10.5 Polymorphism •  QuestionDemo2passedtwoChoiceQuestionobjectstothepresentQuestion()method•  CanwewriteapresentQuestion()methodthatdisplaysbothQuestionandChoiceQuestiontypes?

•  Withinheritance,thisgoalisveryeasytorealize!•  Inordertopresentaques9ontotheuser,weneednotknowtheexacttypeoftheques9on.

•  Wejustdisplaytheques9onandcheckwhethertheusersuppliedthecorrectanswer.

defpresentQuestion(q):q.display()response=input("Youranswer:")print(q.checkAnswer(response))

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Which Display() method was called? •  presentQuestion()simplycallsthedisplay()methodofwhatevertypeispassed:

defpresentQuestion(q):q.display()...

!  IfpassedanobjectoftheQuestionclass:"  Questiondisplay()

!  IfpassedanobjectoftheChoiceQuestionclass:"  ChoiceQuestiondisplay()

display()

__

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•  Thevariableqdoesnotknowthetypeofobjecttowhichitrefers:

Why Does This Work? •  AsdiscussedinSec9on10.1,wecansubs9tuteasubclassobjectwheneverasuperclassobjectisexpected:

second=ChoiceQuestion()presentQuestion(second)#OKtopassaChoiceQuestion

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•  Notehoweveryoucannotsubs9tuteasuperclassobjectwhenasubclassobjectisexpected.•  AnAttributeErrorexcep9onwillberaised.•  Theparentclasshasfewercapabili9esthanthechildclass(youcannotinvokeamethodonanobjectthathasnotbeendefinedbythatobject’sclass).

Polymorphism Benefits •  InPython,methodcallsarealwaysdeterminedbythetypeoftheactualobject,notthetypeofthevariablecontainingtheobjectreference•  Thisiscalleddynamicmethodlookup•  Dynamicmethodlookupallowsustotreatobjectsofdifferentclassesinauniformway

•  Thisfeatureiscalledpolymorphism

•  Weaskmul9pleobjectstocarryoutatask,andeachobjectdoessoinitsownway

•  Polymorphismmakesprogramseasilyextensible

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Questiondemo3.py (1)

Creates an object of the Question class

Creates an object of the ChoiceQuestion class, uses new addChoice() method.

Calls presentQuestion()- next page - passed both types of objects.

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Questiondemo3.py (2)

Receives a parameter of the super-class type

Uses appropriate display method.

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Special Topic 10.2

•  willreturnTrueifqisaninstanceoftheQuestionclassorofany

subclassthatextendstheQuestionclass,•  otherwise,itreturnsFalse.

isinstance(q,Question)

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•  SubclassesandInstances:•  Youlearnedthattheisinstance()func9oncanbeusedtodetermineifanobjectisaninstanceofaspecificclass.

•  Buttheisinstance()func9oncanalsobeusedtodetermineifanobjectisaninstanceofasubclass.

•  Forexample,thefunc9oncall:

Use of Isinstance()!  Acommonuseoftheisinstance()func9onistoverifythattheargumentspassedtoafunc9onormethodareofthecorrecttype.

defpresentQuestion(q):ifnotisintance(q,Question):raiseTypeError("TheargumentisnotaQuestionoroneofitssubclasses.")

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Special Topic 10.3 •  DynamicMethodLookup

•  SupposewemovethepresentQuestion()methodtoinsidetheQuestionclassandcallitasfollows:

cq=ChoiceQuestion()cq.setText("InwhichcountrywastheinventorofPythonborn?")...cq.presentQuestion()

defpresentQuestion(self):self.display()response=input("Your answer: ")print(self.checkAnswer(response))

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•  Whichdisplay()andcheckAnswer()methodswillbecalled?

Dynamic Method Lookup •  IfyoulookatthecodeofthepresentQuestion()method,youcanseethatthesemethodsareexecutedontheselfreferenceparameter.•  Becauseofdynamicmethodlookup,theChoiceQuestionversionsofthedisplay()andcheckAnswer()methodsarecalledautoma9cally.

•  ThishappenseventhoughthepresentQuestion()methodisdeclaredintheQuestionclass,whichhasnoknowledgeoftheChoiceQuestionclass.

classQuestion:defpresentQuestion(self):self.display()response=input("Youranswer:")print(self.checkAnswer(response))

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Special Topic 10.4 •  AbstractClassesandmethods

•  Ifitisdesirabletoforcesubclassestooverrideamethodofabaseclass,youcandeclareamethodasabstract.•  Youcannotinstan9ateanobjectthathasabstractmethods

•  Thereforetheclassisconsideredabstract(ithas1+abstractmethods)

•  It’satooltoforceprogrammerstocreatesubclasses(avoidsthetroubleofcomingupwithuselessdefaultmethodsthatothersmightinheritbyaccident).

