def test_recipientsFromHeaders(self):
        """
        The I{-t} flags specifies that recipients should be obtained
        from headers.
        """
        stdin = NativeStringIO(
            'To: Curly <invaliduser2@example.com>\n'
            'Cc: Larry <invaliduser1@example.com>\n'
            'Bcc: Moe <invaliduser3@example.com>\n'
            '\n'
            'Oh, a wise guy?\n')
        self.patch(sys, 'stdin', stdin)
        o = parseOptions("-t")
        self.assertEqual(len(o.to), 3)

        The I{-t} flags specifies that recipients should be obtained
        from headers.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_mailmail.py