gh-64502: Fix Argument Clinic support of optional groups with defaults (GH-155191)

https://github.com/python/cpython/commit/caac9278306d3cb96b22f51f5003e20b7a6aa18c
commit: caac9278306d3cb96b22f51f5003e20b7a6aa18c
branch: main
author: Serhiy Storchaka <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-08-05T09:21:21+03:00
summary:

gh-64502: Fix Argument Clinic support of optional groups with defaults (GH-155191)

Parameters with a default value which are not in any group were always
required in the generated argument parsing code, although they were
rendered as optional in the signature.  They can now be omitted, and
ambiguous combinations of optional groups and parameters with a default
value are rejected.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

files:
A Misc/NEWS.d/next/Tools-Demos/2026-08-04-17-12-33.gh-issue-64502.Qv7mLp.rst
M Lib/test/clinic.test.c
M Lib/test/test_clinic.py
M Modules/_testclinic.c
M Modules/clinic/_testclinic.c.h
M Tools/c-analyzer/cpython/_parser.py
M Tools/clinic/libclinic/clanguage.py

diff --git a/Lib/test/clinic.test.c b/Lib/test/clinic.test.c
index 171570588e7a2b5..870e4f6956ce4fd 100644
--- a/Lib/test/clinic.test.c
+++ b/Lib/test/clinic.test.c
@@ -5769,6 +5769,67 @@ Test___init___impl(TestObj *self, PyObject *a, int group_right_1,
 /*[clinic end generated code: output=2bbb8ea60e8f57a6 input=10f5d0f1e8e466ef]*/
 
 
+/*[clinic input]
+group_and_optional_parameter
+    [
+    a: object
+    b: object
+    ]
+    c: object = None
+    /
+The optional parameter can be omitted with or without the group.
+[clinic start generated code]*/
+
+PyDoc_STRVAR(group_and_optional_parameter__doc__,
+"group_and_optional_parameter([a, b,] c=None)\n"
+"The optional parameter can be omitted with or without the group.");
+
+#define GROUP_AND_OPTIONAL_PARAMETER_METHODDEF    \
+    {"group_and_optional_parameter", (PyCFunction)group_and_optional_parameter, METH_VARARGS, group_and_optional_parameter__doc__},
+
+static PyObject *
+group_and_optional_parameter_impl(PyObject *module, int group_left_1,
+                                  PyObject *a, PyObject *b, PyObject *c);
+
+static PyObject *
+group_and_optional_parameter(PyObject *module, PyObject *args)
+{
+    PyObject *return_value = NULL;
+    int group_left_1 = 0;
+    PyObject *a = NULL;
+    PyObject *b = NULL;
+    PyObject *c = Py_None;
+
+    switch (PyTuple_GET_SIZE(args)) {
+        case 0:
+        case 1:
+            if (!PyArg_ParseTuple(args, "|O:group_and_optional_parameter", &c)) {
+                goto exit;
+            }
+            break;
+        case 2:
+        case 3:
+            if (!PyArg_ParseTuple(args, "OO|O:group_and_optional_parameter", &a, &b, &c)) {
+                goto exit;
+            }
+            group_left_1 = 1;
+            break;
+        default:
+            PyErr_SetString(PyExc_TypeError, "group_and_optional_parameter requires 0 to 3 arguments");
+            goto exit;
+    }
+    return_value = group_and_optional_parameter_impl(module, group_left_1, a, b, c);
+
+exit:
+    return return_value;
+}
+
+static PyObject *
+group_and_optional_parameter_impl(PyObject *module, int group_left_1,
+                                  PyObject *a, PyObject *b, PyObject *c)
+/*[clinic end generated code: output=3faea69eafd5bbbe input=7f0fbb6124f5a972]*/
+
+
 /*[clinic input]
 Test._pyarg_parsestackandkeywords
     cls: defining_class
diff --git a/Lib/test/test_clinic.py b/Lib/test/test_clinic.py
index 93c284e58764f46..a9b3273ae2d7e13 100644
--- a/Lib/test/test_clinic.py
+++ b/Lib/test/test_clinic.py
@@ -330,6 +330,24 @@ def __init__(self):
         """
         self.expect_failure(block, err, lineno=8)
 
