diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -1,15 +1,15 @@
 """Test cases for traceback module"""
 
 from io import StringIO
 import sys
 import unittest
 import re
-from test.support import run_unittest, Error, captured_output
+from test.support import Error, captured_output, captured_stdout
 from test.support import TESTFN, unlink, cpython_only
 from test.script_helper import assert_python_ok
 import textwrap
 
 import traceback
 
 
 class SyntaxTracebackCases(unittest.TestCase):
@@ -471,14 +471,24 @@ class MiscTracebackCases(unittest.TestCa
         self.assertEqual(len(inner_frame.f_locals), 1)
 
         # Clear traceback frames
         traceback.clear_frames(tb)
 
         # Local variable dict should now be empty.
         self.assertEqual(len(inner_frame.f_locals), 0)
 
-
-def test_main():
-    run_unittest(__name__)
+    def test_constructors(self):
+        output = StringIO()
+        traceback.print_exc(file=output)
+        self.assertEqual(output.getvalue(), 'NoneType\n')
+        output = StringIO()
+        traceback.print_exception(None, None, None, file=output)
+        self.assertEqual(output.getvalue(), 'NoneType\n')
+        with captured_stdout() as output:
+            print(traceback.format_exc())
+        self.assertEqual(output.getvalue(), 'NoneType\n\n')
+        with captured_stdout() as output:
+            print(traceback.format_exception(None, None, None))
+        self.assertEqual(output.getvalue(), "['NoneType\\n']\n")
 
 if __name__ == "__main__":
-    test_main()
+    unittest.main()
diff --git a/Lib/traceback.py b/Lib/traceback.py
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -117,27 +117,27 @@ def extract_tb(tb, limit=None):
     "\nDuring handling of the above exception, "
     "another exception occurred:\n")
 
 def _iter_chain(exc, custom_tb=None, seen=None):
     if seen is None:
         seen = set()
     seen.add(exc)
     its = []
-    context = exc.__context__
-    cause = exc.__cause__
+    context = getattr(exc, '__context__', None)
+    cause = getattr(exc, '__cause__', None)
     if cause is not None and cause not in seen:
         its.append(_iter_chain(cause, False, seen))
         its.append([(_cause_message, None)])
     elif (context is not None and
-          not exc.__suppress_context__ and
+          not getattr(exc, '__suppress_context__', None) and
           context not in seen):
         its.append(_iter_chain(context, None, seen))
         its.append([(_context_message, None)])
-    its.append([(exc, custom_tb or exc.__traceback__)])
+    its.append([(exc, custom_tb or getattr(exc, '__traceback__', None))])
     # itertools.chain is in an extension module and may be unavailable
     for it in its:
         yield from it
 
 def _format_exception_iter(etype, value, tb, limit, chain):
     if chain:
         values = _iter_chain(value, tb)
     else: