11.5. CASESTUDY: PYTHONCALCULATOR 191

Theclearkey blanksthedisplay.

self.display.setText("")

Thebackspacestripsoff onecharacter.

self.display.setText(text[:-1])

Finally, theequalkey causestheexpressioninthedisplaytobeevaluatedandtheresultdisplayed.

try:

result = eval(text)

except:

result = ’ERROR’

self.display.setText(str(result))

Thetry-excepthereisnecessarytocatchrun-timeerrorscausedbyentriesthatarenotlegalPython
expressions.If anerroroccurs,thecalculatorwilldisplayERRORratherthancausingtheprogramtocrash.
Hereis thecompleteprogram.

calc.pyw -- A fourfunction calculator using Python arithmetic.

from graphics import*
from buttons importButton

class Calculator:

This class implementsa simple calculator GUI

def __init__(self):

# create the windowfor the calculator

win = GraphWin("calculator")

win.setCoords(0,0,6,7)

win.setBackground("slategray")

self.win = win

# Now createthe widgets

self.__createButtons()

self.__createDisplay()

def __createButtons(self):

# create listof buttons

# buttonSpecgives center, width and label of a button

buttonSpecs= [(2,1,.75,’0’), (3,1,.75,’.’), (4.5,1,2,’=’),

(1,2,.75,’1’),(1,3,.75,’4’), (1,4,.75,’7’),

(2,2,.75,’2’),(2,3,.75,’5’), (2,4,.75,’8’),

(3,2,.75,’3’),(3,3,.75,’6’), (3,4,.75,’9’),

(4,2,.75,’+’),(4,3,.75,’*’), (4,4,.75,’<-’),

(5,2,.75,’-’),(5,3,.75,’/’), (5,4,.75,’C’)

]

yinc = .75/2.0 # half of a "standard" button height

self.buttons= []

for cx,cy,bwidth,label in buttonSpecs:

xinc = bwidth/2.0

p1 = Point(cx-xinc,cy-yinc)

p2 = Point(cx+xinc,cy+yinc)

b = Button(self.win,p1, p2, label)

b.activate()

self.buttons.append(b)