<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_backup(self, target_file='unifi-backup.unf'):
 """Get a backup archive from a controller. Arguments: target_file -- Filename or full path to download the backup archive to, should have .unf extension for restore. """

        download_path = self.create_backup()

        opener = self.opener.open(self.url + download_path)
        unifi_archive = opener.read()

        backupfile = open(target_file, 'w')
        backupfile.write(unifi_archive)
        backupfile.close()