•  InPython,thereisnoexplicitwaytospecifythatamethodisanabstractmethod.Instead,thecommonprac9ceamongPythonprogrammersistohavethemethodraiseaNotImplementedErrorexcep9onasitsonlystatement:

classAccount:...defdeductFees(self):raiseNotImplementedError

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Common Error 10.3 •  Don’tUseTypeTests

•  Someprogrammersusespecifictypetestsinordertoimplementbehaviorthatvarieswitheachclass:

ifisinstance(q,ChoiceQuestion):#Don’tdothis.#DothetasktheChoiceQuestionway.elifisinstance(q,Question):#DothetasktheQuestionway.

elifisinstance(q,NumericQuestion):#DothetasktheNumericQuestionway.

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•  Thisisapoorstrategy.•  IfanewclasssuchasNumericQuestionisadded,thenyouneedtoreviseallpartsofyourprogramthatmakeatypetest,addinganothercase:

Alternate to Type Tests •  Polymorphism

•  Wheneveryoufindyourselftryingtousetypetestsinahierarchyofclasses,reconsiderandusepolymorphisminstead.

•  DeclareamethoddoTheTask()inthesuperclass,overrideitinthesubclasses,andcall

q.doTheTask()

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Steps to Using Inheritance •  Asanexample,wewillconsiderabankthatofferscustomersthefollowingaccounttypes:1) A savings account that earns interest. The interest compounds monthly and is

based on the minimum monthly balance. 2) A checking account that has no interest, gives you three free withdrawals per

month, and charges a $1 transaction fee for each additional withdrawal. •  Theprogramwillmanageasetofaccountsofbothtypes

•  Itshouldbestructuredsothatotheraccounttypescanbeaddedwithoutaffec9ngthemainprocessingloop.

•  Themenu:D)epositW)ithdrawM)onthendQ)uit•  Fordepositsandwithdrawals,querytheaccountnumberandamount.Printthebalanceoftheaccounta`ereachtransac9on.

•  Inthe“Monthend”command,accumulateinterestorclearthetransac9oncounter,dependingonthetypeofthebankaccount.Thenprintthebalanceofallaccounts.

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Steps to Using Inheritance 1)Listtheclassesthatarepartofthehierarchy.SavingsAccountCheckingAccount2)Organizetheclassesintoaninheritance.hierarchyBaseonsuperclassBankAccount

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3)Determinethecommonresponsibili9es.a.WritePseudocodeforeachtaskb.Findcommontasks

Using Inheritance: Pseudocode

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Steps to Using Inheritance 4)Decidewhichmethodsareoverriddeninsubclasses.

•  Foreachsubclassandeachofthecommonresponsibili9es,decidewhetherthebehaviorcanbeinheritedorwhetheritneedstobeoverridden

5)Declarethepublicinterfaceofeachsubclass.•  Typically,subclasseshaveresponsibili9esotherthanthoseofthesuperclass.Listthose,aswellasthemethodsthatneedtobeoverridden.

•  Youalsoneedtospecifyhowtheobjectsofthesubclassesshouldbeconstructed.

6)Iden9fyinstancevariables.•  Listtheinstancevariablesforeachclass.Placeinstancevariablesthatarecommontoallclassesinthebaseofthehierarchy.

7)Implementconstructorsandmethods.8)Constructobjectsofdifferentsubclassesandprocessthem.

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Eye Break

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10.6 Application •  Crea9ngageometricshapeclasshierarchy

•  TocreatecomplexscenesbeyondthesimplegraphicsintroducedinChapter2,youmayneedalargenumberofshapesthatvaryincolor,size,orloca9on.

•  Ratherthancallingthevariousmethodsagainandagain,itwouldbeusefultohaveclassesthatmodelthevariousgeometricshapes.

•  Usingshapeclasses,aprogrammercancreateashapeobjectwithspecificcharacteris9cs,thenusethesameobjecttodrawmul9pleinstancesoftheshapewithonlyminorchanges.

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Inheritance Diagram of Geometric Shapes

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The Base Class •  TheGeometricShapeclassshouldprovidethefunc9onalitythatiscommonamongthevarioussubclasses:•  Sedngthecolorsusedtodrawtheshape.•  Gedngandsedngthecoordinatesfortheupper-le`cornerofaboundingbox.

•  Compu9ngthewidthandheightoftheshape(ortheboundingboxusedtodefinetheshape).

•  Drawingtheshapeonacanvas.

•  Duetotheamountofvaria9onbetweenshapesallsubclasseswillhavetooverridethedraw()method.

