gh-156219: Fix Argument Clinic for an optional argument with **kwds (GH-156220)

The generated code read an optional positional argument even if it was
not passed.

Author: serhiy-storchaka

Lib/test/test_clinic.py

@@ -4981,6 +4981,18 @@ def test_kwds_with_pos_only(self):
         self.assertEqual(ac_tester.kwds_with_pos_only(1, 2, y='y', z='z'), (1, 2, kwds))
         self.assertEqual(ac_tester.kwds_with_pos_only(1, 2, **kwds), (1, 2, kwds))
 
+    def test_kwds_with_optional_pos_only(self):
+        with self.assertRaises(TypeError):
+            ac_tester.kwds_with_optional_pos_only()
+        with self.assertRaises(TypeError):
+            ac_tester.kwds_with_optional_pos_only(y='y')
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1), (1, None, {}))
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, 2), (1, 2, {}))
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, y='y'),
+                         (1, None, {'y': 'y'}))
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, 2, y='y'),
+                         (1, 2, {'y': 'y'}))
+
     def test_kwds_with_stararg(self):
         self.assertEqual(ac_tester.kwds_with_stararg(), ((), {}))
         self.assertEqual(ac_tester.kwds_with_stararg(1, 2), ((1, 2), {}))

Misc/NEWS.d/next/Tools-Demos/2026-08-22-11-00-00.gh-issue-156219.Wq2nRt.rst

@@ -0,0 +1,2 @@
+Fix Argument Clinic generating code which reads an optional positional
+argument which was not passed, if the function has a ``**kwds`` parameter.

Modules/_testclinic.c

@@ -2474,6 +2474,23 @@ kwds_with_pos_only_impl(PyObject *module, PyObject *a, PyObject *b,
 }
 
 
+/*[clinic input]
+kwds_with_optional_pos_only
+    a: object
+    b: object = None
+    /
+    **kwds: dict
+[clinic start generated code]*/
+
+static PyObject *
+kwds_with_optional_pos_only_impl(PyObject *module, PyObject *a, PyObject *b,
+                                 PyObject *kwds)
+/*[clinic end generated code: output=25a8458f5acc1a07 input=0b18b9e1670904ec]*/
+{
+    return pack_arguments_newref(3, a, b, kwds);
+}
+
+
 /*[clinic input]
 kwds_with_stararg
     *args: tuple
@@ -2611,6 +2628,7 @@ static PyMethodDef tester_methods[] = {
 
     LONE_KWDS_METHODDEF
     KWDS_WITH_POS_ONLY_METHODDEF
+    KWDS_WITH_OPTIONAL_POS_ONLY_METHODDEF
     KWDS_WITH_STARARG_METHODDEF
     KWDS_WITH_POS_ONLY_AND_STARARG_METHODDEF
 

Modules/clinic/_testclinic_kwds.c.h

@@ -655,14 +655,24 @@ def parse_var_keyword(self) -> None:
                 }}}}
                 """, indent=4))
 
+        has_optional = False
         for i, p in enumerate(self.parameters):
             parse_arg = p.converter.parse_arg(
                 f'PyTuple_GET_ITEM(args, {i})',
                 p.get_displayname(i+1),
                 limited_capi=self.limited_capi,
             )
             assert parse_arg is not None
+            if has_optional or p.is_optional():
+                has_optional = True
+                parser_code.append(libclinic.normalize_snippet("""
+                    if (%s < %d) {{
+                        goto skip_optional;
+                    }}
+                    """, indent=4) % (nargs, i + 1))
             parser_code.append(libclinic.normalize_snippet(parse_arg, indent=4))
+        if has_optional:
+            parser_code.append("skip_optional:")
 
         if self.varpos:
             parser_code.append(libclinic.normalize_snippet(self._parse_vararg(), indent=4))