gh-155373: Inline the parsing code of optional groups in Argument Clinic (GH-155374)

Instead of one PyArg_ParseTuple() call per number of arguments, generate
the inlined code of the converters, as for all other parsing methods.
Every argument is parsed once.

Such functions now use the fastcall convention, except for constructors
and other functions which need a tuple.

Author: serhiy-storchaka

Lib/test/clinic.test.c

@@ -5786,21 +5786,15 @@ Test___init__(PyObject *self, PyObject *args, PyObject *kwargs)
         !_PyArg_NoKeywords("Test", kwargs)) {
         goto exit;
     }
-    switch (PyTuple_GET_SIZE(args)) {
-        case 1:
-            if (!PyArg_ParseTuple(args, "O:__init__", &a)) {
-                goto exit;
-            }
-            break;
-        case 2:
-            if (!PyArg_ParseTuple(args, "OO:__init__", &a, &b)) {
-                goto exit;
-            }
-            group_right_1 = 1;
-            break;
-        default:
-            PyErr_SetString(PyExc_TypeError, "Test.__init__ requires 1 to 2 arguments");
-            goto exit;
+    Py_ssize_t nargs = PyTuple_GET_SIZE(args);
+    if (nargs < 1 || nargs > 2) {
+        PyErr_SetString(PyExc_TypeError, "Test.__init__ requires 1 to 2 arguments");
+        goto exit;
+    }
+    a = PyTuple_GET_ITEM(args, 0);
+    if (nargs >= 2) {
+        b = PyTuple_GET_ITEM(args, 1);
+        group_right_1 = 1;
     }
     return_value = Test___init___impl((TestObj *)self, a, group_right_1, b);
 
@@ -5811,7 +5805,7 @@ Test___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 static int
 Test___init___impl(TestObj *self, PyObject *a, int group_right_1,
                    PyObject *b)
-/*[clinic end generated code: output=2bbb8ea60e8f57a6 input=10f5d0f1e8e466ef]*/
+/*[clinic end generated code: output=72fdd2de63c05b9e input=10f5d0f1e8e466ef]*/
 
 
 /*[clinic input]
@@ -5828,30 +5822,25 @@ PyDoc_STRVAR(only_optional_group__doc__,
 "The only parameter is in an optional group.");
 
 #define ONLY_OPTIONAL_GROUP_METHODDEF    \
-    {"only_optional_group", (PyCFunction)only_optional_group, METH_VARARGS, only_optional_group__doc__},
+    {"only_optional_group", _PyCFunction_CAST(only_optional_group), METH_FASTCALL, only_optional_group__doc__},
 
 static PyObject *
 only_optional_group_impl(PyObject *module, int group_right_1, PyObject *a);
 
 static PyObject *
-only_optional_group(PyObject *module, PyObject *args)
+only_optional_group(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
     int group_right_1 = 0;
     PyObject *a = NULL;
 
-    switch (PyTuple_GET_SIZE(args)) {
-        case 0:
-            break;
-        case 1:
-            if (!PyArg_ParseTuple(args, "O:only_optional_group", &a)) {
-                goto exit;
-            }
-            group_right_1 = 1;
-            break;
-        default:
-            PyErr_SetString(PyExc_TypeError, "only_optional_group requires 0 to 1 arguments");
-            goto exit;
+    if (nargs > 1) {
+        PyErr_SetString(PyExc_TypeError, "only_optional_group requires 0 to 1 arguments");
+        goto exit;
+    }
+    if (nargs >= 1) {
+        a = args[0];
+        group_right_1 = 1;
     }
     return_value = only_optional_group_impl(module, group_right_1, a);
 
@@ -5861,7 +5850,7 @@ only_optional_group(PyObject *module, PyObject *args)
 
 static PyObject *
 only_optional_group_impl(PyObject *module, int group_right_1, PyObject *a)
-/*[clinic end generated code: output=e7546b9441793d7d input=426c64055af7bcab]*/
+/*[clinic end generated code: output=4c7959fcc06bd216 input=426c64055af7bcab]*/
 
 
 /*[clinic input]
@@ -5880,39 +5869,37 @@ PyDoc_STRVAR(group_and_optional_parameter__doc__,
 "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__},
+    {"group_and_optional_parameter", _PyCFunction_CAST(group_and_optional_parameter), METH_FASTCALL, 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)
+group_and_optional_parameter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     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;
+    Py_ssize_t offset = 0;
+    if (nargs > 3) {
+        PyErr_SetString(PyExc_TypeError, "group_and_optional_parameter requires 0 to 3 arguments");
+        goto exit;
     }
+    if (nargs >= 2) {
+        a = args[0];
+        b = args[1];
+        offset += 2;
+        group_left_1 = 1;
+    }
+    if (nargs <= offset) {
+        goto skip_optional;
+    }
+    c = args[offset];
+skip_optional:
     return_value = group_and_optional_parameter_impl(module, group_left_1, a, b, c);
 
 exit:
@@ -5922,7 +5909,7 @@ group_and_optional_parameter(PyObject *module, PyObject *args)
 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 end generated code: output=651f2361ffc5e256 input=7f0fbb6124f5a972]*/
 
