192 CHAPTER11. DATA COLLECTIONS

def __createDisplay(self):

bg = Rectangle(Point(.5,5.5), Point(5.5,6.5))

bg.setFill(’white’)

bg.draw(self.win)

text = Text(Point(3,6),"")

text.draw(self.win)

text.setFace("courier")

text.setStyle("bold")

text.setSize(16)

self.display= text

def getKeyPress(self):

# Waits for a buttonto be clicked and returns the labelof

# the buttonthe was clicked.

while 1:

p = self.win.getMouse()

for b in self.buttons:

if b.clicked(p):

returnb.getLabel() # method exit

def processKey(self,key):

# Updates the displayof the calculator for press of thiskey

text = self.display.getText()

if key == ’C’:

self.display.setText("")

elif key == ’<-’:

# Backspace,slice off the last character.

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

elif key == ’=’:

# Evaluatethe expresssion and display the result.

# the try...exceptmechanism "catches" errors in the

# formulabeing evaluated.

try:

result = eval(text)

except:

result = ’ERROR’

self.display.setText(str(result))

else:

# Normal key press,append it to the end of the display

self.display.setText(text+key)

def run(self):

# Infinite’event loop’ to process key presses.

while 1:

key = self.getKeyPress()

self.processKey(key)

This runs the program.

if name == "main":

First createa calculator object

theCalc = Calculator()

Now call the calculator’srun method.

theCalc.run()