with asyncio

[pipermerriam]

• Fixes Require explicit marks for async fixtures #124

Opening this up to get feedback from the maintainers.

With this patch existing users of pytest-asyncio should not experience any change in behavior.

This exposes a new API pytest_asyncio.asyncio_fixture that is intended to be used like this:

import pytest_asyncio


@pytest_asyncio.asyncio_fixture
async def server():   # async yield fixture
    await server.start()
    yield
    await server.stop()

@pytest_asyncio.asyncio_fixture
async def client():  # normal async function
    await client.connect_to_server()
    return client

This PR also adds a pytest.ini configuration value: asyncio_mode which defaults to True. When set to False, only fixtures with the above mark (as well as a few other conditions) will be run by the asyncio event loop.

Some notes:

• I did not see a test suite. This was tested manually against a local project. If you'd like me to write up some tests I don't mind, but it felt heavy handed to do that without the blessing of a maintainer.
• Looking for feedback on whether this is a feature that will be accepted before I polish this and make it nice.

[asvetlov]

I'm not a maintainer but have 5 cents to add.

I think we need not a bool flag but three-state:

1. "on": automatically apply pytest.mark.asyncio to all discovered asynchronous fixtures and tests
2. "off": do nothing with autoapplying
3. "compat" (which is default and deprecated): the current behavior. Apply asyncio mark to async fixtures but don't apply to async tests.

Having default "compat" mode deprecated we notice people switch either to "on" or "off" in their pytest configuration to avoid deprecation warning displayed by pytest.
After some deprecation period we can switch default and even drop "compat" mode eventually.

[asvetlov]

@pipermerriam do you want to implement on/off/compat mode?

[pipermerriam]

@asvetlov yes, but it will be about 2 weeks before I pick this back up. Life and work things taking front seat for a bit and won't be able to focus on this for till then. (won't be offended if anyone takes what I started and moves forward with it)

[asvetlov]

I'm personally is overwhelmed as well by my job and fixing CPython 3.8 beta.
I had a hope to finish 3.8 hacking this week but seems I need more time.
There is a low chance that I can pick up your job in a week or two.

[graingert]

@pipermerriam I had a go at resolving the conflicts, but I'm not sure what needs doing in pytest_asyncio/plugin.py

Gentoo are currently having trouble when running the anyio test suite with pytest-asyncio installed

[codecov-commenter]

Codecov Report

Merging #125 (7fb70ed) into master (d48569e) will decrease coverage by 0.58%.
The diff coverage is 94.66%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   96.17%   95.59%   -0.59%     
==========================================
  Files           2        2              
  Lines         157      227      +70     
==========================================
+ Hits          151      217      +66     
- Misses          6       10       +4     

Impacted Files	Coverage Δ
pytest_asyncio/plugin.py	95.53% <94.52%> (-0.62%)	⬇️
pytest_asyncio/__init__.py	100.00% <100.00%> (ø)

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[graingert]

@asvetlov thanks for taking this on!

[asvetlov]

The PR is done and tested.

@Tinche @seifertm I very much appreciate your review.

Please feel free to ask a question if elaboration is needed.

[Tinche]

Left suggestions for a couple of typos, looks good overall. A really good change!

[seifertm]

This is great! Tests shouldn't just break when pytest-asyncio is installed in addition to any other package. The new modes are also more consistent than the current behavior.

I like that we switched from @pytest_asyncio.asyncio_fixture to the shorter @pytest_asyncio.fixture. I also appreciate that asyncio_mode=strict is the new default.

I annotated a few typos and issues in the docs. Other than that: Great job!

[asvetlov]

Thanks, guys!