gh-157407: Argument Clinic: parse positional-only defining_class methods without _PyArg_Parser (#157483)

* gh-136681: Drop _PyArg_Parser for positional-only defining-class methods

A method that takes a defining class and whose other parameters are all
positional-only (e.g. decimal.Decimal.from_float) was routed through
Argument Clinic's generic keyword path: it emitted a static _PyArg_Parser,
an (empty) keyword tuple and an _PyArg_UnpackKeywords() call, even though
it accepts no keyword arguments.

Such methods now go through parse_pos_only(): the wrapper still uses the
mandatory METH_METHOD|METH_FASTCALL|METH_KEYWORDS convention (the only one
that delivers the defining class), but it rejects keywords with
_PyArg_NoKwnames() and reads positional arguments directly, with no
parser, keyword tuple or argsbuf.

This converts 135 methods across 19 clinic headers, removing the static
_PyArg_Parser, the keyword arrays/tuples and the per-call
_PyArg_UnpackKeywords() + parser_init() overhead for each.

_PyArg_NoKwnames() is now exported (PyAPI_FUNC) so generated code in
shared extension modules can use it, like _PyArg_NoKeywords().

Error messages for these methods now match ordinary positional-only
functions ("expected ... arguments" / "takes no keyword arguments"
instead of the _PyArg_UnpackKeywords phrasing); affected stdlib tests
are updated accordingly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* gh-157407: Rename NEWS entry to the correct issue number

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: eendebakpt

Include/internal/pycore_modsupport.h

@@ -10,7 +10,8 @@ extern "C" {
 #endif
 
 
-extern int _PyArg_NoKwnames(const char *funcname, PyObject *kwnames);
+// Export for 'array' shared extension
+PyAPI_FUNC(int) _PyArg_NoKwnames(const char *funcname, PyObject *kwnames);
 #define _PyArg_NoKwnames(funcname, kwnames) \
     ((kwnames) == NULL || _PyArg_NoKwnames((funcname), (kwnames)))
 

Lib/test/clinic.test.c

@@ -6032,23 +6032,13 @@ static PyObject *
 Test__pyarg_parsestackandkeywords(PyObject *self, PyTypeObject *cls, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
 {
     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#:_pyarg_parsestackandkeywords",
-        .kwtuple = KWTUPLE,
-    };
-    #undef KWTUPLE
     const char *key;
     Py_ssize_t key_length;
 
-    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+    if (!_PyArg_NoKwnames("_pyarg_parsestackandkeywords", kwnames)) {
+        goto exit;
+    }
+    if (!_PyArg_ParseStack(args, nargs, "s#:_pyarg_parsestackandkeywords",
         &key, &key_length)) {
         goto exit;
     }
@@ -6062,7 +6052,7 @@ static PyObject *
 Test__pyarg_parsestackandkeywords_impl(TestObj *self, PyTypeObject *cls,
                                        const char *key,
                                        Py_ssize_t key_length)
-/*[clinic end generated code: output=7060c213d7b8200e input=fc72ef4b4cfafabc]*/
+/*[clinic end generated code: output=21bbb9a8d0100d72 input=fc72ef4b4cfafabc]*/
 
 
 /*[clinic input]

Lib/test/test_clinic.py

@@ -5114,7 +5114,7 @@ def test_defclass_posonly_varpos(self):
         cls = ac_tester.TestClass
         obj = cls()
         fn = obj.defclass_posonly_varpos
-        errmsg = 'takes at least 2 positional arguments'
+        errmsg = 'expected at least 2 arguments'
         self.assertRaisesRegex(TypeError, errmsg, fn)
         self.assertRaisesRegex(TypeError, errmsg, fn, 1)
         self.assertEqual(fn(1, 2), (cls, 1, 2, ()))

Lib/test/test_sqlite3/test_hooks.py

@@ -450,7 +450,7 @@ def test_trace_bad_handler(self):
 
     def test_set_trace_callback_keyword_args(self):
         with self.assertRaisesRegex(TypeError,
-                'takes exactly 1 positional argument'):
+                'takes no keyword arguments'):
             self.con.set_trace_callback(trace_callback=lambda: None)
 
     # When a handler has an invalid signature, the exception raised is

Lib/test/test_sqlite3/test_userfunctions.py

@@ -720,7 +720,7 @@ def test_aggr_text(self):
 
     def test_agg_keyword_args(self):
         with self.assertRaisesRegex(TypeError,
-                'takes exactly 3 positional arguments'):
+                'takes no keyword arguments'):
             self.con.create_aggregate("test", 1, aggregate_class=AggrText)
 
 
@@ -767,7 +767,7 @@ def test_clear_authorizer(self):
 
     def test_authorizer_keyword_args(self):
         with self.assertRaisesRegex(TypeError,
-                'takes exactly 1 positional argument'):
+                'takes no keyword arguments'):
             self.con.set_authorizer(authorizer_callback=lambda: None)
 
 

Misc/NEWS.d/next/Core_and_Builtins/2026-05-15-20-13-41.gh-issue-157407.4Lso8S.rst

@@ -0,0 +1,2 @@
+Argument Clinic now parses arguments of positional-only methods with a
+*defining class* directly, without a :c:type:`!_PyArg_Parser`.