def test_overrideFromFlagByFromHeader(self):
        """
        The I{-F} flag specifies the From: value.  However, I{-F} flag is
        overriden by the value of From: in the e-mail header.
        """
        stdin = NativeStringIO(
            'To: Curly <invaliduser4@example.com>\n'
            'From: Shemp <invaliduser4@example.com>\n')
        self.patch(sys, 'stdin', stdin)
        o = parseOptions(["-F", "Groucho <invaliduser5@example.com>", "-t"])
        self.assertEqual(o.sender, "invaliduser4@example.com")

        The I{-F} flag specifies the From: value.  However, I{-F} flag is
        overriden by the value of From: in the e-mail header.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_mailmail.py