A First breezypythongui Program

The following GUI-based application computes a person’s income tax, based on a mythical tax code having a flat tax rate of 15%.

The user can enter the three inputs in any order, and back out of an entry by editing it, before selecting the button to compute and display the result. Because the window stays alive after the Compute button is pressed, the user can subsequently adjust one input and leave the other previous inputs alone to compute and view a different result. Unlike terminal- and dialog-based applications, which guide the user through a determinate sequence of steps, a GUI-based application puts the user in charge of deciding which actions to perform. To give the kind of control to the user just mentioned, a GUI-based program must do four basic things:

1. Layout and pop up a window with the appropriate data fields, buttons, and other controls.

2. Wait for the user to perform an action, such as pressing a button or selecting text in a field.

3. Detect a user’s action (also called an event).

4. Respond appropriately to each type of user action. We call this type of programming event-driven, because the program’s behavior is driven by user events. The good news is that the runtime system handles the second and third of the four tasks – waiting for and detecting user events – mentioned above. The other two tasks, laying out the window and responding to user events, are a matter of defining several methods belonging to a window class. The GUI-based tax calculator program shows clearly where these two tasks are performed.

from breezypythongui import EasyFrame

class TaxCodeDemo(EasyFrame):

"""Application window for the tax calculator."""

def __init__(self):

"""Sets up the window and the widgets."""

EasyFrame.__init__(self, title = "Tax Calculator")

# Label and field for the income

self.addLabel(text = "Income",

row = 0, column = 0)

self.incomeField = self.addFloatField(value = 0.0,

row = 0,

column = 1)

# Label and field for the number of dependents

self.addLabel(text = "Dependents",

row = 1, column = 0)

self.depField = self.addIntegerField(value = 0,

row = 1,

column = 1)

# Label and field for the exemption amount

self.addLabel(text = "Exemption amount",

row = 2, column = 0)

self.exeField = self.addFloatField(value = 0.0,

row = 2,

column = 1)

# The command button

self.addButton(text = "Compute", row = 3, column = 0,

columnspan = 2, command = self.computeTax)

# Label and field for the tax

self.addLabel(text = "Total tax",

row = 4, column = 0)

self.taxField = self.addFloatField(value = 0.0,

row = 4,

column = 1,

precision = 2)

# The event handler method for the button

def computeTax(self):

"""Obtains the data from the input fields and uses

them to compute the tax, which is sent to the

output field."""

income = self.incomeField.getNumber()

numDependents = self.depField.getNumber()

exemptionAmount = self.exeField.getNumber()

tax = (income - numDependents * exemptionAmount) * .15

self.taxField.setNumber(tax)

#Instantiate and pop up the window.

TaxCodeDemo().mainloop()