I now see you've done a similar thing with with regard to ```_find_images``` (but the other way round). It might be clearer if you defined ```find_images()``` such that it explicitly calls the private method but your current approach is very succinct. As for ```_tileextent = tileextent```, I'm still cautious as it's the other way around. On closer inspection I'm a little suspicious of the all the calls to private methods. In fact I suspect the use of inheritance here might be causing more harm than good as you are going out of your way to prevent methods of the subclass being called. I'd be tempted to look at making QuadTreeTiles no longer a subclass of GoogleTiles. If you need functionality from a GoogleTiles instance, consider containment, i.e. instantiate one as a private instance variable within QuadTreeTiles.