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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def document_partition_url(self, partition_key):
 """ Retrieve the design document partition URL. :param str partition_key: Partition key. :return: Design document partition URL. :rtype: str """

        return '/'.join((
            self._database.database_partition_url(partition_key),
            '_design',
            url_quote_plus(self['_id'][8:], safe='')
        ))