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Attributes of the Base Class •  Theinstancevariables_filland_outlinecanstorethefillandoutlinecolorsusedbytheshapes.

•  Coordinatesofthetople`handcorneroftheboundingboxfortheshapecanbestoredintheinstancevariables_xand_y

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Setting the Attributes •  TheconstructoroftheGeometricShapebaseclassneedstodefinethecommoninstancevariables.

classGeometricShape:##Constructabasicgeometricshape.#@paramxthex-coordinateoftheshape#@paramythey-coordinateoftheshapedef__init__(self,x,y):self._x=xself._y=yself._fill=Noneself._outline="black"

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Accessor Methods •  Asexpectedtheywillreturnthevaluesstoredintheinstancevariables.

defgetX(self):returnself._xdefgetY(self):returnself._y

defgetWidth(self):return0defgetHeight(self):return0

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•  BecausethegetWidth()andgetHeight()methodsreturnzerotheyshouldbeoverriddenbysubclasses.

Mutator Methods •  Wedefinethreemutatormethodsforsedngthecolors.

•  Twomethodssettheoutlineorfillcolorindividually,andthethirdmethodsetsbothtothesamecolor:

defsetFill(self,color=None):self._fill=colordefsetOutline(self,color=None):self._outline=colordefsetColor(self,color):self._fill=colorself._outline=color

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Other Methods •  Themove()methodmovestheshapebyagivenamount(x,y)coordinates.

defmoveBy(self,dx,dy):self._x=self._x+dxself._y=self._y+dy

defdraw(self,canvas):canvas.setFill(self._fill)canvas.setOutline(self._outline)

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•  Asindicatedearlier,thedraw()methodhastobeoverriddenforeachsubclass’sspecificshapebutthecommonopera9on(sedngthedrawingcolors)isincludedhere.

The Rectangle Class (1) •  TheRectangleclassinheritsfromGeometricShape.

•  Theconstructorpassestheupper-le`cornertothesuperclassandstoresthewidthandheight.

def__init__(self,x,y,width,height):super().__init__(x,y)self._width=widthself._height=height

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The Rectangle Class (2) •  ThedrawmethodisoverriddenintheRectanglesubclasstoincludethecalltotheappropriatecanvasmethod.

•  Accessorsareoverridden(fromtheonesthatreturnzero).

defdraw(self,canvas):super().draw(canvas)#Parentmethodsetscolorscanvas.drawRect(self.getX(),self.getY(),self._width,self._height)

defgetWidth(self):returnself._widthdefgetHeight(self):returnself._height

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The Line Class (1) •  Alineisspecifiedbyitsstartandendpoints.

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•  Itispossiblethatneitherofthesepointsistheupper-le`corneroftheboundingbox.

•  Instead,weneedtocomputethesmallerofthex-andy-coordinatesandpassthosevaluestothesuperclassconstructor.

The Line Class (2) •  Wealsoneedtostorethestartandendpointsininstancevariablesbecauseweneedthemtodrawtheline.

def__init__(self,x1,y1,x2,y2):super().__init__(min(x1,x2),min(y1,y2))self._startX=x1self._startY=y1self._endX=x2self._endY=y2

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Line Class: Methods (1) •  Thewidthandheightarethedifferencesbetweenthestar9ngandendingx-andy-coordinates.

•  However,ifthelineisn’tslopingdownward,weneedtotaketheabsolutevaluesofthedifference.

defgetWidth(self):returnabs(self._endX-self._startX)defgetHeight(self):returnabs(self._endY-self._startY)

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Line Class: Methods (2) •  Asnotedthedraw()methodmustbeoverriddendefdraw(self,canvas):super().draw(canvas)canvas.drawLine(self._startX,self._startY,self._endX,self._endY)

defmoveBy(self,dx,dy):super().moveBy(dx,dy)self._startX=self._startX+dxself._startY=self._startY+dyself._endX=self._endX+dxself._endY=self._endY+dy

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•  AlsothemoveBy()methodmustbeoverriddensothatitadjuststhestar9ngandendingpoints,inaddi9ontothetop-le`corner.

Wrapper Class: Square•  Awrapperclasswrapsorencapsulatesthefunc9onalityofanotherclasstoprovideamoreconvenientinterface.

•  Forexample,wecoulddrawasquareusingtheRectanglesubclass.•  Butitrequiresthatwesupplyboththewidthandheight.•  Becauseasquareisaspecialcaseofarectangle,wecandefineaSquaresubclassthatextends,orwraps,theRectangleclassandonlyrequiresonevalue,thelengthofaside.

classSquare(Rectangle):def__init__(self,x,y,size):super().__init__(x,y,size,size)

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Testshapes.py

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Groups of Shapes •  TheGroupsubclassdoesnotactuallydrawageometricshape.