+    def test_ambiguous_group_and_optional_parameters(self):
+        err = ("Function 'my_test_func' has an ambiguous group configuration: "
+               "a call with 2 argument(s) can be parsed in more than one way.")
+        block = """
+            /*[clinic input]
+            my_test_func
+
+                [
+                a: object
+                b: object
+                ]
+                c: object = None
+                d: object = None
+                /
+            [clinic start generated code]*/
+        """
+        self.expect_failure(block, err)
+
     def test_star_after_vararg(self):
         err = "'my_test_func' uses '*' more than once."
         block = """
@@ -3865,6 +3883,27 @@ def test_varpos_kwonly_req_opt(self):
         self.assertEqual(fn(1, a=2, b=3), ((1,), 2, 3, False))
         self.assertEqual(fn(1, a=2, b=3, c=4), ((1,), 2, 3, 4))
 
+    def test_group_and_opt(self):
+        # fn([a, b,] c=None)
+        fn = ac_tester.group_and_opt
+        self.assertEqual(fn(), (False, None, None, None))
+        self.assertEqual(fn(1), (False, None, None, 1))
+        self.assertEqual(fn(1, 2), (True, 1, 2, None))
+        self.assertEqual(fn(1, 2, 3), (True, 1, 2, 3))
+        self.assertRaises(TypeError, fn, 1, 2, 3, 4)
+        self.assertRaises(TypeError, fn, c=1)
+
+    def test_group_and_two_opt(self):
+        # fn([a, b, c,] d=None, e=None)
+        fn = ac_tester.group_and_two_opt
+        self.assertEqual(fn(), (False, None, None, None, None, None))
+        self.assertEqual(fn(1), (False, None, None, None, 1, None))
+        self.assertEqual(fn(1, 2), (False, None, None, None, 1, 2))
+        self.assertEqual(fn(1, 2, 3), (True, 1, 2, 3, None, None))
+        self.assertEqual(fn(1, 2, 3, 4), (True, 1, 2, 3, 4, None))
+        self.assertEqual(fn(1, 2, 3, 4, 5), (True, 1, 2, 3, 4, 5))
+        self.assertRaises(TypeError, fn, 1, 2, 3, 4, 5, 6)
+
     def test_gh_32092_oob(self):
         ac_tester.gh_32092_oob(1, 2, 3, 4, kw1=5, kw2=6)
 
diff --git a/Misc/NEWS.d/next/Tools-Demos/2026-08-04-17-12-33.gh-issue-64502.Qv7mLp.rst b/Misc/NEWS.d/next/Tools-Demos/2026-08-04-17-12-33.gh-issue-64502.Qv7mLp.rst
new file mode 100644
index 000000000000000..da9647d1fdd369b
--- /dev/null
+++ b/Misc/NEWS.d/next/Tools-Demos/2026-08-04-17-12-33.gh-issue-64502.Qv7mLp.rst
@@ -0,0 +1,3 @@
+Fix Argument Clinic support of parameters with a default value used together
+with optional groups.
+Such parameters were always required in the generated parsing code.
diff --git a/Modules/_testclinic.c b/Modules/_testclinic.c
index 66a375589ba38e5..5742dd65f6742a7 100644
--- a/Modules/_testclinic.c
+++ b/Modules/_testclinic.c
@@ -1237,6 +1237,52 @@ posonly_poskw_varpos_array_impl(PyObject *module, PyObject *a, PyObject *b,
 }
 
 
+/*[clinic input]
+group_and_opt
+
+    [
+    a: object
+    b: object
+    ]
+    c: object = None
+    /
+
+[clinic start generated code]*/
+
+static PyObject *
+group_and_opt_impl(PyObject *module, int group_left_1, PyObject *a,
+                   PyObject *b, PyObject *c)
+/*[clinic end generated code: output=23413ec545526111 input=8a84d8f44bc8bd0b]*/
+{
+    return pack_arguments_newref(4, group_left_1 ? Py_True : Py_False,
+                                 a, b, c);
+}
+
+
+/*[clinic input]
+group_and_two_opt
+
+    [
+    a: object
+    b: object
+    c: object
+    ]
+    d: object = None
+    e: object = None
+    /
+
+[clinic start generated code]*/
+
+static PyObject *
+group_and_two_opt_impl(PyObject *module, int group_left_1, PyObject *a,
+                       PyObject *b, PyObject *c, PyObject *d, PyObject *e)
+/*[clinic end generated code: output=1427c4b3c35f24ff input=cdda98eec1e365ea]*/
+{
+    return pack_arguments_newref(6, group_left_1 ? Py_True : Py_False,
+                                 a, b, c, d, e);
+}
+
+
 
