diff -r 47bf09270027 Lib/test/test_traceback.py
--- a/Lib/test/test_traceback.py	Sun Dec 11 14:35:07 2016 -0800
+++ b/Lib/test/test_traceback.py	Mon Dec 12 23:59:02 2016 -0800
@@ -19,6 +19,10 @@
 test_tb = namedtuple('tb', ['tb_frame', 'tb_lineno', 'tb_next'])
 
 
+class UnhashableError(Exception):
+    __hash__ = None
+
+
 class TracebackCases(unittest.TestCase):
     # For now, a very minimal set of tests.  I want to be sure that
     # formatting of SyntaxErrors works based on changes for 2.1.
@@ -463,6 +467,29 @@
         err = self.get_report(Exception(''))
         self.assertIn('Exception\n', err)
 
+    def test_unhashable_exception(self):
+        # http://bugs.python.org/issue28603
+        report = self.get_report(UnhashableError())
+        self.assertIn('UnhashableError', report)
+
+        try:
+            raise UnhashableError('err1') from UnhashableError('err2')
+        except UnhashableError as e:
+            report = self.get_report(e)
+            self.assertIn('UnhashableError', report)
+            self.assertIn('err1', report)
+            self.assertIn('err2', report)
+
+        try:
+            try:
+                raise UnhashableError('err1')
+            except UnhashableError:
+                raise UnhashableError('err2')
+        except UnhashableError as e:
+            report = self.get_report(e)
+            self.assertIn('UnhashableError', report)
+            self.assertIn('err2', report)
+
 
 class PyExcReportingTests(BaseExceptionReportingTests, unittest.TestCase):
     #
diff -r 47bf09270027 Lib/traceback.py
--- a/Lib/traceback.py	Sun Dec 11 14:35:07 2016 -0800
+++ b/Lib/traceback.py	Mon Dec 12 23:59:02 2016 -0800
@@ -436,11 +436,15 @@
         # Handle loops in __cause__ or __context__.
         if _seen is None:
             _seen = set()
-        _seen.add(exc_value)
+        try:
+            _seen.add(exc_value)
+        except TypeError:
+            # see issue 28603
+            pass
         # 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 exc_value.__cause__ is not None
-            and exc_value.__cause__ not in _seen):
+            and not self._safe_in(exc_value.__cause__, _seen)):
             cause = TracebackException(
                 type(exc_value.__cause__),
                 exc_value.__cause__,
@@ -452,7 +456,7 @@
         else:
             cause = None
         if (exc_value and exc_value.__context__ is not None
-            and exc_value.__context__ not in _seen):
+            and not self._safe_in(exc_value.__context__, _seen)):
             context = TracebackException(
                 type(exc_value.__context__),
                 exc_value.__context__,
@@ -486,6 +490,14 @@
         if lookup_lines:
             self._load_lines()
 
+    def _safe_in(self, element, sequence):
+        try:
+            return element in sequence
+        except TypeError:
+            # assume it's there if the element is not hashable, so we don't
+            # continue traversing the chain
+            return True
+
     @classmethod
     def from_exception(self, exc, *args, **kwargs):
         """Create a TracebackException from an exception."""