def _flatten_results(result, default_value=None):
    """ Formats results to map to a single value or default value if empty. """
    for k in list(result):
        v = result[k]
        if len(v) > 1:
            raise ValueError(
                'Expected one result, but more returned for "{}": {}'.
                format(k, v))
        if len(v) == 1:
            result[k] = v[0]
        elif default_value is not None:
            result[k] = default_value
        else:
            del result[k]
    return result
