gh-137314: Fix incorrect treatment of format specs in raw fstrings (GH-137328)

(cherry picked from commit 0153d82)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: pablogsal

Lib/test/test_fstring.py

@@ -1821,6 +1821,34 @@ def test_newlines_in_format_specifiers(self):
         for case in valid_cases:
             compile(case, "<string>", "exec")
 
+    def test_raw_fstring_format_spec(self):
+        # Test raw f-string format spec behavior (Issue #137314).
+        #
+        # Raw f-strings should preserve literal backslashes in format specifications,
+        # not interpret them as escape sequences.
+        class UnchangedFormat:
+            """Test helper that returns the format spec unchanged."""
+            def __format__(self, format):
+                return format
+
+        # Test basic escape sequences
+        self.assertEqual(f"{UnchangedFormat():\xFF}", 'ÿ')
+        self.assertEqual(rf"{UnchangedFormat():\xFF}", '\\xFF')
+
+        # Test nested expressions with raw/non-raw combinations
+        self.assertEqual(rf"{UnchangedFormat():{'\xFF'}}", 'ÿ')
+        self.assertEqual(f"{UnchangedFormat():{r'\xFF'}}", '\\xFF')
+        self.assertEqual(rf"{UnchangedFormat():{r'\xFF'}}", '\\xFF')
+
+        # Test continuation character in format specs
+        self.assertEqual(f"""{UnchangedFormat():{'a'\
+                        'b'}}""", 'ab')
+        self.assertEqual(rf"""{UnchangedFormat():{'a'\
+                         'b'}}""", 'ab')
+
+        # Test multiple format specs in same raw f-string
+        self.assertEqual(rf"{UnchangedFormat():\xFF} {UnchangedFormat():\n}", '\\xFF \\n')
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-08-02-23-04-57.gh-issue-137314.wjEdzD.rst

@@ -0,0 +1,5 @@
+Fixed a regression where raw f-strings incorrectly interpreted
+escape sequences in format specifications. Raw f-strings now properly preserve
+literal backslashes in format specs, matching the behavior from Python 3.11.
+For example, ``rf"{obj:\xFF}"`` now correctly produces ``'\\xFF'`` instead of
+``'ÿ'``. Patch by Pablo Galindo.

Parser/action_helpers.c

@@ -1369,7 +1369,15 @@ expr_ty _PyPegen_decoded_constant_from_token(Parser* p, Token* tok) {
     if (PyBytes_AsStringAndSize(tok->bytes, &bstr, &bsize) == -1) {
         return NULL;
     }
-    PyObject* str = _PyPegen_decode_string(p, 0, bstr, bsize, tok);
+
+    // Check if we're inside a raw f-string for format spec decoding
+    int is_raw = 0;
+    if (INSIDE_FSTRING(p->tok)) {
+        tokenizer_mode *mode = TOK_GET_MODE(p->tok);
+        is_raw = mode->raw;
+    }
+
+    PyObject* str = _PyPegen_decode_string(p, is_raw, bstr, bsize, tok);
     if (str == NULL) {
         return NULL;
     }