[issue16832] Expose cache validity checking support in ABCMeta

[Antoine Pitrou]

Antoine Pitrou added the comment:

> I thought about that originally, but there's only ever one object
> graph for the process, and as soon as you break any one edge in that
> graph you pretty much invalidate anything based on caching traversal
> results.

Ah, I was forgetting that a ABC registration also affects the subclasses
of the registered class, and the abstract parents of the ABC class.
So it's a bit more annoying than it looked like.

> So it's not really an implementation detail, it's a promise to provide
> a unique token for the current state of the object graph that can be
> used reliably for cache invalidation.

Sounds fair.

> A callback based system is actually *harder* to use, because you can't
> simply ask the question "Is my cache still valid?" - you have to
> register a callback that sets a flag, or something similar. Your
> lookup code ends up being:

Well, the point of a callback based system is for the callback to
invalidate or recompute the cache *right now*, so you don't have to
"check a flag" each time you do a lookup.

It's a basic cost/benefit compromise: the callback system makes
invalidations more expensive, but lookups cheapers.

Besides, a callback-based system can be more future-proof than exposing
the raw cache tags or tokens.

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<http://bugs.python.org/issue16832>
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