gh-142236: Fix incorrect keyword suggestions for syntax errors

The keyword typo suggestion mechanism in traceback would incorrectly
suggest replacements when the extracted source code was merely incomplete
rather than containing an actual typo. For example, when a missing comma
caused a syntax error, the system would suggest replacing 'print' with
'not' because the incomplete code snippet happened to pass validation.

The fix adds a validation step that first checks whether the original
extracted code raises a SyntaxError. If the code compiles successfully
or is simply incomplete (compile_command returns None), the function
returns early since there is no way to verify that a keyword replacement
would actually fix the problem.

Author: pablogsal

Lib/test/test_traceback.py

@@ -1784,6 +1784,23 @@ def test_keyword_suggestions_from_command_string(self):
                 stderr_text = stderr.decode('utf-8')
                 self.assertIn(f"Did you mean '{expected_kw}'", stderr_text)
 
+    def test_no_keyword_suggestion_for_comma_errors(self):
+        # When the parser identifies a missing comma, don't suggest
+        # bogus keyword replacements like 'print' -> 'not'
+        code = '''\
+import sys
+print(
+    "line1"
+    "line2"
+    file=sys.stderr
+)
+'''
+        source = textwrap.dedent(code).strip()
+        rc, stdout, stderr = assert_python_failure('-c', source)
+        stderr_text = stderr.decode('utf-8')
+        self.assertIn("Perhaps you forgot a comma", stderr_text)
+        self.assertNotIn("Did you mean", stderr_text)
+
 @requires_debug_ranges()
 @force_not_colorized_test_class
 class PurePythonTracebackErrorCaretTests(

Lib/traceback.py

@@ -1340,6 +1340,15 @@ def _find_keyword_typos(self):
         if len(error_code) > 1024:
             return
 
+        # If the original code doesn't raise SyntaxError, we can't validate
+        # that a keyword replacement actually fixes anything
+        try:
+            codeop.compile_command(error_code, symbol="exec")
+        except SyntaxError:
+            pass  # Good - the original code has a syntax error we might fix
+        else:
+            return  # Original code compiles or is incomplete - can't validate fixes
+
         error_lines = error_code.splitlines()
         tokens = tokenize.generate_tokens(io.StringIO(error_code).readline)
         tokens_left_to_process = 10