The documentation on "Background tasks" is wrong

[cecton]

Long story short

In the example provided here: https://github.com/aio-libs/aiohttp/blob/master/docs/web.rst#background-tasks we add a new key to the application which is a background task.

If you activate the warnings using -Wdefault and execute the script, you will get:

/tmp/b.py:15: DeprecationWarning: Changing state of started or joined application is deprecated

Expected behaviour

Not sure. Not having the warning? The example doesn't look bad, only make_handler is called before the on_startup signal which freezes the app before the signal is sent.

Actual behaviour

When the example in the doc is executed, you get the warning.

Steps to reproduce

I made this variant to test it out:

import asyncio
from aiohttp import web


async def listen_to_redis(app):
    try:
        await asyncio.sleep(200)
    except asyncio.CancelledError:
        pass
    finally:
        print("cleanup")


async def start_background_tasks(app):
    app['redis_listener'] = app.loop.create_task(listen_to_redis(app))


async def cleanup_background_tasks(app):
    app['redis_listener'].cancel()
    await app['redis_listener']


app = web.Application()
app.on_startup.append(start_background_tasks)
app.on_cleanup.append(cleanup_background_tasks)
web.run_app(app)

And run: python -Wdefault <script>.py

Your environment

Python 3.6, Arch Linux

[asvetlov]

The warning should be relaxed: see #1947

[cecton]

Okay good to know thanks.

[asvetlov]

Fixed

[lock]

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