def need(self, folder, page=None, perpage=None):
        """ Returns lists of files which are needed by this device in order
        for it to become in sync.

            Args:
                folder (str):
                page (int): If defined applies pagination accross the
                    collection of results.
                perpage (int): If defined applies pagination across the
                    collection of results.

            Returns:
                dict
        """
        assert isinstance(page, int) or page is None
        assert isinstance(perpage, int) or perpage is None
        return self.get('need', params={'folder': folder,
                                 'page': page,
                                 'perpage': perpage})