gh-152490: Fix TypeError when formatting SyntaxError with msg=NotImplemented in traceback.py

Lib/test/test_traceback.py

@@ -587,6 +587,13 @@ def test_format_exception_only_exc(self):
         output = traceback.format_exception_only(Exception("projector"))
         self.assertEqual(output, ["Exception: projector\n"])
 
+    @support.subTests("arg", [NotImplemented, "invalid syntax"])
+    def test_format_syntax_error_message(self, arg):
+        exc = SyntaxError(arg)
+        lines = traceback.format_exception_only(type(exc), exc)
+        result = "".join(lines)
+        self.assertIn(str(arg), result)
+
     def test_exception_is_None(self):
         NONE_EXC_STRING = 'NoneType: None\n'
         excfile = StringIO()

Lib/traceback.py

@@ -1611,7 +1611,10 @@ def _format_syntax_error(self, stype, **kwargs):
                     )
                 else:
                     yield '    {}\n'.format(ltext)
-        msg = self.msg or "<no detail available>"
+        if self.msg in (None, ''):
+            msg = "<no detail available>"
+        else:
+            msg = str(self.msg)
         yield "{}{}{}: {}{}{}{}\n".format(
             theme.type,
             stype,

Misc/NEWS.d/next/Library/2026-06-28-14-37-01.gh-issue-152490.A8qLbF.rst

@@ -0,0 +1 @@
+Fix :exc:`TypeError` when formatting a :exc:`SyntaxError` raised with :data:`NotImplemented` as its argument, which caused unexpected REPL exit and IDLE Shell restart.