gh-78416: Add the "%" format unit for required keyword-only arguments - #155329
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gh-78416: Add the "%" format unit for required keyword-only arguments#155329serhiy-storchaka wants to merge 1 commit into
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…eTupleAndKeywords() The new format unit "%" starts the keyword-only arguments which are required until "|", so that they can be mixed with optional arguments. Unlike "$", it does not depend on whether "|" was specified before it. Argument Clinic uses it if "$" cannot express the signature, so that such functions can now use the limited C API. It also no longer rejects a required keyword-only parameter after an optional positional one.
This was referenced Aug 7, 2026
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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"), sof(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 a new format unit.
%replaces$and starts the keyword-only arguments as required,|making the rest optional:"O|O%O|O"corresponds tof(a, b=None, *, c, d=None). Its meaning does not depend on whether|was specified before it, so the meaning of every unit is fixed by the text to its left and no lookahead is needed except in the fast-return path.Argument Clinic uses
%when$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.The cost is a format character, which can then never be used for anything else, spent on a marker that nearly duplicates
$: without a preceding|they are the same thing,"OO%OO"and"OO$OO"both meaningf(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: #155330, #155331.