Summary

This PR reimplementscached_property in slotted classes as a slot wrapping descriptor replacing the current generated__getattr__ implementation.

Accessing the attributes will probably be slightly slower due to going through this descriptor but if the performance is reasonable this willclose#1288#1325 and#1333.

This idea came up again as part of discussion onhttps://discuss.python.org/t/extensible-cached-property/105188/25. I hadn't realised it would work, but it looks like there was a partial attempt to implement it for attrs before in#1357

I've implemented it for my own attrs/dataclasses-likehere (the _SlottedCachedProperty code is mostly identical).

As I saw there were outstanding issues for it and the code is fairly similar I thought you might also find it useful. (Passing all of your existing tests also gave me some confidence that it does work correctly).

Changes:

• Adds a_SlottedCachedProperty descriptor class that replaces the slot descriptors after attrs has rebuilt the class.
  • This is very similar tocached_property itself, other than reading/writing/deleting a wrapped slot rather than dict entry.
• Removes the customised__getattr__ function you were making previously.
  • There are probably some redundant tests that were checking the__getattr__ behaviour that are now just checking__getattr__ works in Python.

Outstanding issues:

This was already the case for the__getattr__ implementation but I'm now aware of one difference in behaviour this hasn't yet resolved as it would require changing the logic of when you create a new cached property. There's currently a test marked as xfail for this.

importattrsimportfunctoolsforslottedin [False,True]:@attrs.define(slots=slotted)classParent:name:str="Alice"@attrs.define(slots=slotted)classChild(Parent):@functools.cached_propertydefname(self):return"Bob"# This isn't to imply that this is good# just that it's consistentp=Parent()c=Child()assertp.name=="Alice"assertc.name=="Alice"delc.nameassertc.name=="Bob"# True without slots, errors under slots.

This is because the cached_property replacement checkattrs does skips field names that already exist, soname is skipped in the check and doesn't get replaced. The descriptor would behave like this, but it's never placed.

Pull Request Check List

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    The next version is the second number in the current release + 1.
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