 /*[clinic input]
 gh_32092_oob
@@ -2455,6 +2501,8 @@ static PyMethodDef tester_methods[] = {
     POSONLY_VARPOS_ARRAY_METHODDEF
     POSONLY_REQ_OPT_VARPOS_ARRAY_METHODDEF
     POSONLY_POSKW_VARPOS_ARRAY_METHODDEF
+    GROUP_AND_OPT_METHODDEF
+    GROUP_AND_TWO_OPT_METHODDEF
 
     GH_32092_OOB_METHODDEF
     GH_32092_KW_PASS_METHODDEF
diff --git a/Modules/clinic/_testclinic.c.h b/Modules/clinic/_testclinic.c.h
index 05615c1fdd81b9c..19b215b0cca8899 100644
--- a/Modules/clinic/_testclinic.c.h
+++ b/Modules/clinic/_testclinic.c.h
@@ -3477,6 +3477,96 @@ posonly_poskw_varpos_array(PyObject *module, PyObject *const *args, Py_ssize_t n
     return return_value;
 }
 
+PyDoc_STRVAR(group_and_opt__doc__,
+"group_and_opt([a, b,] c=None)");
+
+#define GROUP_AND_OPT_METHODDEF    \
+    {"group_and_opt", (PyCFunction)group_and_opt, METH_VARARGS, group_and_opt__doc__},
+
+static PyObject *
+group_and_opt_impl(PyObject *module, int group_left_1, PyObject *a,
+                   PyObject *b, PyObject *c);
+
+static PyObject *
+group_and_opt(PyObject *module, PyObject *args)
+{
+    PyObject *return_value = NULL;
+    int group_left_1 = 0;
+    PyObject *a = NULL;
+    PyObject *b = NULL;
+    PyObject *c = Py_None;
+
+    switch (PyTuple_GET_SIZE(args)) {
+        case 0:
+        case 1:
+            if (!PyArg_ParseTuple(args, "|O:group_and_opt", &c)) {
+                goto exit;
+            }
+            break;
+        case 2:
+        case 3:
+            if (!PyArg_ParseTuple(args, "OO|O:group_and_opt", &a, &b, &c)) {
+                goto exit;
+            }
+            group_left_1 = 1;
+            break;
+        default:
+            PyErr_SetString(PyExc_TypeError, "group_and_opt requires 0 to 3 arguments");
+            goto exit;
+    }
+    return_value = group_and_opt_impl(module, group_left_1, a, b, c);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(group_and_two_opt__doc__,
+"group_and_two_opt([a, b, c,] d=None, e=None)");
+
+#define GROUP_AND_TWO_OPT_METHODDEF    \
+    {"group_and_two_opt", (PyCFunction)group_and_two_opt, METH_VARARGS, group_and_two_opt__doc__},
+
+static PyObject *
+group_and_two_opt_impl(PyObject *module, int group_left_1, PyObject *a,
+                       PyObject *b, PyObject *c, PyObject *d, PyObject *e);
+
+static PyObject *
+group_and_two_opt(PyObject *module, PyObject *args)
+{
+    PyObject *return_value = NULL;
+    int group_left_1 = 0;
+    PyObject *a = NULL;
+    PyObject *b = NULL;
+    PyObject *c = NULL;
+    PyObject *d = Py_None;
+    PyObject *e = Py_None;
+
+    switch (PyTuple_GET_SIZE(args)) {
+        case 0:
+        case 1:
+        case 2:
+            if (!PyArg_ParseTuple(args, "|OO:group_and_two_opt", &d, &e)) {
+                goto exit;
+            }
+            break;
+        case 3:
+        case 4:
+        case 5:
+            if (!PyArg_ParseTuple(args, "OOO|OO:group_and_two_opt", &a, &b, &c, &d, &e)) {
+                goto exit;
+            }
+            group_left_1 = 1;
+            break;
+        default:
+            PyErr_SetString(PyExc_TypeError, "group_and_two_opt requires 0 to 5 arguments");
+            goto exit;
+    }
+    return_value = group_and_two_opt_impl(module, group_left_1, a, b, c, d, e);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(gh_32092_oob__doc__,
 "gh_32092_oob($module, /, pos1, pos2, *varargs, kw1=None, kw2=None)\n"
 "--\n"
@@ -4600,4 +4690,4 @@ _testclinic_TestClass_posonly_poskw_varpos_array_no_fastcall(PyObject *type, PyO
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=9971dbbc5f62b8d2 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=f6a3b617130c4e3a input=a9049054013a1b77]*/
diff --git a/Tools/c-analyzer/cpython/_parser.py b/Tools/c-analyzer/cpython/_parser.py
index 2875f45cb8d3756..3d755765b967097 100644
--- a/Tools/c-analyzer/cpython/_parser.py
+++ b/Tools/c-analyzer/cpython/_parser.py
@@ -318,6 +318,7 @@ def format_tsv_lines(lines):
     _abs('Modules/_remote_debugging/debug_offsets_validation.h'): (25_000, 1000),
     _abs('Modules/_remote_debugging/*.h'): (20_000, 1000),
     _abs('Modules/_testcapimodule.c'): (20_000, 400),
+    _abs('Modules/_testclinic.c'): (20_000, 400),
     _abs('Modules/expat/expat.h'): (10_000, 400),
     _abs('Objects/stringlib/unicode_format.h'): (10_000, 400),
     _abs('Objects/typeobject.c'): (380_000, 13_000),
diff --git a/Tools/clinic/libclinic/clanguage.py b/Tools/clinic/libclinic/clanguage.py
index 7f02c7790f015aa..a76fddb7602001e 100644
--- a/Tools/clinic/libclinic/clanguage.py
+++ b/Tools/clinic/libclinic/clanguage.py
@@ -1,6 +1,5 @@
 from __future__ import annotations
 import itertools
-import sys
 import textwrap
 from typing import TYPE_CHECKING, Literal, Final
 from operator import attrgetter
@@ -12,7 +11,7 @@
 from libclinic.codegen import CRenderData, TemplateDict, CodeGen
 from libclinic.language import Language
 from libclinic.function import (
-    Module, Class, Function, Parameter,
+    Module, Class, Function, Parameter, ParamTuple,
     permute_optional_groups,
     GETTER, SETTER, METHOD_INIT)
 from libclinic.converters import self_converter
@@ -21,6 +20,20 @@
     from libclinic.app import Clinic
 
