def test_lastOverridesNoneInAdd(self):
        """
        Adding a L{None}, representing C{*}, or a sequence that
        includes L{None} to a L{MessageSet} whose
        L{last<MessageSet.last>} property has been set replaces all
        occurrences of L{None} with the value of
        L{last<MessageSet.last>}.
        """
        hasLast = MessageSet(1)
        hasLast.last = 4

        hasLast.add(None)
        self.assertEqual(list(hasLast), [1, 4])

        self.assertEqual(list(hasLast + (None, 5)), [1, 4, 5])

        hasLast.add(3, None)
        self.assertEqual(list(hasLast), [1, 3, 4])

        Adding a L{None}, representing C{*}, or a sequence that
        includes L{None} to a L{MessageSet} whose
        L{last<MessageSet.last>} property has been set replaces all
        occurrences of L{None} with the value of
        L{last<MessageSet.last>}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py