> CPython will track only one global callback; it is a well-behaved client’s responsibility to check if a callback is already set when setting a new one, and daisy-chain to the previous callback if so.

Hm, this is a bit scary. Could we (or others) end up with unguarded stale caches if some buggy extension forgets to chain the calls correctly?

Core CPython seems most at risk of this, since we would most likely be registered first.