gh-117557: Improve error message for 'C' format if given string is not 1 char long

Lib/test/clinic.test.c

@@ -1028,7 +1028,9 @@ test_int_converter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
         goto exit;
     }
     if (PyUnicode_GET_LENGTH(args[2]) != 1) {
-        _PyArg_BadArgument("test_int_converter", "argument 3", "a unicode character", args[2]);
+        PyErr_SetString(PyExc_TypeError,
+            "test_int_converter(): argument 3 must be a string containing "
+            "exactly one unicode character");
         goto exit;
     }
     c = PyUnicode_READ_CHAR(args[2], 0);
@@ -1048,7 +1050,7 @@ test_int_converter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 
 static PyObject *
 test_int_converter_impl(PyObject *module, int a, int b, int c, myenum d)
-/*[clinic end generated code: output=5aed87a7589eefb2 input=d20541fc1ca0553e]*/
+/*[clinic end generated code: output=9c44e10850d9b44c input=d20541fc1ca0553e]*/
 
 
 /*[clinic input]

Misc/NEWS.d/next/Tools-Demos/2024-04-05-13-08-02.gh-issue-117557.tPewu9.rst

@@ -0,0 +1,3 @@
+Argument Clinic now generates slightly more accurate error messages for
+``int(accept={str})`` converters if the given string is not exactly one
+character long.

Python/getargs.c

@@ -575,11 +575,13 @@ static const char *
 converterr(const char *expected, PyObject *arg, char *msgbuf, size_t bufsize)
 {
     assert(expected != NULL);
-    assert(arg != NULL);
     if (expected[0] == '(') {
         PyOS_snprintf(msgbuf, bufsize,
                       "%.100s", expected);
     }
+    else if (arg == NULL) {
+        PyOS_snprintf(msgbuf, bufsize, "must be %.100s", expected);
+    }
     else {
         PyOS_snprintf(msgbuf, bufsize,
                       "must be %.50s, not %.50s", expected,
@@ -812,7 +814,8 @@ convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags,
             return converterr("a unicode character", arg, msgbuf, bufsize);
 
         if (PyUnicode_GET_LENGTH(arg) != 1)
-            return converterr("a unicode character", arg, msgbuf, bufsize);
+            return converterr("a string containing exactly one unicode character",
+                              NULL, msgbuf, bufsize);
 
         kind = PyUnicode_KIND(arg);
         data = PyUnicode_DATA(arg);

Tools/clinic/libclinic/converters.py

@@ -272,11 +272,14 @@ def parse_arg(self, argname: str, displayname: str, *, limited_capi: bool) -> st
                     goto exit;
                 }}}}
                 if (PyUnicode_GET_LENGTH({argname}) != 1) {{{{
-                    {bad_argument}
+                    PyErr_SetString(PyExc_TypeError,
+                        "{{name}}(): {displayname} must be a string containing "
+                        "exactly one unicode character");
                     goto exit;
                 }}}}
                 {paramname} = PyUnicode_READ_CHAR({argname}, 0);
                 """,
+                displayname=displayname,
                 argname=argname,
                 bad_argument=self.bad_argument(displayname, 'a unicode character', limited_capi=limited_capi),
             )