gh-157407: Argument Clinic: parse positional-only defining_class methods without _PyArg_Parser

[eendebakpt]

Methods taking a defining_class whose other parameters are all positional-only (e.g. Decimal.from_float) used the keyword path: a static _PyArg_Parser and a _PyArg_UnpackKeywords() call. They now use _PyArg_NoKwnames().

Typical code change:

-    static const char * const _keywords[] = {"", NULL};
-    static _PyArg_Parser _parser = {
-        .keywords = _keywords,
-        .fname = "from_float",
-        .kwtuple = KWTUPLE,
-    };
-    PyObject *argsbuf[1];
     PyObject *pyfloat;
 
-    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser,
-            /*minpos*/ 1, /*maxpos*/ 1, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
-    if (!args) {
+    if (!_PyArg_NoKwnames("from_float", kwnames)) {
+        goto exit;
+    }
+    if (!_PyArg_CheckPositional("from_float", nargs, 1, 1)) {

This converts 135 methods in 19 clinic headers. _PyArg_NoKwnames() is now exported for shared extension modules.

Size change in bytes:

	.text	read-only data	.data
Windows x64 (MSVC, LTCG)	−6352	−4708	−10512
Linux x86-64 (gcc -O3)*	−2592	−1144	−5664

* excluding _decimal, _sqlite3, _dbm and _gdbm, which were not built. Linux also drops 4872 bytes of .rela.dyn.

Changed error messages: these methods now raise the same TypeError messages as other positional-only methods,:

call	before	after
compressobj().compress()	compress() takes exactly 1 positional argument (0 given)	compress expected 1 argument, got 0
compressobj().compress(b'', b'')	compress() takes at most 1 argument (2 given)	compress expected 1 argument, got 2
Pickler(f).dump(obj=1)	dump() takes exactly 1 positional argument (0 given)	dump() takes no keyword arguments

This is arguably more consistent, but a small change in error messages.

Performance seems slightly improved (but within noise, no not quoting any numbers).

Generated with Claude Code

• Issue: Reduce some code sizes #157407