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Hi, as I stated, the original patch was simply our original implementation.
Here is a new patch. It is simpler:
1) it exposes a gc.callbacks list where users can register themselves, in the spirit of sys.meta_path
2) One can have multiple callbacks
3) Improve error handling
4) The callback is called with a "phase" argument, currently 0 for start, and 1 for the end.
Let's start bikeshedding the calling signature. I like having a single callback, since multiple callables are a nuisance to manage.
Once we agree, I'll post a patch for the documentation, and unittests.