[3.6] bpo-28603: Fix formatting tracebacks for unhashable exceptions (GH-4014) (#4024)

(cherry picked from commit de86073a76)

Lib/idlelib/idle_test/test_run.py

@@ -0,0 +1,35 @@
+import unittest
+from unittest import mock
+
+from test.support import captured_stderr
+import idlelib.run as idlerun
+
+
+class RunTest(unittest.TestCase):
+    def test_print_exception_unhashable(self):
+        class UnhashableException(Exception):
+            def __eq__(self, other):
+                return True
+
+        ex1 = UnhashableException('ex1')
+        ex2 = UnhashableException('ex2')
+        try:
+            raise ex2 from ex1
+        except UnhashableException:
+            try:
+                raise ex1
+            except UnhashableException:
+                with captured_stderr() as output:
+                    with mock.patch.object(idlerun,
+                                           'cleanup_traceback') as ct:
+                        ct.side_effect = lambda t, e: t
+                        idlerun.print_exception()
+
+        tb = output.getvalue().strip().splitlines()
+        self.assertEqual(11, len(tb))
+        self.assertIn('UnhashableException: ex2', tb[3])
+        self.assertIn('UnhashableException: ex1', tb[10])
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)