python · Aniketsy · Jun 28, 2026 · Jun 28, 2026 · Jun 28, 2026 · Jun 28, 2026
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
@@ -587,6 +587,18 @@ def test_format_exception_only_exc(self):
         output = traceback.format_exception_only(Exception("projector"))
         self.assertEqual(output, ["Exception: projector\n"])
 
+    def test_format_syntax_error_msg(self):
+        exc = SyntaxError(NotImplemented)
+        lines = traceback.format_exception_only(type(exc), exc)
+        result = "".join(lines)
+        self.assertIn("NotImplemented", result)
+
+        exc = SyntaxError("invalid syntax")
+        lines = traceback.format_exception_only(type(exc), exc)
+        result = "".join(lines)
+        self.assertIn("SyntaxError", result)
+        self.assertIn("invalid syntax", result)
+
     def test_exception_is_None(self):
         NONE_EXC_STRING = 'NoneType: None\n'
         excfile = StringIO()

diff --git a/Lib/traceback.py b/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 is None or self.msg == '':
+            msg = "<no detail available>"
+        else:
+            msg = str(self.msg)
         yield "{}{}{}: {}{}{}{}\n".format(
             theme.type,
             stype,

diff --git a/Misc/NEWS.d/next/Library/2026-06-28-14-37-01.gh-issue-152490.A8qLbF.rst b/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.
Original file line number	Diff line number	Diff line change
		@@ -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.