Python Programming: An Introduction to Computer Science

5.3.USINGGRAPHICALOBJECTS 67

Circle

y:

x:

100

100

center: Point

radius:

draw( )

.

.

.

GraphWin

.

.

.

Low-level drawing commands

win:

30

circ:

Figure5.5:Objectinteractionstodraw a circle.

Incorrect way to createtwo circles.

leftEye = Circle(Point(80,50), 5)
leftEye.setFill(’yellow’)
leftEye.setOutline(’red’)
rightEye = leftEye
rightEye.move(20,0)

Thebasicideais tocreatethelefteyeandthencopy thatintoa righteyewhichis thenmovedover 20units.
Thisdoesn’t work.Theproblemhereis thatonlyoneCircleobjectis created.Theassignment

rightEye = leftEye

simplymakesrightEyerefertotheverysamecircleasleftEye. Figure5.6showsthesituation.When
theCircleis movedinthelastlineofcode,bothrightEyeandleftEyerefertoit initsnew location
ontherightside.Thissituationwheretwo variablesrefertothesameobjectis calledaliasing, andit can
sometimesproduceratherunexpectedresults.

rightEye:

leftEye: Circle

y:

x:

50

80

center: Point

radius: 10

Figure5.6:VariablesleftEyeandrightEyearealiases.

Onesolutiontothisproblemwouldbetocreatea separatecircleforeacheye.

A correct way to createtwo circles.

leftEye = Circle(Point(80,50), 5)
leftEye.setFill(’yellow’)