gh-78416: Support required keyword-only arguments in PyArg_ParseTupleAndKeywords()

The new format unit "$$" starts the keyword-only arguments which are
required until "|", so that they can be mixed with optional arguments.
Unlike "$", it does not depend on whether "|" was specified before it.
It was previously rejected as "$ specified twice".

Argument Clinic uses it if "$" 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.

Author: serhiy-storchaka

Doc/c-api/arg.rst

@@ -402,18 +402,33 @@ inside nested parentheses.  They are:
    For example, the format string ``"OO|OO"`` corresponds to the Python
    signature ``f(a, b, c=None, d=None)``.
 
+   :c:func:`PyArg_ParseTupleAndKeywords` only:
+   after ``$$`` it indicates that the remaining keyword-only arguments are optional.
+
 ``$``
    :c:func:`PyArg_ParseTupleAndKeywords` only:
    Indicates that the remaining arguments in the Python argument list are
    keyword-only.
    They are optional if ``|`` was specified before ``$``, and required otherwise.
    ``|`` cannot be specified after ``$``.
+   Use ``$$`` to mix required and optional keyword-only arguments.
    For example, the format string ``"O|O$O"`` corresponds to the Python
    signature ``f(a, b=None, *, c=None)``,
    and the format string ``"OO$OO"`` corresponds to ``f(a, b, *, c, d)``.
 
    .. versionadded:: 3.3
 
+``$$``
+   :c:func:`PyArg_ParseTupleAndKeywords` only:
+   Indicates that the remaining arguments in the Python argument list are
+   keyword-only and required.
+   They become optional after ``|``.
+   Unlike ``$``, it does not depend on whether ``|`` was specified before it.
+   For example, the format string ``"O|O$$O|O"`` corresponds to the Python
+   signature ``f(a, b=None, *, c, d=None)``.
+
+   .. versionadded:: next
+
 ``:``
    The list of format units ends here; the string after the colon is used as the
    function name in error messages (the "associated value" of the exception that

Doc/whatsnew/3.16.rst

@@ -845,7 +845,13 @@ C API changes
 New features
 ------------
 
-* TODO
+* :c:func:`PyArg_ParseTupleAndKeywords` now supports required keyword-only
+  arguments mixed with optional arguments.
+  The new format unit ``$$`` starts the keyword-only arguments which are
+  required until ``|``.
+  For example, the format string ``"O|O$$O|O"`` corresponds to the Python
+  signature ``f(a, b=None, *, c, d=None)``.
+  (Contributed by Serhiy Storchaka in :gh:`78416`.)
 
 Porting to Python 3.16
 ----------------------

Include/cpython/modsupport.h

@@ -31,6 +31,7 @@ typedef struct _PyArg_Parser {
     int pos;                /* number of positional-only arguments */
     int min;                /* minimal number of arguments */
     int max;                /* maximal number of positional arguments */
+    int minkw;              /* index of the first optional keyword-only argument */
     PyObject *kwtuple;      /* tuple of keyword parameter names */
     struct _PyArg_Parser *next;
 } _PyArg_Parser;

Lib/test/test_capi/test_getargs.py

@@ -1,6 +1,7 @@
 import string
 import sys
 import unittest
+from functools import partial
 from test import support
 from test.support import import_helper
 from test.support import script_helper
@@ -825,6 +826,80 @@ def __hash__(self):
             getargs_keyword_only(1, 2, **{BadStr("monster"): 666})
 
