[issue21145] Add the @cached_property decorator

[Carl Meyer]

Carl Meyer added the comment:

Uploaded a patch updated per Nick's comment.

Not opposed to waiting to see if someone is motivated to implement a version in C that supports __slots__, but if that doesn't happen by the Python 3.7 feature deadline, I don't think it should block this proven version.

It also occurred to me that we could probably support __slots__ in pure Python without harming the non-slots case by implementing a fallback cache in the descriptor itself, keyed by instance in a WeakKeyDictionary. I don't love having the behavior differ so much between the slots and non-slots case, but maybe it's better than not supporting slots at all.

Re setting and deleting: under the current patch, if you set or delete a cached property, you set or delete the cached value. I think this is fine and useful behavior, but it could perhaps be added explicitly to the documentation.

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