GUI Programming with Tkinter: Windows, Widgets and Events

What is a GUI and how does Tkinter start?

A GUI (graphical user interface) shows windows, buttons and text boxes. A CLI (command-line interface) only shows text. Tkinter is Python's built-in GUI library, so nothing extra needs installing.

import tkinter as tk
root = tk.Tk()            # the main window
root.title("My App")      # text in the title bar
root.geometry("400x300")  # width x height in pixels
root.mainloop()           # show it and wait for events

mainloop() is the event loop. It keeps the window open and keeps checking: did the user click? type? close? Code after mainloop() runs only when the window closes.

Widgets are objects with properties

Each widget is an object. You create it from a class and give it a parent (the window it lives in) and properties (options).

Common properties: text, bg (background colour), fg (text colour), font, width. Change them later with widget.config(bg="yellow").

Messages: from tkinter import messagebox, then messagebox.showinfo("Title", "Saved!"), showwarning, showerror or askyesno (returns True/False).

Layout: pack, grid and place

A widget is invisible until a layout manager places it.

• pack(): stacks widgets top to bottom (or side="left"). Quick and simple.
• grid(row=r, column=c): a table of rows and columns. Best for forms.
• place(x=…, y=…): exact pixel position. Precise but does not adapt when the window resizes.

Do not mix pack and grid inside the same parent; use a Frame for each part instead.

Events, handlers and a small game

A GUI program is event-driven: it does not run top to bottom and stop. It waits for events (click, key press, mouse move) and runs a handler function for each.

def say_hi():
    name = ent.get()
    lab.config(text="Hello, " + name + "!")

btn = tk.Button(root, text="Say hi", command=say_hi)

Note: command=say_hi without brackets. With brackets the function would run at once, not on click.

For keys use bind; the handler receives an event object:

cv = tk.Canvas(root, width=400, height=300, bg="black")
ball = cv.create_oval(10, 10, 40, 40, fill="orange")
def right(event):
    cv.move(ball, 20, 0)
root.bind("<Right>", right)

A game adds a score variable, checks if two shapes overlap (with cv.coords), and uses root.after(ms, function) to move things on a timer.

Try it yourself

Type the say_hi program above into a .py file and run it. Then: (1) change the button colour with bg; (2) add a Checkbutton for 'Loud' that shows the greeting in capitals with .upper(); (3) switch from pack to grid and see the form line up.

Key formulas and definitions

• root = tk.Tk() … root.mainloop()
• w = tk.Widget(parent, option=value)
• w.pack() | w.grid(row=r, column=c) | w.place(x=, y=)
• w.config(option=value) — change a property
• entry.get() — read the text
• tk.Button(root, command=function_name) — no brackets
• root.bind("<Key>", handler) — handler(event)
• messagebox.showinfo(title, message)

Worked examples

Common mistakes

• Writing command=say_hi() with brackets: the function runs once at start and the button then does nothing.
• Forgetting pack/grid/place, so the widget never appears.
• Forgetting root.mainloop(), so the window flashes and closes (or never shows).
• Mixing pack() and grid() in the same parent, which can freeze the program.