Improve `asyncio.loop.add_read` and `asyncio.loop.add_writer` documentation

[jmgurney]

Documentation

The documentation for add_read/add_writer (https://docs.python.org/3/library/asyncio-eventloop.html#watching-file-descriptors ) does not specify what happens if you add two different callbacks for the same fd. Do both callbacks get called? Or does only one get called? Does the first one or the last callback get called (or maybe a random one, unlikely, but could be valid per current documentation)?

Linked PRs

• gh-126137: improve docs for loop.add_reader and loop.add_writer #128666
• [3.13] gh-126137: improve docs for loop.add_reader and loop.add_writer (GH-128666) #128667
• [3.12] gh-126137: improve docs for loop.add_reader and loop.add_writer (GH-128666) #128668

[jmgurney]

Looks like it replaces the last callback: https://floss.social/@gwidion/113392129452013893

We better check the source! [me open some .py files in cpython/Lib/asyncio] [parse some code. confirm docs to "selector.modify" at https://docs.python.org/3/library/selectors.html#selectors.BaseSelector.modify]

Yep: asyncio will just register a single reader/writer CB per FD. The previous one is cancelled when adding another.

[picnixz]

Thanks! You're welcome to submit a PR for patching the docs! (but check whether there isn't more explanation in the tutorial or somewhere else; asyncio docs are a bit spread around)