bpo-28603 Fix tracebacks for unhashable exceptions

traceback2/__init__.py

@@ -438,11 +438,11 @@ def __init__(self, exc_type, exc_value, exc_traceback, limit=None,
         # Handle loops in __cause__ or __context__.
         if _seen is None:
             _seen = set()
-        _seen.add(exc_value)
+        _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
         if (exc_value and getattr(exc_value, '__cause__', None) is not None
-            and exc_value.__cause__ not in _seen):
+            and id(exc_value.__cause__) not in _seen):
             cause = TracebackException(
                 type(exc_value.__cause__),
                 exc_value.__cause__,
@@ -454,7 +454,7 @@ def __init__(self, exc_type, exc_value, exc_traceback, limit=None,
         else:
             cause = None
         if (exc_value and getattr(exc_value, '__context__', None) is not None
-            and exc_value.__context__ not in _seen):
+            and id(exc_value.__context__) not in _seen):
             context = TracebackException(
                 type(exc_value.__context__),
                 exc_value.__context__,

traceback2/tests/test_traceback.py

@@ -747,6 +747,30 @@ def test_context(self):
         self.assertEqual(exc_info[0], exc.exc_type)
         self.assertEqual(str(exc_info[1]), str(exc))
 
+    def test_unhashable(self):
+        class UnhashableException(Exception):
+            __hash__ = None
+
+            def __eq__(self, other):
+                return True
+
+        ex1 = UnhashableException('ex1')
+        ex2 = UnhashableException('ex2')
+        try:
+            raise_from(ex2, ex1)
+        except UnhashableException:
+            try:
+                raise ex1
+            except UnhashableException:
+                exc_info = sys.exc_info()
+        exc = traceback.TracebackException(*exc_info)
+        formatted = list(exc.format())
+        if six.PY2:
+            self.assertIn('UnhashableException: ex1\n', formatted[2])
+        else:
+            self.assertIn('UnhashableException: ex2\n', formatted[3])
+            self.assertIn('UnhashableException: ex1\n', formatted[7])
+
     def test_limit(self):
         def recurse(n):
             if n: