gh-136681: Reduce the size of _PyArg_Parser

Narrow pos, min, max and is_kwtuple_owned, which are small counts, and
remove custom_msg: it is only needed to report a conversion error, and
can then be looked up in the format string.  Keep format, keywords and
fname first, so that existing positional initializers still work.
_PyArg_Parser shrinks from 72 to 48 bytes on 64-bit platforms.

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

Author: eendebakpt

Include/cpython/modsupport.h

@@ -25,14 +25,13 @@ typedef struct _PyArg_Parser {
     const char *format;
     const char * const *keywords;
     const char *fname;
-    const char *custom_msg;
-    _PyOnceFlag once;       /* atomic one-time initialization flag */
-    int is_kwtuple_owned;   /* does this parser own the kwtuple object? */
-    int pos;                /* number of positional-only arguments */
-    int min;                /* minimal number of arguments */
-    int max;                /* maximal number of positional arguments */
     PyObject *kwtuple;      /* tuple of keyword parameter names */
     struct _PyArg_Parser *next;
+    _PyOnceFlag once;       /* atomic one-time initialization flag */
+    uint8_t is_kwtuple_owned;  /* does this parser own the kwtuple object? */
+    uint16_t pos;           /* number of positional-only arguments */
+    uint16_t min;           /* minimal number of arguments */
+    uint16_t max;           /* maximal number of positional arguments */
 } _PyArg_Parser;
 
 PyAPI_FUNC(int) _PyArg_ParseTupleAndKeywordsFast(PyObject *, PyObject *,

Python/getargs.c

@@ -2045,21 +2045,11 @@ scan_keywords(const char * const *keywords, int *ptotal, int *pposonly)
 
 static int
 parse_format(const char *format, int total, int npos,
-             const char **pfname, const char **pcustommsg,
-             int *pmin, int *pmax)
+             const char **pfname, int *pmin, int *pmax)
 {
-    /* grab the function name or custom error msg first (mutually exclusive) */
-    const char *custommsg;
     const char *fname = strchr(format, ':');
     if (fname) {
         fname++;
-        custommsg = NULL;
-    }
-    else {
-        custommsg = strchr(format,';');
-        if (custommsg) {
-            custommsg++;
-        }
     }
 
     int min = INT_MAX;
@@ -2118,7 +2108,6 @@ parse_format(const char *format, int total, int npos,
     }
 
     *pfname = fname;
-    *pcustommsg = custommsg;
     *pmin = min;
     *pmax = max;
     return 0;
@@ -2153,22 +2142,25 @@ _parser_init(void *arg)
     const char * const *keywords = parser->keywords;
     assert(keywords != NULL);
     assert((parser->format == NULL || parser->fname == NULL) &&
-           parser->custom_msg == NULL &&
            parser->min == 0 &&
            parser->max == 0);
 
     int len, pos;
     if (scan_keywords(keywords, &len, &pos) < 0) {
         return -1;
     }
+    if (len > UINT16_MAX) {
+        PyErr_SetString(PyExc_SystemError, "Too many keyword parameters");
+        return -1;
+    }
     assert(parser->pos == 0 || parser->pos == pos);  // may be set statically
 
-    const char *fname, *custommsg = NULL;
+    const char *fname;
     int min = 0, max = 0;
     if (parser->format) {
         assert(parser->fname == NULL);
         if (parse_format(parser->format, len, pos,
-                         &fname, &custommsg, &min, &max) < 0) {
+                         &fname, &min, &max) < 0) {
             return -1;
         }
     }
@@ -2206,11 +2198,10 @@ _parser_init(void *arg)
         owned = 0;
     }
 
-    parser->pos = pos;
+    parser->pos = (uint16_t)pos;
     parser->fname = fname;
-    parser->custom_msg = custommsg;
-    parser->min = min;
-    parser->max = max;
+    parser->min = (uint16_t)min;
+    parser->max = (uint16_t)max;
     parser->is_kwtuple_owned = owned;
     // Set last: see _PyArg_UnpackKeywords()
     _Py_atomic_store_ptr_release(&parser->kwtuple, kwtuple);
@@ -2243,7 +2234,6 @@ parser_clear(struct _PyArg_Parser *parser)
     else {
         assert(parser->fname != NULL);
     }
-    parser->custom_msg = NULL;
     parser->min = 0;
     parser->max = 0;
     parser->is_kwtuple_owned = 0;
@@ -2412,7 +2402,15 @@ vgetargskeywordsfast_impl(PyObject *const *args, Py_ssize_t nargs,
                 levels, msgbuf, sizeof(msgbuf), &freelist);
             Py_DECREF(current_arg);
             if (msg) {
-                seterror(i+1, msg, levels, parser->fname, parser->custom_msg);
+                const char *custom_msg = NULL;
+                if (parser->fname == NULL) {
+                    // The format has ";message" instead of ":name"
+                    custom_msg = strchr(parser->format, ';');
+                    if (custom_msg != NULL) {
+                        custom_msg++;
+                    }
+                }
+                seterror(i+1, msg, levels, parser->fname, custom_msg);
                 return cleanreturn(0, &freelist);
             }
             continue;