168 CHAPTER10. DEFININGCLASSES

AsI mentionedabove, inorderforthiscodetowork,ourconstructorwillhave toinitializeself.label
asanapproprateTextobjectandself.rectasaRectangleobject.Inaddition,theself.active
instancevariablestoresa Booleanvalue(1fortrue,0 forfalse)torememberwhetherornotthebuttonis
currentlyactive.
Ourdeactivatemethodwilldotheinverseofactivate. It lookslike this:

def deactivate(self):

"Sets thisbutton to ’inactive’."

self.label.setFill(’darkgrey’)

self.rect.setWidth(1)

self.active= 0

Ofcourse,themainpointofa buttonis beingabletodetermineif it hasbeenclicked.Let’s trytowrite
theclickedmethod.Asyouknow, thegraphicspackageprovidesagetMousemethodthatreturns
thepointwherethemousewasclicked.If anapplicationneedstogeta buttonclick,it willfirsthave tocall
getMouseandthenseewhichactive button(ifany)thepointis insideof.We couldimaginethebutton
processingcodelookingsomethinglike thefollowing:

pt = win.getMouse()
if button1.clicked(pt):

Do button1 stuff

elif button2.clicked(pt):

Do button2 stuff

elif button2.clicked(pt)

Do button3 stuff

...

Themainjoboftheclickedmethodis todeterminewhethera givenpointis insidetherectangular
button.Thepointis insidetherectangleif itsxandycoordinatesliebetweentheextremexandyvalues
oftherectangle. Thiswouldbeeasiesttofigureoutif wejustassumethatthebuttonobjecthasinstance
variablesthatrecordtheminandmaxvaluesofxandy.
Assumingtheexistenceofinstancevariablesxmin,xmax,ymin, andymax, wecanimplementthe
clickedmethodwitha singleBooleanexpression.

def clicked(self,p):

"RETURNS trueif button is active and p is inside"

return self.activeand \

self.xmin<= p.getX() <= self.xmax and \

self.ymin<= p.getY() <= self.ymax

Herewehave a singlelargeBooleanexpressioncomposedbyandingtogetherthreesimplerexpressions;all
threemustbetrueforthefunctiontoreturna truevalue.Recallthatthebackslashat theendofa lineis used
toextenda statementovermultiplelines.
Thefirstofthethreesubexpressionssimplyretrievesthevalueoftheinstancevariableself.active.
Thisensuresthatonlyactive buttonswillreportthatthey have beenclicked.Ifself.activeis false,then
clickedwillreturnfalse.Thesecondtwo subexpressionsarecompoundconditionstocheckthatthexand
yvaluesofthepointfallbetweentheedgesofthebuttonrectangle.(Remember,x <= y <= zmeansthe
sameasthemathematicalexpressionx y z(section7.5.1)).
Now thatwehave thebasicoperationsofthebuttonironedout,wejustneeda constructortogetallthe
instancevariablesproperlyinitialized.It’s nothard,butit is a bittedious.Hereis thecompleteclasswitha
suitableconstructor.

button.py

from graphics import*

class Button: