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

Antoine Pitrou solipsis at pitrou.net
Mon Aug 5 20:56:29 CEST 2013

On Sun, 04 Aug 2013 17:59:57 +0200 Stefan Behnel <stefan_ml at behnel.de> wrote:

> I continued my implementation and found that calling up the base type > hierarchy is essentially the same code as calling up the hierarchy for > tpdealloc(), so that was easy to adapt to in Cython and is also more > efficient than a generic loop (because it can usually benefit from > inlining). So I'm personally ok with leaving the super type calling code to > the user side, even though manual implementers may not be entirely happy. > > I think it should get explicitly documented how subtypes should deal with a > tpfinalize() in (one of the) super types. It's not entirely trivial > because the tpfinalize slot is not guaranteed to be filled for a super > type IIUC, as opposed to tpdealloc. I assume the recursive invariant that > PyTypeReady() copies it would still hold, though.

Not only it could be NULL (if no upper type has a finalizer), but it could also not exist at all (if Py_TPFLAGS_HAVE_FINALIZE isn't in tp_flags). If an API is needed to make this easier then why not. But I'm not sure anyone else than Cython really has such concerns. Usually, the class hierarchy for C extension types is known at compile-time and therefore you know exactly which upper finalizers to call.

Hmm, it seems to me by now that the only safe way of handling this is to let each tpdealloc() level in the hierarchy call tpfinalize() through PyObjectCallFinalizerFromDealloc(), instead of calling up the stack in tpfinalize(). Otherwise, it's a bit fragile for arbitrary tpdealloc() functions in base types and subtypes.

I'm not following you. Why is it "a bit fragile" to call the base tp_finalize from a derived tp_finalize? It should actually be totally safe, since tp_finalize is a regular function called in a safe environment (unlike tp_dealloc and tp_del).

Regards

Antoine.

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