GeezerVison

PyArg_ParseTupleAndKeywords() supports only three of the four kinds of parameters: keyword-only arguments are either all required ("OO$OO") or all optional ("O|O$O"), so f(a, b=None, *, c, d=None) cannot be expressed.

This is one of three alternative implementations, made for comparison as suggested in the issue; this one adds no new syntax at all (taleinat's proposal). | may now follow $, ending the required keyword-only arguments: "O|O$O|O" corresponds to f(a, b=None, *, c, d=None). Without a | after it, $ keeps its current meaning exactly. Every format it gives meaning to is a SystemError today, so nothing that works can change.

Argument Clinic uses the trailing | when $ alone cannot express the signature, so such functions can now use the limited C API, and it no longer rejects a required keyword-only parameter after an optional positional one.

It is the smallest change to Python/getargs.c of the three, because the "$ before |" error disappears entirely. Against that: whether an argument after $ is required now depends on text further right, so the scanner must look ahead; and a forgotten trailing | silently makes required keyword-only arguments optional, since "O|O$O|" is f(a, b=None, *, c) while "O|O$O" is f(a, b=None, *, c=None) — both legal, neither diagnosable.

The other two alternatives: #155329, #155330.

• Issue: PyArg_ParseTupleAndKeywords: support required keyword arguments #78416