Add __slots__ to AsyncResource

[injust]

Changes

AsyncResource should have __slots__ so child classes can use __slots__.

Similar to python/cpython#105726 and python/cpython#106771 for contextlib.AbstractAsyncContextManager.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

[agronholm]

Do you have a use case for this? None of the subclasses in AnyIO itself use __slots__.

[injust]

I'm creating my own child classes of AsyncResource with attrs, and was surprised that they weren't slotted classes. I'm assuming that's a supported use case, similar to how one might use contextlib.AbstractAsyncContextManager.

[agronholm]

Attrs doesn't require the superclasses to have __slots__ in order to create a slotted class:

>>> import attrs
>>> class Foo:
...   pass
... 
>>> @attrs.define(slots=True)
... class Bar:
...   x: int
... 
>>> b = Bar(1)
>>> b.y = 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Bar' object has no attribute 'y'

[injust]

You forgot to subclass Bar from Foo in your example.

From https://threeofwands.com/attrs-ii-slots/ (which is a really good write-up on slotted classes btw):

Slot classes lose a bunch of their advantages if the inheritance chain contains even a single dict class, because there will be an instance dictionary associated with every instance. Slot classes all the way!

[agronholm]

Right, I forgot to do that. It seems that attrs has changed the way it operates, because it used to work this way (although I will say it was the wrong thing to do). The only question that remains then, is whether we want to include the __weakref__ slot here.

[injust]

It seems that attrs has changed the way it operates, because it used to work this way

I haven't been using attrs for too long so I can't comment on that. Only thing that comes to mind is that slotted classes are default for attrs.define and attrs.frozen,

whether we want to include the __weakref__ slot here

I'm -0 on this. If someone wanted a weak referenceable-AsyncResource, they could create their own subclass (and have it be slotted or not).

[agronholm]

Alright, you convinced me.