we usually call classes higher in the tree (like and superclassesclasses lower in the tree (like are known as subclasses these terms refer to both relative tree positions and roles superclasses provide behavior shared by all their subclassesbut because the search proceeds from the bottom upsubclasses may override behavior defined in their superclasses by redefining superclass names lower in the tree as these last few words are really the crux of the matter of software customization in ooplet' expand on this concept suppose we build up the tree in figure - and then say thisi right awaythis code invokes inheritance because this is an object attribute expressionit triggers search of the tree in figure - --python will search for the attribute by looking in and above specificallyit will search the linked objects in this orderi and stop at the first attached it finds (or raise an error if isn' found at allin this casew won' be found until is searched because it appears only in that object in other wordsi resolves to by virtue of the automatic search in oop terminologyi "inheritsthe attribute from ultimatelythe two instances inherit four attributes from their classeswxyand other attribute references will wind up following different paths in the tree for examplei and both find in and stop because is lower than and both find in because that' the only place appears and both find in because is further to the left than name finds name in without climbing the tree at all trace these searches through the tree in figure - to get feel for how inheritance searches work in python the first item in the preceding list is perhaps the most important to notice--because redefines the attribute lower in the treeit effectively replaces the version above it in as you'll see in momentsuch redefinitions are at the heart of software customization in oop--by redefining and replacing the attributec effectively customizes what it inherits from its superclasses in other literature and circlesyou may also occasionally see the terms base classes and derived classes used to describe superclasses and subclassesrespectively python people and this book tend to use the latter terms oop from , feet