comment and refactoring on optional directories

        Check if we are a valid SourceImage directories

        '''

        # setup and payload are the only needed dirs

        # setup are mandatory to do something

        # payload directory is needed because build script chroot into payload directory

        for d in (self.setup_path, self.payload_path):

            if not os.path.exists(d):

                raise ISError(u"Invalid source image: directory %s is missing" % d)

        # remove list

        rl = set()

        # run build script

        if os.path.exists(self.build_path):

            if script:

        if script and os.path.exists(self.build_path):

            rl |= set(self.run_build())

        if force_payload:

            rl |= set(self.select_payloads())

            tarball.add_str("format", self.format, tarfile.REGTYPE, 0644)

            # add setup scripts

            self.add_scripts(tarball, self.setup_path)

            # add optional scripts

            for d in (self.build_path, self.parser_path, self.lib_path):

                if os.path.exists(d):

                    # add scripts

                    self.add_scripts(tarball, d)

            # closing tarball file

            tarball.close()