[Python-Dev] PEP 442 clarification for type hierarchies (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Mon Aug 5 20:51:09 CEST 2013

Hi,

On Sun, 04 Aug 2013 09:23:41 +0200 Stefan Behnel <stefan_ml at behnel.de> wrote:

I'm currently catching up on PEP 442, which managed to fly completely below my radar so far. It's a really helpful change that could end up fixing a major usability problem that Cython was suffering from: user provided deallocation code now has a safe execution environment (well, at least in Py3.4+). That makes Cython a prime candidate for testing this, and I've just started to migrate the implementation.

That's great to hear. "Safe execution environment" for finalization code is exactly what the PEP is about.

One thing that I found to be missing from the PEP is inheritance handling. The current implementation doesn't seem to care about base types at all, so it appears to be the responsibility of the type to call its super type finalisation function. Is that really intended?

Yes, it is intended that users have to call super().del() in their del implementation, if they want to call the upper-level finalizer. This is exactly the same as in init() and (most?) other special functions.

Another bit is the exception handling. According to the documentation, tpfinalize() is supposed to first save the current exception state, then do the cleanup, then call WriteUnraisable() if necessary, then restore the exception state.

http://docs.python.org/3.4/c-api/typeobj.html#PyTypeObject.tpfinalize Is there a reason why this is left to the user implementation, rather than doing it generically right in PyObjectCallFinalizer() ? That would also make it more efficient to call through the super type hierarchy, I guess. I don't see a need to repeat this exception state swapping at each level.

I didn't give much thought to this detail. Originally I was simply copying this bit of semantics from tp_dealloc and tp_del, but indeed we could do better. Do you want to open an issue about it?

Regards

Antoine.

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