def _basicFields(self):
        """
        Return a list of the basic fields for a single-part message.
        """
        headers = self.message.getHeaders(False, *self._HEADERS)

        # Number of octets total
        size = self.message.getSize()

        major, minor = self.main, self.subtype

        # content-type parameter list
        unquotedAttrs = self._unquotedAttrs()

        return [
            major, minor, unquotedAttrs,
            headers.get('content-id'),
            headers.get('content-description'),
            headers.get('content-transfer-encoding'),
            size,
            ]

        Return a list of the basic fields for a single-part message.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/imap4.py