gh-144175: Add PyArg_ParseVector() function

[vstinner]

• Issue: Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL #144175

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📚 Documentation preview 📚: https://cpython-previews--144283.org.readthedocs.build/

[vstinner]

The change doesn't include tests because my plan is to write a following PR to rename _PyArg_ParseStack() to PyArg_ParseVector(). Many modules using Argument Clinic are calling _PyArg_ParseStack(). I prefer to write a separated PR since it changes a lot of code and so is harder to review.

[vstinner]

Ok, I completed the PR to add also PyArg_ParseVectorAndKeywords() function. The PR is now ready for review :-)

[vstinner]

I propose the following API:

int PyArg_ParseVector(
    PyObject *const *args,
    Py_ssize_t nargs,
    const char *format,
    ...);

int PyArg_ParseVectorAndKeywords(
    PyObject *const *args,
    Py_ssize_t nargs,
    PyObject *kwnames,
    const char *format,
    char * const *kwlist,
    ...);

[picnixz]

Any reason to have PY_CXX_CONST? why now always const char *const *kwlist? (are there occurrences where we don't have a const?)

[vstinner]

If kwlist is declared as const char * const *kwlist, I get a compiler error on the following code:

    int arg, arg2 = 0;
    char *kwlist[] = {"arg", "arg2", NULL};
    if (!PyArg_ParseVectorAndKeywords(args, nargs, kwnames, "i|i", kwlist,
                                      &arg, &arg2)) {
        return NULL;
    }

Error:

./Modules/_testcapimodule.c: In function 'fastcall_kwnames':
./Modules/_testcapimodule.c:2603:68: error: passing argument 5 of 'PyArg_ParseVectorAndKeywords' from incompatible pointer type [-Wincompatible-pointer-types]
 2603 |     if (!PyArg_ParseVectorAndKeywords(args, nargs, kwnames, "i|i", kwlist,
      |                                                                    ^~~~~~
      |                                                                    |
      |                                                                    char **
In file included from ./Include/modsupport.h:153,
                 from ./Include/Python.h:132,
                 from ./Modules/_testcapi/parts.h:25,
                 from ./Modules/_testcapimodule.c:13:
./Include/cpython/modsupport.h:15:25: note: expected 'const char * const*' but argument is of type 'char **'
   15 |     const char * const *kwlist,
      |     ~~~~~~~~~~~~~~~~~~~~^~~~~~

PY_CXX_CONST is also used for kwlist by PyArg_ParseTupleAndKeywords() for example.

PY_CXX_CONST is only defined as const if building C++.