I've just released logot - a log capture and assertion plugin 🚀

[etianen]

Hello! 👋 I've just released logot, a log capture and assertion plugin for pytest.

I've posted a show and tell in the pytest forum about using it as a replacement for caplog. But that's actually one of the less-interesting parts of the plugin, to me at least! What I'd like to show here is an example of using it to test highly concurrent asynchronous code. 🙇

Testing concurrent async code 🧶

Imagine the following code running in an asyncio.Task:

async def poll_daemon(app: App) -> None:
    while not app.stopping:
        await asyncio.sleep(app.poll_interval)
        logger.debug("Poll started")
        try:
            app.data = await app.get("http://is-everything-ok.com/")
        except HTTPError:
            logger.exception("Poll error")
        else:
            logger.debug("Poll finished")

Testing this sort of code is tricky, as it's running in a loop and not returning a value. You probably want to check that app.data has been fetched correctly, but with this code running in a background task, you have no way of knowing when that is.

While it’s possible to rewrite this code in a way that can be tested without logot, that risks making the code less clear or more verbose. For complex asynchronous code, this can quickly become burdensome. 👎

But testing this code with logot is easy!

from logot import Logot, logged

def test_poll_daemon(logot: Logot, app: MyApp) -> None:
    with asyncio.TaskGroup() as tasks:
        # Start the poll daemon.
        poll_task = tasks.create_task(poll_daemon(app))
        # Wait for a poll cycle to complete.
        await logot.await_for(logged.info("Poll started"))
        await logot.await_for(logged.info("Poll finished"))
        # Test the app data.
        assert app.data = {"this_is": "great data"}
        # Cancel the poll task.
        poll_task.cancel()

You'll see a couple of things here. A logot fixture, and a logged API. Use these together to make neat little log assertions. The logot.await_for(...) method pauses your async test task until the expected logs arrive, or a configurable timeout expires.

Bells and whistles 🔔

The logot.await_for(...) API is pretty powerful and includes:

• Support for log message matching using %-style placeholders.
• Support for log pattern matching using log pattern operators.
• Support for 3rd-party logging frameworks (e.g. loguru).

I hope you like it! ❤️

This is only a v1 release, but it's building on a lot of ideas I've been developing in different projects for a while now. I hope you like it, and find it useful.

The project documentation and pytest integeration guide are there if you'd like to find out more. 🙇