•  Insteaditcanbeusedtogroupbasicgeometricshapestocreateacomplexshape.•  Forexample,supposeyouconstructadoorusingarectangle,acircleforthedoorknob,andacircleforthepeephole.

•  ThethreecomponentscanbestoredinaGroupinwhichtheindividualshapesaredefinedrela9vetotheposi9onofthegroup.

•  Thisallowstheen9regrouptobemovedtoadifferentposi9onwithouthavingtomoveeachindividualshape.

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The Group Class •  TocreateaGroup,youprovidethecoordinatesoftheupper-le`cornerofitsboundingbox.

•  Theclassdefinesaninstancevariablethatstorestheshapesinalist.•  AsnewshapesareaddedtoaGroupobject,thewidthandheightoftheboundingboxexpandstoenclosethenewshapes.

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Group Class Methods (1) •  Createthegroupwithitsboundingboxposi9onedat(x,y).

def__init__(self,x=0,y=0):super().__init__(x,y)self._shapeList=[]

defadd(self,shape):self._shapeList.append(shape)

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•  Addingashapetothegroupinvolvesseveralsteps.•  First,theshapehastobeappendedtothelist:

Group Class Methods (2) •  Theindividualshapesareposi9onedrela9vetotheupper-le`cornerofthegroup’sboundingbox.

•  Wemustensurethateachshapeisposi9onedbelowandtotherightofthispoint.Ifitisnot,itmustbemoved.

#Keeptheshapewithintopandleftedgesofthe#boundingbox.ifshape.getX()<0:shape.moveBy(-shape.getX(),0)ifshape.getY()<0:shape.moveBy(0,-shape.getY())

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Group Class Methods (3) •  Thewidthofthegroupisdeterminedbytherightmostextentofanyofthegroup’smembers.

•  Therightmostextentofashapeisshape.getX()+shape.getWidth().Thefollowingmethodcomputesthemaximumoftheseextents.

defgetWidth(self):width=0forshapeinself._shapeList:width=max(width,shape.getX()+shape.getWidth())returnwidth

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Group Class Methods (4) •  Theheightofthegroup(thebohommostextent)iscomputedinthesamewayasthewidth.

defgetHeight(self):height=0forshapeinself._shapeList:height=max(height,shape.getY()+shape.getHeight())returnheight

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Group Class Methods (5) •  Theen9regroupcanbedrawnonthecanvas.•  Theshapescontainedinthegrouparedefinedrela9vetotheupper-le`cornerofitsboundingbox.

•  Beforeashapecanbedrawn,ithastobemovedtoitsposi9onrela9vetotheupper-le`cornerofthegroup’sboundingbox.

defdraw(self,canvas):forshapeinself._shapeList:shape.moveBy(self.getX(),self.getY())shape.draw(canvas)shape.moveBy(-self.getX(),-self.getY())

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Summary: Inheritance •  Asubclassinheritsdataandbehaviorfromasuperclass.•  Youcanalwaysuseasubclassobjectinplaceofasuperclassobject.•  Asubclassinheritsallmethodsthatitdoesnotoverride.•  Asubclasscanoverrideasuperclassmethodbyprovidinganewimplementa9on.

•  InPythonaclassnameinsideparenthesesintheclassheaderindicatesthataclassinheritsfromasuperclass.

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Summary: Overriding Methods

•  Anoverridingmethodcanextendorreplacethefunc9onalityofthesuperclassmethod.

•  Usethereservedwordsupertocallasuperclassmethod.•  Tocallasuperclassconstructor,usethesuperreservedwordbeforethesubclassdefinesitsowninstancevariables.

•  Theconstructorofasubclasscanpassargumentstoasuperclassconstructor,usingthereservedwordsuper.

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Summary: Polymorphism •  Asubclassreferencecanbeusedwhenasuperclassreferenceisexpected.•  Polymorphism(“havingmul9pleshapes”)allowsustomanipulateobjectsthatshareasetoftasks,eventhoughthetasksareexecutedindifferentways.

•  Anabstractmethodisamethodwhoseimplementa9onisnotspecified.

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Summary: Use Inheritance for Designing a Hierarchy of Shapes

•  TheGeometricShapeclassprovidesmethodsthatarecommontoallshapes.

•  •EachsubclassofGeometricShapemustoverridethedraw()method.

•  Ashapeclassconstructormustini9alizethecoordinatesofitsupper-le`corner.

•  Eachshapesubclassmustoverridethemethodsforcompu9ngthewidthandheight.

•  AGroupcontainsshapesthataredrawnandmovedtogether.

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