Think Python - Denison University

176 Chapter 18. InheritanceThey are also highly detailed; for some purposes, too detailed. A class diagram is a more abstractrepresentation of the structure of a program. Instead of showing individual objects, it shows classesand the relationships between them.There areseveral kinds of relationship between classes:• Objects in one class might contain references to objects in another class. For example, eachRectangle contains a reference to a Point, and each Deck contains references to many Cards.This kind of relationship iscalled HAS-A, as in, “aRectangle has aPoint.”• Oneclassmightinheritfromanother. ThisrelationshipiscalledIS-A,asin,“aHandisakindof aDeck.”• One class might depend on another in the sense that changes in one class would requirechanges intheother.A class diagram is a graphical representation of these relationships 2 . For example, this diagramshows the relationships betweenCard,DeckandHand.Deck*CardHandThe arrow with a hollow triangle head represents an IS-A relationship; in this case it indicates thatHand inherits from Deck.ThestandardarrowheadrepresentsaHAS-Arelationship;inthiscaseaDeckhasreferencestoCardobjects.The star (*) near the arrow head is a multiplicity; it indicates how many Cards a Deck has. Amultiplicity can be a simple number, like 52, a range, like 5..7 or a star, which indicates that aDeck can have any number ofCards.AmoredetaileddiagrammightshowthataDeckactuallycontainsalistofCards,butbuilt-intypeslikelistand dict areusually not included inclass diagrams.Exercise18.4 ReadTurtleWorld.py,World.pyandGui.pyanddrawaclassdiagramthatshowstherelationships among theclasses defined there.18.9 DebuggingInheritance can make debugging a challenge because when you invoke a method on an object, youmight not know which method willbe invoked.Suppose you are writing a function that works with Hand objects. You would like it to work withall kinds of Hands, like PokerHands, BridgeHands, etc. If you invoke a method like shuffle, you2 The diagrams I am using here are similar to UML (see wikipedia.org/wiki/Unified_Modeling_Language),with afew simplifications.