'Parse is_public into something usable.'

def _parse_is_public(self, is_public):

    if (is_public is None):
        return True
    elif utils.is_none_string(is_public):
        return None
    else:
        try:
            return strutils.bool_from_string(is_public, strict=True)
        except ValueError:
            msg = (_('Invalid    is_public    filter    [%s]') % is_public)
            raise webob.exc.HTTPBadRequest(explanation=msg)