 
+def count_required(subset: ParamTuple) -> int:
+    """Return the number of arguments which cannot be omitted.
+
+    A parameter in an optional group is passed together with its group,
+    so only trailing parameters with a default value can be omitted.
+    """
+    count = len(subset)
+    for p in reversed(subset):
+        if p.group or not p.is_optional():
+            break
+        count -= 1
+    return count
+
+
 def c_id(name: str) -> str:
     if len(name) == 1 and ord(name) < 256:
         if name.isalnum():
@@ -301,18 +314,26 @@ def render_option_group_parsing(
             assert group is not None
             group.append(p)
 
-        count_min = sys.maxsize
-        count_max = -1
+        # Map the number of arguments to the subset which accepts it.
+        subsets: dict[int, ParamTuple] = {}
+        for subset in permute_optional_groups(left, required, right):
+            for count in range(count_required(subset), len(subset) + 1):
+                if count in subsets:
+                    fail(f"Function {f.full_name!r} has an ambiguous group "
+                         f"configuration: a call with {count} argument(s) "
+                         f"can be parsed in more than one way.")
+                subsets[count] = subset
 
         if limited_capi:
             nargs = 'PyTuple_Size(args)'
         else:
             nargs = 'PyTuple_GET_SIZE(args)'
         out.append(f"switch ({nargs}) {{\n")
-        for subset in permute_optional_groups(left, required, right):
-            count = len(subset)
-            count_min = min(count_min, count)
-            count_max = max(count_max, count)
+        for count, subset in sorted(subsets.items()):
+            if count < len(subset):
+                # The omitted parameters are parsed by the following case.
+                out.append(f"    case {count}:\n")
+                continue
 
             if count == 0:
                 out.append("""    case 0:
@@ -324,7 +345,11 @@ def render_option_group_parsing(
             d: dict[str, str | int] = {}
             d['count'] = count
             d['name'] = f.name
-            d['format_units'] = "".join(p.converter.format_unit for p in subset)
+            format_units = [p.converter.format_unit for p in subset]
+            n_required = count_required(subset)
+            if n_required < count:
+                format_units.insert(n_required, '|')
+            d['format_units'] = "".join(format_units)
 
             parse_arguments: list[str] = []
             for p in subset:
@@ -351,7 +376,7 @@ def render_option_group_parsing(
 
         out.append("    default:\n")
         s = '        PyErr_SetString(PyExc_TypeError, "{} requires {} to {} arguments");\n'
-        out.append(s.format(f.full_name, count_min, count_max))
+        out.append(s.format(f.full_name, min(subsets), max(subsets)))
         out.append('        goto exit;\n')
         out.append("}")
 

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