 
+class RequiredKeywordOnly_TestCase(unittest.TestCase):
+    # "$$" marks the start of the keyword-only arguments, which are required
+    # until "|", so that required and optional ones can be mixed.
+
+    def parse(self, format, args, kwargs, keywords=('a', 'b', 'c', 'd')):
+        return _testcapi.parse_tuple_and_keywords(args, kwargs,
+                                                  format, list(keywords))
+
+    def test_required_after_optional(self):
+        # f(a, b=None, *, c, d=None)
+        parse = partial(self.parse, "O|O$$O|O")
+        self.assertEqual(parse((1, 2), {'c': 3, 'd': 4}), (1, 2, 3, 4))
+        self.assertEqual(parse((1,), {'c': 3}), (1, None, 3, None))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            parse((1,), {})
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            parse((1,), {'d': 4})
+        with self.assertRaisesRegex(TypeError, "at most 2 positional"):
+            parse((1, 2, 3), {'c': 3})
+
+    def test_all_keyword_only_required(self):
+        # f(a, b=None, *, c, d)
+        parse = partial(self.parse, "O|O$$OO")
+        self.assertEqual(parse((1,), {'c': 3, 'd': 4}), (1, None, 3, 4))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'd'"):
+            parse((1,), {'c': 3})
+
+    def test_all_positional_required(self):
+        # f(a, b, *, c, d=None)
+        parse = partial(self.parse, "OO$$O|O")
+        self.assertEqual(parse((1, 2), {'c': 3}), (1, 2, 3, None))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            parse((1, 2), {})
+
+    def test_cached_parser(self):
+        # The same format, parsed once and cached in a _PyArg_Parser.
+        f = _testcapi.getargs_fast_required_kwonly
+        self.assertEqual(f(1, 2, c=3, d=4), (1, 2, 3, 4))
+        self.assertEqual(f(1, c=3), (1, None, 3, None))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            f(1)
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            f(1, d=4)
+
+    def test_invalid_format(self):
+        for format, msg in (
+            ("O$$O$$O", r"\$ specified twice"),
+            ("O$$O$O", r"\$ specified twice"),
+            ("O$$O|O|O", r"\| specified twice"),
+            ("O$O|O", r"\$ before \|"),
+        ):
+            with self.subTest(format=format):
+                n = format.count('O')
+                npos = len(format) - len(format.lstrip('O'))
+                args = tuple(range(npos))
+                kwargs = {'abcd'[i]: i for i in range(npos, n)}
+                with self.assertRaisesRegex(SystemError, msg):
+                    self.parse(format, args, kwargs, 'abcd'[:n])
+
+    def test_unchanged_meaning_of_dollar(self):
+        # A single "$" still inherits the state of the positional arguments.
+        parse = partial(self.parse, "O|O$O", keywords=('a', 'b', 'c'))
+        self.assertEqual(parse((1,), {}), (1, None, None))
+        parse = partial(self.parse, "OO$O", keywords=('a', 'b', 'c'))
+        self.assertEqual(parse((1, 2), {'c': 3}), (1, 2, 3))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            parse((1, 2), {})
+        # The same with a cached parser.
+        f = _testcapi.getargs_fast_kwonly
+        self.assertEqual(f(1, 2, c=3, d=4), (1, 2, 3, 4))
+        with self.assertRaisesRegex(TypeError, "missing required argument 'c'"):
+            f(1, 2)
+
+
 class PositionalOnlyAndKeywords_TestCase(unittest.TestCase):
     from _testcapi import getargs_positional_only_and_keywords as getargs
 
@@ -1269,8 +1344,6 @@ def test_bad_use(self):
                           (1,), {}, '||O', ['a'])
         self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
                           (1, 2), {}, '|O|O', ['a', 'b'])
-        self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
-                          (), {'a': 1}, '$$O', ['a'])
         self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,
                           (), {'a': 1, 'b': 2}, '$O$O', ['a', 'b'])
         self.assertRaises(SystemError, _testcapi.parse_tuple_and_keywords,

Lib/test/test_clinic.py

@@ -2207,6 +2207,36 @@ def test_depr_slash_duplicate2(self):
         err = "Function 'bar': '/ [from 3.14]' must precede '/ [from 3.15]'"
         self.expect_failure(block, err, lineno=5)
 
+    def test_required_keyword_only_after_optional(self):
+        function = self.parse_function("""
+            module foo
+            foo.bar
+                a: int
+                b: int = 0
+                *
+                c: int
+                d: int = 0
+            Docstring.
+        """)
+        _, a, b, c, d = function.parameters.values()
+        self.assertFalse(a.converter.is_optional())
+        self.assertTrue(b.converter.is_optional())
+        self.assertFalse(c.converter.is_optional())
+        self.assertTrue(d.converter.is_optional())
+
+    def test_optional_before_required_keyword_only(self):
+        block = """
+            module foo
+            foo.bar
+                *
+                a: int = 0
+                b: int
+            Docstring.
+        """
+        err = ("Can't have a parameter without a default ('b') "
+               "after a parameter with a default!")
+        self.expect_failure(block, err, lineno=4)
+
     def test_single_slash(self):
         block = """
             module foo
@@ -4664,6 +4694,41 @@ def test_limited_capi_float(self):
         self.assertIn("float f;", generated)
         self.assertIn("f = (float) PyFloat_AsDouble", generated)
 
+    def test_limited_capi_required_keyword_only(self):
+        block = self.wrap_clinic_input("""
+            func
+                a: object
+                b: object = None
+                *
+                c: object
+                d: object = None
+        """)
+        generated = self.clinic.parse(block)
+        # "$" cannot express this, only "$$" can.
+        self.assertIn('"O|O$$O|O:func"', generated)
+
+    def test_limited_capi_optional_after_required_keyword_only(self):
+        block = self.wrap_clinic_input("""
+            func
+                a: object
+                *
+                b: object
+                c: object = None
+        """)
+        generated = self.clinic.parse(block)
+        self.assertIn('"O$$O|O:func"', generated)
+
+    def test_limited_capi_keyword_only(self):
+        # "$" is still used if it can express the signature.
+        block = self.wrap_clinic_input("""
+            func
+                a: object
+                *
+                b: object
+        """)
+        generated = self.clinic.parse(block)
+        self.assertIn('"O$O:func"', generated)
+
     def test_limited_capi_double(self):
         block = self.wrap_clinic_input("""
             func
@@ -4720,6 +4785,18 @@ def test_my_double_sum(self):
                 with self.assertRaises(TypeError):
                     func(1., "2")
 
