def _format_bounding_box(bbox, output_format="%(lat1)s,%(lon1)s,%(lat2)s,%(lon2)s"):
        """
        Transform bounding box boundaries to a string matching
        `output_format` from the following formats:

            - [Point(lat1, lon1), Point(lat2, lon2)]
            - [[lat1, lon1], [lat2, lon2]]
            - ["lat1,lon1", "lat2,lon2"]

        It is guaranteed that lat1 <= lat2 and lon1 <= lon2.
        """
        if len(bbox) != 2:
            raise GeocoderQueryError("Unsupported format for a bounding box")
        p1, p2 = bbox
        p1, p2 = Point(p1), Point(p2)
        return output_format % dict(lat1=min(p1.latitude, p2.latitude),
                                    lon1=min(p1.longitude, p2.longitude),
                                    lat2=max(p1.latitude, p2.latitude),
                                    lon2=max(p1.longitude, p2.longitude))