How to use the pefile.retrieve_flags function in pefile

fobj += ' {:4}{:<16} {}\n'.format('', "Size", self.filesize)
        fobj += ' {:4}{:<16} {}\n\n'.format('', "First Bytes", self.getbytestring(0, 16))
        fobj += ' Hashes\n'
        fobj += ' {:4}{:<16} {}\n'.format('', "MD5", self.gethash('md5'))
        fobj += ' {:4}{:<16} {}\n'.format('', "SHA1", self.gethash('sha1'))
        fobj += ' {:4}{:<16} {}\n'.format('', "SHA256", self.gethash('sha256'))
        fobj += ' {:4}{:<16} {}\n'.format('', "Import Hash", self.getimphash())
        fobj += ' {:4}{:<16} {}\n\n'.format('', "ssdeep", self.getfuzzyhash())
        fobj += ' Headers\n'

        if self.pe is not None:
            for str_key in self.metadata:
                fobj += ' {:4}{:<16} {}\n'.format('', str_key, self.metadata[str_key])

            fobj += ' {:4}{:<16} {}\n'.format('', "PDB Path", self.get_pdb_path())
            iflags = pefile.retrieve_flags(pefile.IMAGE_CHARACTERISTICS, 'IMAGE_FILE_')

            fobj += ' {:4}{:<16} \n'.format('', "Characteristics")
            for flag in iflags:
                if getattr(self.pe.FILE_HEADER, flag[0]):
                    fobj += " {:20s} {:<20s}\n".format('', str(flag[0]))

        return fobj