Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL

[rdb]

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

[vstinner]

Python has a private API but this private API has been removed 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()

[vstinner]

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;

[vstinner]

cc @serhiy-storchaka