+    def test_required_kwonly(self):
+        # test a required keyword-only parameter after an optional one
+        func = _testclinic_limited.required_kwonly
+        self.assertEqual(func(1, 2, c=3, d=4), (1, 2, 3, 4))
+        self.assertEqual(func(1, c=3), (1, None, 3, None))
+        with self.assertRaisesRegex(TypeError, "argument 'c'"):
+            func(1, 2)
+        with self.assertRaisesRegex(TypeError, "argument 'c'"):
+            func(1, 2, d=4)
+        with self.assertRaises(TypeError):
+            func(1, 2, 3)
+
     def test_get_file_descriptor(self):
         # test 'file descriptor' converter: call PyObject_AsFileDescriptor()
         get_fd = _testclinic_limited.get_file_descriptor

Misc/NEWS.d/next/C_API/2026-08-06-17-11-54.gh-issue-78416.jsJUNi.rst

@@ -0,0 +1,3 @@
+:c:func:`PyArg_ParseTupleAndKeywords` now supports required keyword-only arguments mixed with optional arguments.
+The new format unit ``$$`` starts the keyword-only arguments which are required until ``|``.
+For example, the format string ``"O|O$$O|O"`` corresponds to the Python signature ``f(a, b=None, *, c, d=None)``.

Modules/_testcapi/getargs.c

@@ -765,6 +765,42 @@ gh_99240_clear_args(PyObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
+/* f(a, b=None, *, c, d=None) parsed with a cached _PyArg_Parser. */
+static PyObject *
+getargs_fast_required_kwonly(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    static const char * const keywords[] = {"a", "b", "c", "d", NULL};
+    static _PyArg_Parser parser = {
+        .format = "O|O$$O|O:getargs_fast_required_kwonly",
+        .keywords = keywords,
+    };
+    PyObject *a, *b = Py_None, *c, *d = Py_None;
+    if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &parser,
+                                          &a, &b, &c, &d))
+    {
+        return NULL;
+    }
+    return Py_BuildValue("OOOO", a, b, c, d);
+}
+
+/* f(a, b, *, c, d) parsed with a cached _PyArg_Parser. */
+static PyObject *
+getargs_fast_kwonly(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    static const char * const keywords[] = {"a", "b", "c", "d", NULL};
+    static _PyArg_Parser parser = {
+        .format = "OO$OO:getargs_fast_kwonly",
+        .keywords = keywords,
+    };
+    PyObject *a, *b, *c, *d;
+    if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &parser,
+                                          &a, &b, &c, &d))
+    {
+        return NULL;
+    }
+    return Py_BuildValue("OOOO", a, b, c, d);
+}
+
 static PyMethodDef test_methods[] = {
     {"get_args",                get_args,                        METH_VARARGS},
     {"get_kwargs", _PyCFunction_CAST(get_kwargs), METH_VARARGS|METH_KEYWORDS},
@@ -809,6 +845,10 @@ static PyMethodDef test_methods[] = {
     {"getargs_z_hash",          getargs_z_hash,                  METH_VARARGS},
     {"getargs_z_star",          getargs_z_star,                  METH_VARARGS},
     {"parse_tuple_and_keywords", parse_tuple_and_keywords,       METH_VARARGS},
+    {"getargs_fast_required_kwonly",
+     _PyCFunction_CAST(getargs_fast_required_kwonly), METH_VARARGS|METH_KEYWORDS},
+    {"getargs_fast_kwonly", _PyCFunction_CAST(getargs_fast_kwonly),
+                                                 METH_VARARGS|METH_KEYWORDS},
     {"gh_99240_clear_args",     gh_99240_clear_args,             METH_VARARGS},
     {"test_w_code_invalid",     test_w_code_invalid,             METH_NOARGS},
     {NULL},

Modules/_testclinic_limited.c

@@ -129,9 +129,31 @@ static PyMethodDef tester_methods[] = {
     MY_FLOAT_SUM_METHODDEF
     MY_DOUBLE_SUM_METHODDEF
     GET_FILE_DESCRIPTOR_METHODDEF
+    REQUIRED_KWONLY_METHODDEF
     {NULL, NULL}
 };
 
+/*[clinic input]
+required_kwonly
+
+    a: object
+    b: object = None
+    *
+    c: object
+    d: object = None
+
+Mix an optional parameter with a required keyword-only one.
+[clinic start generated code]*/
+
+static PyObject *
+required_kwonly_impl(PyObject *module, PyObject *a, PyObject *b, PyObject *c,
+                     PyObject *d)
+/*[clinic end generated code: output=8e9a974afa614cd6 input=297ec6487f6373dd]*/
+{
+    return Py_BuildValue("OOOO", a, b, c, d);
+}
+
+
 static struct PyModuleDef _testclinic_module = {
     PyModuleDef_HEAD_INIT,
     .m_name = "_testclinic_limited",