 
 /*[clinic input]
@@ -5944,15 +5931,15 @@ PyDoc_STRVAR(two_groups_on_the_same_level__doc__,
 "Groups on the same level are independent of each other.");
 
 #define TWO_GROUPS_ON_THE_SAME_LEVEL_METHODDEF    \
-    {"two_groups_on_the_same_level", (PyCFunction)two_groups_on_the_same_level, METH_VARARGS, two_groups_on_the_same_level__doc__},
+    {"two_groups_on_the_same_level", _PyCFunction_CAST(two_groups_on_the_same_level), METH_FASTCALL, two_groups_on_the_same_level__doc__},
 
 static PyObject *
 two_groups_on_the_same_level_impl(PyObject *module, int group_left_1,
                                   PyObject *a, PyObject *b, int group_left_2,
                                   PyObject *c, PyObject *d);
 
 static PyObject *
-two_groups_on_the_same_level(PyObject *module, PyObject *args)
+two_groups_on_the_same_level(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
     int group_left_1 = 0;
@@ -5962,35 +5949,20 @@ two_groups_on_the_same_level(PyObject *module, PyObject *args)
     PyObject *c = NULL;
     PyObject *d;
 
-    switch (PyTuple_GET_SIZE(args)) {
-        case 1:
-            if (!PyArg_ParseTuple(args, "O:two_groups_on_the_same_level", &d)) {
-                goto exit;
-            }
-            break;
-        case 2:
-            if (!PyArg_ParseTuple(args, "OO:two_groups_on_the_same_level", &c, &d)) {
-                goto exit;
-            }
-            group_left_2 = 1;
-            break;
-        case 3:
-            if (!PyArg_ParseTuple(args, "OOO:two_groups_on_the_same_level", &a, &b, &d)) {
-                goto exit;
-            }
-            group_left_1 = 1;
-            break;
-        case 4:
-            if (!PyArg_ParseTuple(args, "OOOO:two_groups_on_the_same_level", &a, &b, &c, &d)) {
-                goto exit;
-            }
-            group_left_1 = 1;
-            group_left_2 = 1;
-            break;
-        default:
-            PyErr_SetString(PyExc_TypeError, "two_groups_on_the_same_level requires 1 to 4 arguments");
-            goto exit;
+    if (nargs < 1 || nargs > 4) {
+        PyErr_SetString(PyExc_TypeError, "two_groups_on_the_same_level requires 1 to 4 arguments");
+        goto exit;
+    }
+    if (nargs >= 3) {
+        a = args[0];
+        b = args[1];
+        group_left_1 = 1;
+    }
+    if (nargs == 2 || nargs == 4) {
+        c = args[nargs - 2];
+        group_left_2 = 1;
     }
+    d = args[nargs - 1];
     return_value = two_groups_on_the_same_level_impl(module, group_left_1, a, b, group_left_2, c, d);
 
 exit:
@@ -6001,7 +5973,7 @@ static PyObject *
 two_groups_on_the_same_level_impl(PyObject *module, int group_left_1,
                                   PyObject *a, PyObject *b, int group_left_2,
                                   PyObject *c, PyObject *d)
-/*[clinic end generated code: output=508a61ee582da21e input=1b45d9b675b32d1a]*/
+/*[clinic end generated code: output=737c3f543296e916 input=1b45d9b675b32d1a]*/
 
 
 /*[clinic input]

Misc/NEWS.d/next/Tools-Demos/2026-08-08-12-40-00.gh-issue-155373.Wq3xTm.rst

@@ -0,0 +1,6 @@
+Argument Clinic now generates the inlined code of the converters for functions
+with optional groups, instead of a ``PyArg_ParseTuple()`` call for every number
+of arguments.
+Every argument is now parsed once, most such functions now use the fastcall
+convention, and the errors for a wrong type of an argument are the same as in
+other functions.

Modules/_cursesmodule.c

@@ -5303,7 +5303,8 @@ static PyMethodDef PyCursesWindow_methods[] = {
     {"standout", PyCursesWindow_wstandout, METH_NOARGS,
      "standout($self, /)\n--\n\n"
      "Turn on the A_STANDOUT attribute."},
-    {"subpad",          _curses_window_subwin, METH_VARARGS, _curses_window_subwin__doc__},
+    {"subpad", _PyCFunction_CAST(_curses_window_subwin), METH_FASTCALL,
+     _curses_window_subwin__doc__},
     _CURSES_WINDOW_SUBWIN_METHODDEF
     {"syncdown", PyCursesWindow_wsyncdown, METH_NOARGS,
      "syncdown($self, /)\n--\n\n"