def test_singlePartWithMissing(self):
        """
        For fields with no information contained in the message headers,
        L{imap4.getBodyStructure} fills in L{None} values in its result.
        """
        major = 'image'
        minor = 'jpeg'
        body = b'hello, world'
        msg = FakeyMessage({
                'content-type': major + '/' + minor
                }, (), b'', body, 123, None)
        structure = imap4.getBodyStructure(msg, extended=True)
        self.assertEqual(
            [major, minor, None, None, None, None, len(body), None, None,
             None, None],
            structure)

        For fields with no information contained in the message headers,
        L{imap4.getBodyStructure} fills in L{None} values in its result.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py