def _parse_filter_string(filter_string):
    """ parse a filter string into a key-value pair """
    assert "=" in filter_string, "filter string requires an '=', got {0}".format(filter_string)
    split_values = filter_string.split('=')
    assert len(split_values) == 2, "more than one equals found in filter string {0}!".format(filter_string)
    return split_values