gh-58124: Avoid CP_UTF8 in UnicodeDecodeError (#137415)

Fix name of the Python encoding in Unicode errors of the code page
codec: use "cp65000" and "cp65001" instead of "CP_UTF7" and "CP_UTF8"
which are not valid Python code names.

Author: vstinner

Lib/test/test_codecs.py

@@ -3293,7 +3293,7 @@ def test_code_page_name(self):
             codecs.code_page_encode, 932, '\xff')
         self.assertRaisesRegex(UnicodeDecodeError, 'cp932',
             codecs.code_page_decode, 932, b'\x81\x00', 'strict', True)
-        self.assertRaisesRegex(UnicodeDecodeError, 'CP_UTF8',
+        self.assertRaisesRegex(UnicodeDecodeError, 'cp65001',
             codecs.code_page_decode, self.CP_UTF8, b'\xff', 'strict', True)
 
     def check_decode(self, cp, tests):

Misc/NEWS.d/next/Core_and_Builtins/2025-08-05-17-22-24.gh-issue-58124.q1__53.rst

@@ -0,0 +1,3 @@
+Fix name of the Python encoding in Unicode errors of the code page codec:
+use "cp65000" and "cp65001" instead of "CP_UTF7" and "CP_UTF8" which are not
+valid Python code names. Patch by Victor Stinner.

Objects/unicodeobject.c

@@ -7684,10 +7684,6 @@ code_page_name(UINT code_page, PyObject **obj)
     *obj = NULL;
     if (code_page == CP_ACP)
         return "mbcs";
-    if (code_page == CP_UTF7)
-        return "CP_UTF7";
-    if (code_page == CP_UTF8)
-        return "CP_UTF8";
 
     *obj = PyBytes_FromFormat("cp%u", code_page);
     if (*obj == NULL)

Python/codecs.c

@@ -1204,7 +1204,7 @@ get_standard_encoding_impl(const char *encoding, int *bytelength)
             }
         }
     }
-    else if (strcmp(encoding, "CP_UTF8") == 0) {
+    else if (strcmp(encoding, "cp65001") == 0) {
         *bytelength = 3;
         return ENC_UTF8;
     }