On 2018-04-04 17:56, Guido van Rossum wrote:

It would be helpful if you explained the context of your request.

The context is PEP 575. I guess my question is mostly about PyCFunction_Check(). I will not be able to keep it 100% backwards compatible simply because the goal of that PEP is precisely changing the classes of some objects. Now the question is: am I allowed to change the implementation of PyCFunction_Check()? If it's considered part of the stable ABI, then the answer is immediately "no". By the way, does anybody happen to know why the PyCFunction_* functions are undocumented? Is it just an oversight in the docs or is it intentional? But regardless of the context, I think that the question "Are undocumented functions part of the stable ABI?" should be answered in PEP 384. Jeroen.