Think Python - Denison University

174 Chapter 18. InheritanceIn this case we are defining a “thin” method that expresses a list operation in terms that are appropriatefordecks.Asanotherexample,wecanwriteaDeckmethodnamedshuffleusingthefunctionshufflefromtherandommodule:# inside class Deck:def shuffle(self):random.shuffle(self.cards)Don’t forget toimportrandom.Exercise 18.2 Write a Deck method namedsortthat uses the list methodsortto sort the cards inaDeck. sortuses the__cmp__method wedefined todetermine sortorder.18.7 InheritanceThe language feature most often associated with object-oriented programming is inheritance. Inheritanceistheabilitytodefine a new classthat isamodified version of anexisting class.Itiscalled“inheritance”becausethenewclassinheritsthemethodsoftheexistingclass. Extendingthismetaphor, the existing classiscalled the parent and thenew classiscalled the child.As an example, let’s say we want a class to represent a “hand,” that is, the set of cards held by oneplayer. A hand is similar to a deck: both are made up of a set of cards, and both require operationslikeadding and removing cards.A hand is also different from a deck; there are operations we want for hands that don’t make sensefor a deck. For example, in poker we might compare two hands to see which one wins. In bridge,wemight compute ascorefor ahand inorder tomake a bid.This relationship between classes—similar,but different—lends itselftoinheritance.Thedefinitionofachildclassislikeotherclassdefinitions,butthenameoftheparentclassappearsinparentheses:class Hand(Deck):"""represents a hand of playing cards"""ThisdefinitionindicatesthatHandinheritsfromDeck;thatmeanswecanusemethodslikepop_cardandadd_cardfor Hands as wellas Decks.Handalsoinherits__init__fromDeck,butitdoesn’treallydowhatwewant: insteadofpopulatingthehand with52 new cards, theinit method for Hands should initializecardswithan empty list.Ifweprovide an initmethod intheHandclass, itoverrides the one intheDeckclass:# inside class Hand:def __init__(self, label=''):self.cards = []self.label = label