pythongh-142236: Fix incorrect keyword suggestions for syntax errors (python#142328)

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", flags=codeop.PyCF_ONLY_AST)
+        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

Misc/NEWS.d/next/Core_and_Builtins/2025-12-06-00-16-43.gh-issue-142236.m3EF9E.rst

@@ -0,0 +1,4 @@
+Fix incorrect keyword suggestions for syntax errors in :mod:`traceback`. The
+keyword typo suggestion mechanism would incorrectly suggest replacements when
+the extracted source code was incomplete rather than containing an actual typo.
+Patch by Pablo Galindo.