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

(cherry picked from commit de86073a76)

Lib/test/test_traceback.py

@@ -443,6 +443,33 @@ class TracebackFormatTests(unittest.TestCase):
             '    return traceback.format_stack()\n' % (__file__, lineno+1),
         ])
 
+    @cpython_only
+    def test_unhashable(self):
+        from _testcapi import exception_print
+
+        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:
+                exc_type, exc_val, exc_tb = sys.exc_info()
+
+        with captured_output("stderr") as stderr_f:
+            exception_print(exc_val)
+
+        tb = stderr_f.getvalue().strip().splitlines()
+        self.assertEqual(11, len(tb))
+        self.assertEqual(context_message.strip(), tb[5])
+        self.assertIn('UnhashableException: ex2', tb[3])
+        self.assertIn('UnhashableException: ex1', tb[10])
+
 
 cause_message = (
     "\nThe above exception was the direct cause "
@@ -994,6 +1021,25 @@ class TestTracebackException(unittest.TestCase):
         self.assertEqual(exc_info[0], exc.exc_type)
         self.assertEqual(str(exc_info[1]), str(exc))
 
+    def test_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:
+                exc_info = sys.exc_info()
+        exc = traceback.TracebackException(*exc_info)
+        formatted = list(exc.format())
+        self.assertIn('UnhashableException: ex2\n', formatted[2])
+        self.assertIn('UnhashableException: ex1\n', formatted[6])
+
     def test_limit(self):
         def recurse(n):
             if n: