15.26. The Design of GUI Programs

For very simple GUI programs, no special program design is needed, as demonstrated in the previous “Hello World” example programs. However, any non-trivial GUI program will require extensive use of global variables if the structure of the code does not use a Python class. You have learned in previous lessons that global variables are bad because they make debugging programs more difficult. Therefore we want a design for GUI programs that avoids global variables as much as possible.

To demonstrate this, let’s look at two versions of a simple program that increments a counter each time a user clicks a button. The first version of this code does not use a class definition and requires that a global variable called my_counter be used. This is because the label that represents the counter is created in the create_user_interface function but it must be accessed in the event handler function increment_counter. In fact, the event handlers of a GUI program almost always need access to multiple widgets in the program’s interface and the values can’t be passed as parameters because an command event handler function receives no parameters and a bind event handler function receives exactly one parameter – an event object. Study the following example and pay close attention to where the my_counter global variable is used.

import tkinter as tk
from tkinter import ttk

global my_counter


def create_user_interface(application_window):
    global my_counter

    my_counter = ttk.Label(application_window, text="0")
    my_counter.grid(row=0, column=0)

    increment_button = ttk.Button(application_window, text="Add 1 to counter")
    increment_button.grid(row=1, column=0)
    increment_button['command'] = increment_counter

    quit_button = ttk.Button(application_window, text="Quit")
    quit_button.grid(row=2, column=0)
    quit_button['command'] = window.destroy


def increment_counter():
    global my_counter
    my_counter['text'] = str(int(my_counter['text']) + 1)

# Create the application window
window = tk.Tk()

create_user_interface(window)

# Start the GUI event loop
window.mainloop()

Let’s compare the above program to an identical application that is designed as a Python class. The class encapsulates all of the values needed for the GUI interface and the event handlers and we don’t need global variables!

import tkinter as tk
from tkinter import ttk

def main():
    # Create the entire GUI program
    program = CounterProgram()

    # Start the GUI event loop
    program.window.mainloop()

class CounterProgram:

    def __init__(self):
        self.window = tk.Tk()
        self.my_counter = None  # All attributes should be initialize in init
        self.create_widgets()

    def create_widgets(self):
        self.my_counter = ttk.Label(self.window, text="0")
        self.my_counter.grid(row=0, column=0)

        increment_button = ttk.Button(self.window, text="Add 1 to counter")
        increment_button.grid(row=1, column=0)
        increment_button['command'] = self.increment_counter

        quit_button = ttk.Button(self.window, text="Quit")
        quit_button.grid(row=2, column=0)
        quit_button['command'] = self.window.destroy

    def increment_counter(self):
        self.my_counter['text'] = str(int(self.my_counter['text']) + 1)

if __name__ == "__main__":
    main()

Notice the following about this design:

• The application’s window is created in the constructor (__init__) of the CounterProgram class and then the interface widgets are created by a call to create_widgets.

• The event handler, increment_counter can access the label self.my_counter using the object’s attributes.

• The code creates an instance of the class CounterProgram and starts the GUI event-loop.

It is recommended that you develop all of your GUI programs as Python Classes. For complex designs, a Python Class can help manage the complexity of the code and the scoping of variables.

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