> Proposed patch renames METH_FASTCALL to METH_FASTCALL|METH_KEYWORDS and makes bare METH_FASTCALL be used for functions with positional-only parameters.

While I tried to keep everything related to FASTCALL private, it seems like Cython uses some FASTCALL features. I don't know which ones exactly. Well, if only one project in the world uses FASTCALL, we can help them to support such backward incompatible change ;-)


> This eliminates small cost that these functions pay for handling empty keywords: calling _PyStack_UnpackDict() and _PyArg_NoStackKeywords(), passing kwnames.

My idea when I designed FASTCALL was to move code to parse arguments in the function body rather than in _PyCFunction_FastCallKeywords(), and to have a single calling function METH_FASTCALL, rather than two (METH_FASTCALL and METH_FASTCALL|METH_KEYWORDS).

The long term plan is also to support passing arguments by keyword in more functions. IMHO many functions don't accept keywords for technical reasons, but once we converted a module, function or type to Argument Clinic, it becomes trivial to accept keywords. If most functions accept keywords, I'm not sure that having a special case for positional-only is still worth it. But this plan was before I had discussions on supporting keywords in unicode methods. In fact, it's deliberate to not accept keywords in many functions or methods.

Well, when I see your patch, I see that it removes a lot of code. So it's likely to be a good idea :-)


> This also can slightly reduce stack consumption.

You mean the removal of the "PyObject *kwnames" for METH_FASTCALL (positional arguments only)? Do you have an idea of the stack usage? Try maybe testcapi_stacksize.patch of the issue #28870? It would help to take a decision on this change.