gh-157890: Raise SyntaxError for malformed Unicode escapes in format specifiers

Raise `SyntaxError` instead of leaking `UnicodeDecodeError` when malformed Unicode character name escapes occur in f-string or t-string format specifiers.

Author: lpyu001

Lib/test/test_fstring.py

@@ -1045,6 +1045,12 @@ def test_misformed_unicode_character_name(self):
                              r"f'\N{'",
                              r"f'\N{GREEK CAPITAL LETTER DELTA'",
 
+                             # Malformed Unicode escapes in format specifiers
+                             #  are decoded through a separate parser path.
+                             r"f'{x:\N}'",
+                             r"f'{x:\N{'",
+                             r"f'{x:{y}\N{'",
+
                              # Here are the non-f-string versions,
                              #  which should give the same errors.
                              r"'\N'",

Lib/test/test_tstring.py

@@ -354,6 +354,11 @@ def test_syntax_errors(self):
             with self.subTest(case), self.assertRaisesRegex(SyntaxError, err):
                 eval(case)
 
+        # Malformed Unicode escapes in format specifiers are decoded
+        # through a separate parser path.
+        with self.assertRaises(SyntaxError):
+            eval(r"t'{x:\N}'")
+
     def test_runtime_errors(self):
         # Test missing variables
         with self.assertRaises(NameError):

Misc/NEWS.d/next/Core_and_Builtins/2026-09-21-16-28-16.gh-issue-157890.OzIanA.rst

@@ -0,0 +1,2 @@
+Raise :exc:`SyntaxError` instead of :exc:`UnicodeDecodeError` for malformed
+Unicode character name escapes in f-string and t-string format specifiers.

Parser/action_helpers.c

@@ -1504,8 +1504,9 @@ expr_ty _PyPegen_decoded_constant_from_token(Parser* p, Token* tok) {
 
     int is_raw = tok->is_raw;
 
-    PyObject* str = _PyPegen_decode_string(p, is_raw, bstr, bsize, tok);
+    PyObject *str = _PyPegen_decode_string(p, is_raw, bstr, bsize, tok);
     if (str == NULL) {
+        _Pypegen_raise_decode_error(p);
         return NULL;
     }
     if (_PyArena_AddPyObject(p->arena, str) < 0) {