def METHOD_NAME(self, app):
    def callback(img_src, service, layers, environ, query_extent):
        assert isinstance(img_src, ImageSource)
        assert "wmts" == service
        assert "wms_cache" == layers[0]
        assert isinstance(environ, dict)
        assert len(query_extent) == 2
        assert len(query_extent[1]) == 4
        assert query_extent[0] == "EPSG:900913"
        return img_src
    app.get(
        str(self.common_tile_req), extra_environ={"mapproxy.decorate_img": callback}
    )