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 spends no new character. The doubled marker $$ starts the keyword-only arguments, which are required until |: "O|O$$O|O" corresponds to f(a, b=None, *, c, d=None). Unlike $, its meaning does not depend on whether | was specified before it, so the meaning of every unit is fixed by the text to its left and no lookahead is needed except in the fast-return path.

Argument Clinic uses $$ when $ 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.

$$ currently raises SystemError ("$ specified twice"), so no working code can be affected. Consuming two characters costs a little more code than a single-character marker.

$$ also nearly duplicates $: without a preceding | they are the same thing, "OO$$OO" and "OO$OO" both meaning f(a, b, *, c, d). The two diverge only when | precedes the marker, where $ makes the keyword-only arguments optional and $$ keeps them required until the next |.

The other two alternatives: #155329, #155331.

• Issue: PyArg_ParseTupleAndKeywords: support required keyword arguments #78416