<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create_tmp_rootdir(self):
 """ create a rootdir that we can destroy later again Returns ------- """

        if self.rootdir:
            tmp_rootdir = tempfile.mkdtemp(prefix='bcolz-')
            self._dir_clean_list.append(tmp_rootdir)
        else:
            tmp_rootdir = None
        return tmp_rootdir