gh-154711: Strip comments after escaped quotes in f-string and t-string debug text

Lib/test/test_fstring.py

@@ -1678,6 +1678,11 @@ def __repr__(self):
         self.assertEqual(f'{" # nooo "=}', '" # nooo "=\' # nooo \'')
         self.assertEqual(f'{" \" # nooo \" "=}', '" \\" # nooo \\" "=\' " # nooo " \'')
 
+        # A real comment after a string that ends with an escaped quote must
+        # still be stripped (gh-154711).
+        self.assertEqual(f"{'\'' = # comment
+}", "'\\'' = \n" + repr("'"))
+
         self.assertEqual(f'{ # some comment goes here
   """hello"""=}',  ' \n  """hello"""=\'hello\'')
         self.assertEqual(f'{"""# this is not a comment

Lib/test/test_tstring.py

@@ -140,6 +140,18 @@ def test_debug_specifier(self):
         )
         self.assertEqual(fstring(t), "Value: value = 42")
 
+    def test_comment_after_escaped_quote(self):
+        # A comment after a string that ends with an escaped quote must be
+        # stripped from the interpolation expression (gh-154711).
+        t = t"{'a\'b' # comment
+}"
+        self.assertEqual(t.interpolations[0].expression, "'a\\'b'")
+
+        t = t"{'a\'b' = # comment
+}"
+        self.assertEqual(t.interpolations[0].expression, "'a\\'b'")
+        self.assertEqual(t.strings, ("'a\\'b' = \n", ""))
+
     def test_raw_tstrings(self):
         path = r"C:\Users"
         t = rt"{path}\Documents"

Misc/NEWS.d/next/Core_and_Builtins/2026-07-30-14-40-00.gh-issue-154711.nUxYXc.rst

@@ -0,0 +1,3 @@
+Fix a bug where a comment placed after a string that ends with an escaped
+quote could leak into the output of a debug f-string and into t-string
+metadata. Now the comment is stripped as expected.

Parser/lexer/string.c

@@ -74,6 +74,19 @@ _PyLexer_set_ftstring_expr(struct tok_state* tok, struct token *token, char c) {
         while (i < tok_mode->last_expr_size - tok_mode->last_expr_end) {
             char ch = tok_mode->last_expr_buffer[i];
 
+            // Copy escaped characters as-is. This keeps an escaped quote from
+            // flipping the in_string state, which would otherwise stop a real
+            // comment from being detected (see the detection loop above).
+            if (ch == '\\') {
+                result[j++] = ch;
+                i++;
+                if (i < tok_mode->last_expr_size - tok_mode->last_expr_end) {
+                    result[j++] = tok_mode->last_expr_buffer[i];
+                    i++;
+                }
+                continue;
+            }
+
             // Handle string quotes
             if (ch == '"' || ch == '\'') {
                 // See comment above to understand this part