def make_slice_key(cls, start_string, size_string):
        """
        Converts the given start and size query parts to a slice key.

        :return: slice key
        :rtype: slice
        """
        try:
            start = int(start_string)
        except ValueError:
            raise ValueError('Query parameter "start" must be a number.')
        if start < 0:
            raise ValueError('Query parameter "start" must be zero or '
                             'a positive number.')
        try:
            size = int(size_string)
        except ValueError:
            raise ValueError('Query parameter "size" must be a number.')
        if size < 1:
            raise ValueError('Query parameter "size" must be a positive '
                             'number.')
        return slice(start, start + size)
Converts the given start and size query parts to a slice key.

        :return: slice key
        :rtype: slice