15.1. Graphical User Interfaces

A graphical user interface (GUI) allows a user to interact with a computer program using a pointing device that manipulates small pictures on a computer screen. The small pictures are called icons or widgets. Various types of pointing devices can be used, such as a mouse, a stylus pen, or a human finger on a touch screen.

We refer to programs that use a graphical user interface as “GUI programs.” A GUI program is very different from a program that uses a command line interface which receives user input from typed characters on a keyboard. Typically programs that use a command line interface perform a series of tasks in a predetermined order and then terminate. However, a GUI program creates the icons and widgets that are displayed to a user and then it simply waits for the user to interact with them. The order that tasks are performed by the program is under the user’s control – not the program’s control! This means a GUI program must keep track of the “state” of its processing and respond correctly to user commands that are given in any order the user chooses. This style of programming is called “event driven programming.” In fact, by definition, all GUI programs are event-driven programs.

15.2. GUI Programming

An GUI program has the following structure:

A GUI program’s interface is composed of widgets displayed in a window. Your computer’s operating system controls the creation and manipulation of windows on your computer’s display screen. The operating system also controls the pointing devices on your computer, such as a mouse or a touch screen. Therefore, your computer’s operating system is what recognizes events that happen in a window. Your operating system sends events to your program in the order they are generated by a user. Your program’s event-loop responds to these events. All GUI programs have the same event-loop, so there is an event-loop provided for you and it looks something like this:

while True:
  # Get the next event from the operating system
  event = get_next_event()

  # Get the function that is assigned to handle this event
  a_function_to_handle_the_event = event-handlers[event]

  # If a function has been assigned to handle this event, call the function
  if a_function_to_handle_the_event:
    a_function_to_handle_the_event()  # Call the event-handler function

  # Stop processing events if the user gives a command to stop the application
  if window_needs_to_close:
    break  # out of the event-loop

Again, you do not implement an event-loop in a GUI program. The event loop has already been written for you. You make this event-loop work by associating a function (which is called an event-handler or a callback function) to a specific event. We will show you how to do this in a few lessons. First, let’s learn how to create a GUI interface which is the widgets a user sees when a GUI program runs.

15.3. GUI Programming Options

Python does not implement GUI, event-driven-programming in its core functionality. GUI programming is implemented using imported modules which are often referred to as “toolkits.” Anyone can implement external modules that facilitate GUI programming, and many people have. Therefore you have many options available to you for GUI programming. A partial list of options can be found at https://docs.python.org/3/faq/gui.html. The following lessons explain how to use the Tkinter toolkit to create GUI programs. Once you understand how GUI programming works, you should be able to learn how to use any of the other available toolkits without much difficulty.

15.4. TKinter

TKinter is an abbreviation for “TK interface”. “TK” is a platform independent, customizable, and configurable GUI library. The Python module TKinter allows Python programs to use the TK libraries. An overview of TK can be found at https://en.wikipedia.org/wiki/Tk_(software)_

15.5. Tkinter Pre-programmed Interfaces

Tkinter provides a set of standard GUI dialog boxes that can be used with minimal programming. These are described in the next lesson. (A dialog box is a small window on a computer screen in which a user is prompted to provide information or select commands.)

15.6. Tkinter Custom Interfaces

Tkinter also provides the functionality to create any user interface imaginable. To create a custom GUI program you basically do five things:

Each of these tasks are explain in detail in the following lessons.

Note: All coding examples in these lessons assume you are using Python 3.5 or greater.

15.7. Hello World

Many programming languages are introduced to new users by showing them how to display “Hello world!” on the screen. This is considered to be the simplest possible program you can write in the language. In that spirit, here is a GUI program that displays “Hello World!:

import tkinter as tk
from tkinter import ttk

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

# Create the user interface
my_label = ttk.Label(window, text="Hello World!")
my_label.grid(row=1, column=1)

# Start the GUI event loop
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