def items_to_extract(self, offset=0, length=None):
        """
        Return an iterable of specific items to extract.
        As a side-effect, set self.items_to_extract_length.

        :param offset: where to start extracting
        :param length: how many to extract
        :return: An iterable of the specific
        """
        endoffset = length and offset + length
        qs = self.origin_data()[offset:endoffset]
        self.items_to_extract_length = qs.count()
        return qs
Return an iterable of specific items to extract.
        As a side-effect, set self.items_to_extract_length.

        :param offset: where to start extracting
        :param length: how many to extract
        :return: An iterable of the specific