gh-78416: Allow "|" after "$" for required keyword-only arguments - #155331
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This was referenced Aug 7, 2026
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…eTupleAndKeywords() "|" can now be specified after "$" to end the required keyword-only arguments, so that they can be mixed with optional arguments. The keyword-only arguments are required until "|", if it follows, and inherit the state of the positional arguments otherwise. Argument Clinic uses this if "$" alone cannot express the signature, so that such functions can now use the limited C API. It also no longer rejects a required keyword-only parameter after an optional positional one.
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PyArg_ParseTupleAndKeywords()supports only three of the four kinds of parameters: keyword-only arguments are either all required ("OO$OO") or all optional ("O|O$O"), sof(a, b=None, *, c, d=None)cannot be expressed.This is one of three alternative implementations, made for comparison as suggested in the issue; this one adds no new syntax at all (taleinat's proposal).
|may now follow$, ending the required keyword-only arguments:"O|O$O|O"corresponds tof(a, b=None, *, c, d=None). Without a|after it,$keeps its current meaning exactly. Every format it gives meaning to is aSystemErrortoday, so nothing that works can change.Argument Clinic uses the trailing
|when$alone cannot express the signature, so such functions can now use the limited C API, and it no longer rejects a required keyword-only parameter after an optional positional one.It is the smallest change to
Python/getargs.cof the three, because the"$ 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|"isf(a, b=None, *, c)while"O|O$O"isf(a, b=None, *, c=None)— both legal, neither diagnosable.The other two alternatives: #155329, #155330.