def test_createWithSingleMessageNumber(self):
        """
        Creating a L{MessageSet} with a single message number adds
        only that message to the L{MessageSet}; its serialized form
        includes only that message number, its length is one, and it
        yields only that message number.
        """
        m = MessageSet(1)
        self.assertEqual(str(m), "1")
        self.assertEqual(len(m), 1)
        self.assertEqual(list(m), [1])

        Creating a L{MessageSet} with a single message number adds
        only that message to the L{MessageSet}; its serialized form
        includes only that message number, its length is one, and it
        yields only that message number.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py