Graphical User Interfaces with Tkinter

Note

Source: Contributed by PhD students in COMP 501 at Loyola University Chicago.

Todo

This chapter covers Tkinter, Python’s classic built-in GUI library. While Tkinter remains fully functional and widely taught, the Python GUI landscape has evolved. Modern alternatives (e.g., PyQt, PySide, Dear PyGui, or web-based interfaces) may be more appropriate for production work. Consider modernizing this chapter in a future revision to survey current options or to replace Tkinter examples with a more contemporary toolkit.

A Graphical User Interface (GUI) lets users interact with a program using visual elements — windows, buttons, text boxes, and menus — rather than typing commands. GUIs make software more accessible and intuitive, especially for users who are not comfortable with the command line.

You encounter GUIs every day: the desktop environment, web browsers, file explorers, calculators, and games all use them.

What Is Tkinter?

Tkinter is Python’s built-in library for creating desktop GUIs. It comes pre-installed with Python, so no extra installation is required. Tkinter acts as a bridge between Python and the Tcl/Tk GUI toolkit.

The simplest possible Tkinter program:

import tkinter as tk

root = tk.Tk()
root.title("My First GUI")

label = tk.Label(root, text="Hello, Tkinter!")
label.pack()

root.mainloop()

• tk.Tk() creates the main application window.

• Label is a widget that displays text.

• pack() places the widget inside the window.

• mainloop() starts the event loop — it keeps the window open and listens for user actions until the window is closed.

Widgets

Widgets are the building blocks of a Tkinter GUI.

Widget	Purpose
Label	Display text or an image.
Button	Perform an action when clicked.
Entry	Accept a single line of text input.
Text	Accept multiple lines of text input.
Frame	Container for grouping other widgets.
Listbox	Display a scrollable list of items.
Canvas	Draw shapes, lines, or custom graphics.
Checkbutton	A checkbox that can be on or off.

Example — a button that reacts to a click:

import tkinter as tk

def on_click():
    print("Button clicked!")

root = tk.Tk()
btn = tk.Button(root, text="Click Me", command=on_click)
btn.pack()
root.mainloop()

The command attribute points to a function that is called when the button is clicked.

Handling User Input

Use an Entry widget to collect text from the user, and entry.get() to retrieve what they typed:

import tkinter as tk

def show_greeting():
    name = entry.get()
    label.config(text=f"Hello, {name}!")

root = tk.Tk()

entry = tk.Entry(root)
entry.pack()

btn = tk.Button(root, text="Submit", command=show_greeting)
btn.pack()

label = tk.Label(root, text="")
label.pack()

root.mainloop()

label.config(text=...) updates the label’s text dynamically without recreating the widget.

Layout Managers

Tkinter offers three geometry managers for positioning widgets:

Manager	Description
pack()	Stacks widgets vertically or horizontally; simplest to use.
grid()	Arranges widgets in a table of rows and columns.
place()	Positions widgets at exact pixel coordinates.

For most programs, pack() or grid() is sufficient. place() is useful when you need precise control over widget positions.

Error Messages with messagebox

Display pop-up dialogs for errors or confirmations:

from tkinter import messagebox

messagebox.showerror("Error", "Invalid input. Please try again.")
messagebox.showinfo("Success", "Data saved successfully.")
messagebox.askyesno("Confirm", "Are you sure you want to delete this?")

Integrating an API with a GUI

Tkinter and the requests module can be combined to build interactive data apps. Here is a minimal weather-lookup window:

import tkinter as tk
from tkinter import messagebox
import requests

def fetch_weather():
    city = entry.get().strip()
    if not city:
        messagebox.showerror("Error", "Please enter a city name.")
        return
    url = f"https://wttr.in/{city}?format=3"
    try:
        response = requests.get(url, timeout=5)
        if response.status_code == 200:
            result_label.config(text=response.text)
        else:
            result_label.config(text="City not found.")
    except requests.exceptions.ConnectionError:
        messagebox.showerror("Error", "No internet connection.")

root = tk.Tk()
root.title("Weather Lookup")

tk.Label(root, text="City:").pack()
entry = tk.Entry(root)
entry.pack()

tk.Button(root, text="Get Weather", command=fetch_weather).pack()
result_label = tk.Label(root, text="")
result_label.pack()

root.mainloop()

Exercises

1. Create a Tkinter window that displays “Welcome to Tkinter!” and a button that closes the application when clicked.

2. Build a greeting app with an Entry for the user’s name and a Button that updates a Label to say "Hello, <name>!".

3. Create a simple counter app with “+” and “−” buttons and a label that shows the current count.

4. Build a color-changer: three buttons (“Red”, “Green”, “Blue”) that each change the window’s background color when clicked.

5. Create a GUI calculator that adds two numbers entered in Entry widgets and displays the result in a Label when a “Calculate” button is clicked.

6. Extend one of the exercises above to display a pop-up error message if the user provides invalid input (e.g., non-numeric values in the calculator).