Support filtering the messages on a receiver

[shsms]

A new filter method is added to the Receiver interface, which allows the application of a filter function on the messages on a receiver.

Example:

async for message in receiver.filter(lambda num: num % 2):
    print(f"An even number: {message}")

[llucax]

What about adding an async_filter to the new core repo instead? This would be useful with any async iterator. I would also remove map() and add an async_map() to core.

[llucax]

There is also https://snyk.io/advisor/python/asyncstdlib. Maybe we should start using that? It is strange that this is not part of Python stdlib already.

[llucax]

https://asyncstdlib.readthedocs.io/en/latest/source/api/builtins.html#asyncstdlib.builtins.filter

[llucax]

If we agree it is a good idea we could add it to repo-config so it is used by default, like we do with typing_extensions.

[shsms]

What about adding an async_filter to the new core repo instead? This would be useful with any async iterator. I would also remove map() and add an async_map() to core.

This gives out Receivers, we can do all the things we can do with receivers with them. Maybe we can have an additional async_filter or use asyncstd, but having it here is still valuable.

[shsms]

If we agree it is a good idea we could add it to repo-config so it is used by default, like we do with typing_extensions.

I do agree, it would be very useful, but I don't think it can replace this PR, because the filter in this PR returns receivers, which we can put in select or any of the many possibilities.

[llucax]

Right. OK, and I guess it is useful to get a receiver because it can be used in select(). It came to my mind a couple of times that maybe we should have a utility receiver that converts any async iterator into a receiver. So we could potentially do:

filtered_receiver = as_receiver(a.filter(lambda num: num % 2), original_receiver)
async for selected in select(filtered_receiver, ...):
    print(f"An even number: {message}")

But if we want to move in that direction, we'll eventually have to remove map() too, and that will be a breaking change, so I'm happy to leave that for 2.0 and add filter now if it is useful.

Will start the review now then...

[llucax]

What about adding an async_filter to the new core repo instead? This would be useful with any async iterator. I would also remove map() and add an async_map() to core.

This gives out Receivers, we can do all the things we can do with receivers with them. Maybe we can have an additional async_filter or use asyncstd, but having it here is still valuable.

• Consider adding asyncstdlib by default to the dependencies in the template frequenz-repo-config-python#282

[shsms]

Based on #301

[llucax]

LGTM. Not approving until it's not a draft anymore.

[llucax]

Also tests are failing:

src/frequenz/channels/_receiver.py:451:101: E501 line too long (138 > 100 characters)

[shsms]

Fixed the long line