Message Boxes

A message box can be used to announce a significant event, such as a user input error. One place where this might occur is when the program attempts to get a number from a numeric entry field. If the user’s input text cannot be converted to a number, the getNumber method raises a ValueError. A robust application can respond with a standard try-except pattern, which pops up a message box and allows her to try again. The next code segment shows the event handler from a circle area program (circlearea3.py) that recovers from both number format errors and invalid radius errors.

def computeArea(self):

"""Inputs the radius, computes the area,

and outputs the area. Responds to number format

errors and to invalid data errors (input < 0) with

message boxes."""

try:

radius = self.radiusField.getNumber()

if radius < 0:

self.messageBox(title = "ERROR",

message = "Radius must be >= 0.")

else:

area = radius ** 2 * math.pi

self.areaField.setNumber(area)

except ValueError:

self.messageBox(title = "ERROR",

message = "Radius must be a number.")

Here is a screen shot of the program’s main window and the message box resulting from an input error (an o was typed instead of a 0):