gh-13731: Fix incorrect treatment of format specs in raw fstrings

pablogsal · pablogsal · commit b15aa0480a52 · 2025-08-02T23:10:05.000+01:00
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
@@ -1831,6 +1831,42 @@ 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 Unicode escape
+        non_raw = f"{UnchangedFormat():\xFF}"
+        self.assertEqual(non_raw, 'ÿ')
+        
+        raw = rf"{UnchangedFormat():\xFF}"
+        self.assertEqual(raw, '\\xFF')
+        
+        # Test newline escape
+        non_raw = f"{UnchangedFormat():\n}"
+        self.assertEqual(non_raw, '\n')
+        
+        raw = rf"{UnchangedFormat():\n}"
+        self.assertEqual(raw, '\\n')
+        
+        # Test tab escape
+        non_raw = f"{UnchangedFormat():\t}"
+        self.assertEqual(non_raw, '\t')
+        
+        raw = rf"{UnchangedFormat():\t}"
+        self.assertEqual(raw, '\\t')
+        
+        # Test multiple format specs in same raw f-string
+        result = rf"{UnchangedFormat():\xFF} {UnchangedFormat():\n}"
+        self.assertEqual(result, '\\xFF \\n')
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-08-02-23-04-57.gh-issue-137314.wjEdzD.rst b/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.
diff --git a/Parser/action_helpers.c b/Parser/action_helpers.c
@@ -1404,7 +1404,15 @@ 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;
     }
