Address Serhiy's review

- try to make the error message even clearer

- sync with PyArg_Parse

Author: erlend-aasland

Lib/test/clinic.test.c

@@ -1029,7 +1029,8 @@ test_int_converter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
     }
     if (PyUnicode_GET_LENGTH(args[2]) != 1) {
         PyErr_SetString(PyExc_TypeError,
-            "test_int_converter(): argument 3 must be exactly one character long");
+            "test_int_converter(): argument 3 must be a string containing "
+            "exactly one unicode character");
         goto exit;
     }
     c = PyUnicode_READ_CHAR(args[2], 0);
@@ -1049,7 +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=fd8e758049e1d836 input=d20541fc1ca0553e]*/
+/*[clinic end generated code: output=9c44e10850d9b44c input=d20541fc1ca0553e]*/
 
 
 /*[clinic input]

Modules/clinic/_testclinic.c.h

@@ -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);

Modules/clinic/arraymodule.c.h

@@ -273,7 +273,8 @@ def parse_arg(self, argname: str, displayname: str, *, limited_capi: bool) -> st
                 }}}}
                 if (PyUnicode_GET_LENGTH({argname}) != 1) {{{{
                     PyErr_SetString(PyExc_TypeError,
-                        "{{name}}(): {displayname} must be exactly one character long");
+                        "{{name}}(): {displayname} must be a string containing "
+                        "exactly one unicode character");
                     goto exit;
                 }}}}
                 {paramname} = PyUnicode_READ_CHAR({argname}, 0);