Feature or enhancement

ThePyType_FromSpec family of functions provides a convenient and forward-compatible way mechanism for creating new types. It could in principle also be used to createcallables providing aPEP-590 vector call interface, which has significant performance benefits for binding libraries (see therelated discussion here).

One can already specify a member named__vectorcalloffset__ inPyType_FromSpec. This isn't fully working in the limited API, however. I encountered the following problems.

1. ThePy_TPFLAGS_HAVE_VECTORCALL flag is not part of the limited API.
2. ThePyVectorcall_NARGS() helper function is not part of the limited API.
3. One would normally settp_call to the compatibility dispatch routinePyVectorcall_Call. It is, however, also not part of the public ABI.
4. Leavingtp_call unspecified is not an option.PyType_Ready() even throws an exception intype_ready_pre_checks() whentp_call is unspecified.

Pitch

I propose the following changes:

1. AddingPy_TPFLAGS_HAVE_VECTORCALL,PyVectorcall_NARGS(), andPyVectorcall_Call() to the limited API.
2. Redundant, but nice: Settingtp_call toPyVectorcall_Call whenPyType_Ready encounters a type that doesn't have this field set.

Note that vector calls can bereceived andperformed. This issue is just about the receiving end.

Previous discussion

See the discord threadhttps://discuss.python.org/t/ideas-for-forward-compatible-and-fast-extension-libraries-in-python-3-12/15993/12.