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 23:55:47 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 +467,30 @@
         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('err1', 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 23:55:47 2016 -0800
@@ -435,8 +435,8 @@
         # need stub thunk objects just to glue it together.
         # Handle loops in __cause__ or __context__.
         if _seen is None:
-            _seen = set()
-        _seen.add(exc_value)
+            _seen = []
+        _seen.append(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 exc_value.__cause__ is not None
diff -r 47bf09270027 Python/pythonrun.c
--- a/Python/pythonrun.c	Sun Dec 11 14:35:07 2016 -0800
+++ b/Python/pythonrun.c	Mon Dec 12 23:55:47 2016 -0800
@@ -807,13 +807,14 @@
 
     if (seen != NULL) {
         /* Exception chaining */
-        if (PySet_Add(seen, value) == -1)
+        if (PyList_Append(seen, value) == -1) {
             PyErr_Clear();
+        }
         else if (PyExceptionInstance_Check(value)) {
             cause = PyException_GetCause(value);
             context = PyException_GetContext(value);
             if (cause) {
-                res = PySet_Contains(seen, cause);
+                res = PySequence_Contains(seen, cause);
                 if (res == -1)
                     PyErr_Clear();
                 if (res == 0) {
@@ -825,7 +826,7 @@
             }
             else if (context &&
                 !((PyBaseExceptionObject *)value)->suppress_context) {
-                res = PySet_Contains(seen, context);
+                res = PySequence_Contains(seen, context);
                 if (res == -1)
                     PyErr_Clear();
                 if (res == 0) {
@@ -870,7 +871,7 @@
         /* We choose to ignore seen being possibly NULL, and report
            at least the main exception (it could be a MemoryError).
         */
-        seen = PySet_New(NULL);
+        seen = PyList_New(0);
         if (seen == NULL)
             PyErr_Clear();
         print_exception_recursive(f, value, seen);