gh-128894: Fix TracebackException._format_syntax_error on custom SyntaxError metadata

Lib/test/test_traceback.py

@@ -376,6 +376,30 @@ def f():
             '   ValueError: 0\n',
         ])
 
+    def test_format_exception_group_syntax_error_with_custom_values(self):
+        # See https://github.com/python/cpython/issues/128894
+        for exc in [
+            SyntaxError('error', 'abcd'),
+            SyntaxError('error', [None] * 4),
+            SyntaxError('error', (1, 2, 3, 4)),
+            SyntaxError('error', (1, 2, 3, 4)),
+            SyntaxError('error', (1, 'a', 'b', 2)),
+            # with end_lineno and end_offset:
+            SyntaxError('error', 'abcdef'),
+            SyntaxError('error', [None] * 6),
+            SyntaxError('error', (1, 2, 3, 4, 5, 6)),
+            SyntaxError('error', (1, 'a', 'b', 2, 'c', 'd')),
+        ]:
+            with self.subTest(exc=exc):
+                err = traceback.format_exception_only(exc, show_group=True)
+                # Should not raise an exception:
+                if exc.lineno is not None:
+                    self.assertEqual(len(err), 2)
+                    self.assertTrue(err[0].startswith('  File'))
+                else:
+                    self.assertEqual(len(err), 1)
+                self.assertEqual(err[-1], 'SyntaxError: error\n')
+
     @requires_subprocess()
     @force_not_colorized
     def test_encoded_file(self):

Lib/traceback.py

@@ -1283,7 +1283,7 @@ def _format_syntax_error(self, stype, **kwargs):
             filename_suffix = ' ({})'.format(self.filename)
 
         text = self.text
-        if text is not None:
+        if isinstance(text, str):
             # text  = "   foo\n"
             # rtext = "   foo"
             # ltext =    "foo"
@@ -1292,10 +1292,17 @@ def _format_syntax_error(self, stype, **kwargs):
             spaces = len(rtext) - len(ltext)
             if self.offset is None:
                 yield '    {}\n'.format(ltext)
-            else:
+            elif isinstance(self.offset, int):
                 offset = self.offset
                 if self.lineno == self.end_lineno:
-                    end_offset = self.end_offset if self.end_offset not in {None, 0} else offset
+                    end_offset = (
+                        self.end_offset
+                        if (
+                            isinstance(self.end_offset, int)
+                            and self.end_offset != 0
+                        )
+                        else offset
+                    )
                 else:
                     end_offset = len(rtext) + 1
 

Misc/NEWS.d/next/Library/2025-01-17-17-20-51.gh-issue-128894.gX1-8J.rst

@@ -0,0 +1,2 @@
+Fix ``traceback.TracebackException._format_syntax_error`` not to fail on
+exceptions with custom metadata.