[Python-Dev] Are undocumented functions part of the stable ABI? (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Tue Apr 10 07:49:03 EDT 2018

On 10 April 2018 at 15:34, Jeroen Demeyer <J.Demeyer at ugent.be> wrote:

On 2018-04-08 05:17, Nick Coghlan wrote:

Changing macro definitions doesn't break the stable ABI, as long as the old macro expansions still do the right thing. To me, it looks like a bad idea to change macros. Imagine that the PyCFunctionCheck macro changes in Python 3.8. Then an extension module compiled on 3.7 (but run on 3.8) would behave differently from the same extension compiled on 3.8. I cannot imagine that this is in line with the "stable ABI" philosophy.

It's fine, as the stable ABI philosophy is simply that extension modules compiled against a given version of the stable ABI (as defined by Py_LIMITED_API) will be binary compatible with all interpreter versions that support that ABI.

So if a macro is changed to reference a newly exported symbol, then the new definition must be put behind an "#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03080000" guard.

If it's only a semantic level change in the way the macro gets expanded, then whether or not it needs an ABI version guard gets judged on a case-by-case basis, and in this particular case, my view would be that developers should be able to write extensions using the stable ABI that accept function subclasses on 3.8+, without having to require the use of 3.8+ to import their module.

For folks that explicitly don't want to allow subclasses, even when compiled with 3.8+, we can offer the following snippet in the Python 3.8 porting guide:

#ifndef PyCFunction_CheckExact
    #define PyCFunction_CheckExact PyCFunction_Check
#endif

And then doing a search-and-replace on their code to use the "*Exact" variant. Ideally stable ABI users won't need to do that though, since the stable ABI hides struct internals, and all the public PyCFunction APIs should be updated to cope with subclasses properly.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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