Description

rdb

opened on Jan 23, 2026

Issue body actions

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

• gh-144175: Add PyArg_ParseArray() function #144283
• gh-144175: Remove internal _PyArg_ParseStack() function #159150

Activity

1. rdb

   added

   type-featureA feature request or enhancementA feature request or enhancement

   on Jan 23, 2026
2. vstinner commented on Jan 23, 2026

   

   vstinner

   on Jan 23, 2026

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   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()
3. vstinner commented on Jan 23, 2026

   

   vstinner

   on Jan 23, 2026

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   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;
4. vstinner commented on Jan 23, 2026

   

   vstinner

   on Jan 23, 2026

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   cc @serhiy-storchaka
5. StanFromIreland

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   topic-C-API

   on Jan 23, 2026
6. vstinner

   added 2 commits that reference this issue on Jan 27, 2026

   pythongh-144175: Add PyArg_ParseVector() function

   b35a234

   pythongh-144175: Add PyArg_ParseVector() function

   769e40b
7. vstinner commented on Jan 27, 2026

   

   vstinner

   on Jan 27, 2026

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

   pinned and unpinned a comment on Mar 6, 2026
9. vstinner

   added a commit that references this issue on Mar 6, 2026

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

   9159287
10. vstinner commented on Mar 9, 2026

    

    vstinner

    on Mar 9, 2026

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    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.
11. vstinner

    closed this as completedon Mar 9, 2026
12. rdb commented on Mar 10, 2026

    

    rdb

    on Mar 10, 2026

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    Thanks a lot!

    Reacted by Victor Stinner
13. ljfp

    added a commit that references this issue on Apr 25, 2026

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

    4593b5d
14. vstinner

    added a commit that references this issue on Oct 12, 2026

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

    8992704

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