In method cook$(),$ ApplePie.num$\_$pies is used to reference the class attribute num$\_$pies, and is incremented to record the number of ApplePie instances created. Using self.num$\_$pies to modify class attribute num$\_$pies would only create an instance attribute with the same name for each instance.

The instance attribute num$\_$apples is referenced using self.num$\_$apples.