Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL

@vstinner

Feature or enhancement

Proposal:

Opening an issue for this as requested by @vstinner in #110964.

METH_FASTCALL is part of the stable ABI as of Python 3.10. However, using it in extension module is quite difficult, because there is no equivalent API for PyArg_ParseTupleAndKeywords (which is the usual way to parse complex signatures with keywords in METH_VARARGS-style functions), and parsing vectorcall-style keywords is not a trivial problem to begin with. There used to be an undocumented public API (_PyArg_ParseStackAndKeywords) that solves this problem that is used by Argument Clinic, but it was removed from the public API as part of a wider effort to unexpose undocumented APIs in #110964.

So this issue requests a new public, stable API similar to PyArg_ParseTuple and PyArg_ParseTupleAndKeywords but accepting vectorcall-style parameters instead.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

Python has a private API but this private API has been moved to the internal C API in Python 3.13:

_PyArg_BadArgument()
_PyArg_CheckPositional()
_PyArg_NoKeywords()
_PyArg_NoPositional()
_PyArg_ParseStack()
_PyArg_ParseStackAndKeywords()
_PyArg_Parser structure
_PyArg_UnpackKeywords()
_PyArg_UnpackKeywordsWithVararg()
_PyArg_UnpackStack()
_Py_ANY_VARARGS()

The problem of _PyArg_ParseStackAndKeywords() is that it uses _PyArg_Parser structure which is complex to initialize.

Example of code generated by Argument Clinic for _gdbm.gdbm.nextkey() function to parse a string (with its length):

    PyObject *return_value = NULL;
    #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
    #  define KWTUPLE (PyObject *)&_Py_SINGLETON(tuple_empty)
    #else
    #  define KWTUPLE NULL
    #endif

    static const char * const _keywords[] = {"", NULL};
    static _PyArg_Parser _parser = {
        .keywords = _keywords,
        .format = "s#:nextkey",
        .kwtuple = KWTUPLE,
    };
    #undef KWTUPLE
    const char *key;
    Py_ssize_t key_length;

    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
        &key, &key_length)) {
        goto exit; 
    }
    Py_BEGIN_CRITICAL_SECTION(self);
    return_value = _gdbm_gdbm_nextkey_impl((gdbmobject *)self, cls, key, key_length);
    Py_END_CRITICAL_SECTION();
    
exit:
    return return_value;

@serhiy-storchaka

cc @serhiy-storchaka

pythongh-144175: Add PyArg_ParseVector() function

I wrote #144283 to add PyArg_ParseVector() function: a first incomplete solution to this issue.

gh-144175: Add PyArg_ParseArray() function (#144283)

@rdb

Thanks for the report @rdb, it's now implemented in Python 3.15.

I wrote #144283 to add PyArg_ParseVector() function: a first incomplete solution to this issue.

The final change adds PyArg_ParseArray() and PyArg_ParseArrayAndKeywords(), so keywords are supported as well.

Thanks a lot!

pythongh-144175: Add PyArg_ParseArray() function (python#144283)

gh-144175: Remove internal _PyArg_ParseStack() function (#159150)

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Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL #144175

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Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL #144175

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Feature or enhancement

Proposal:

Has this already been discussed elsewhere?

Links to previous discussion of this feature:

Linked PRs

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vstinner commented on Jan 23, 2026

vstinner commented on Jan 23, 2026

vstinner commented on Jan 23, 2026

vstinner commented on Jan 27, 2026

vstinner commented on Mar 9, 2026

rdb commented on Mar 10, 2026

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