Updates the current document's expiration value.

        :param expiration: Expiration in seconds for the document to be removed by
            couchbase server, defaults to 0 - will never expire.
        :type expiration: int
        :returns: Response from CouchbaseClient.
        :rtype: unicode
        :raises: :exc:'cbwrapper.errors.DoesNotExist' or
            :exc:'couchbase.exception.TemporaryFailError'

def touch(self, expiration):
        """Updates the current document's expiration value.

        :param expiration: Expiration in seconds for the document to be removed by
            couchbase server, defaults to 0 - will never expire.
        :type expiration: int
        :returns: Response from CouchbaseClient.
        :rtype: unicode
        :raises: :exc:'cbwrapper.errors.DoesNotExist' or
            :exc:'couchbase.exception.TemporaryFailError'
        """
        if not self.cas_value or not self.doc_id:
            raise self.DoesNotExist(self)
        return self.bucket.touch(self.doc_id, expiration)