Python Tkinter drawing App - Freehand drawing and undo

Last update on August 25 2025 12:33:24 (UTC/GMT +8 hours)

Write a Python program to design a drawing application with Tkinter that allows users to draw freehand on a Canvas widget. Implement an "Undo" button to remove the last drawn line.

Sample Solution:

Python Code:

import tkinter as tk

class DrawingApp:
    def __init__(self, root):
        self.root = root
        self.root.title("Drawing App")

        self.canvas = tk.Canvas(root, bg="white")
        self.canvas.pack(fill=tk.BOTH, expand=True)

        self.pen_color = "black"
        self.drawing = False
        self.last_x, self.last_y = None, None

        self.lines = []  # List to store drawn lines

        self.undo_button = tk.Button(root, text="Undo", command=self.undo)
        self.erase_button = tk.Button(root, text="Erase", command=self.erase)
        
        self.undo_button.pack(side=tk.LEFT)
        self.erase_button.pack(side=tk.LEFT)

        self.canvas.bind("<Button-1>", self.start_draw)
        self.canvas.bind("<B1-Motion>", self.draw)
        self.canvas.bind("<ButtonRelease-1>", self.stop_draw)

    def start_draw(self, event):
        self.drawing = True
        self.last_x, self.last_y = event.x, event.y

    def draw(self, event):
        if self.drawing:
            x, y = event.x, event.y
            line = self.canvas.create_line(self.last_x, self.last_y, x, y, fill=self.pen_color, width=2)
            self.lines.append(line)  # Store the drawn line
            self.last_x, self.last_y = x, y

    def stop_draw(self, event):
        self.drawing = False

    def undo(self):
        if self.lines:
            last_line = self.lines.pop()  # Remove the last drawn line
            self.canvas.delete(last_line)

    def erase(self):
        self.canvas.delete("all")

if __name__ == "__main__":
    root = tk.Tk()
    app = DrawingApp(root)
    root.mainloop()

Explanation:

In the exercise above -

Import the "tkinter" library.
"DrawingApp" class represents the main application.
Initializing the application:

In the init method, the application's main window (root) is created and titled "Drawing App."
A Canvas widget is created.

Setting up initial attributes:

pen_color: It represents the drawing pen color and is initially set to black.
drawing: A Boolean variable that keeps track of whether the user is currently drawing.
last_x and last_y: These variables store the coordinates of the last drawn point.
lines: A list to store references to the lines drawn on the canvas.

Creating buttons:

Two buttons, "Undo" and "Erase," are created using the "Button" widget. The command parameter specifies the functions to be executed when the buttons are clicked.
The buttons are scattered along the left side of the main window.

Binding mouse events:

The Canvas widget is bound to three mouse events:

<Button-1>: Triggered when the left mouse button is pressed. It calls the start_draw method.
<B1-Motion>: Triggered when the left mouse button is moved while pressed. It calls the draw method.
<ButtonRelease-1>: Triggered when the left mouse button is released. It calls the stop_draw method.

Define event handling methods:

start_draw(event): Sets the drawing flag to True and records the starting coordinates of the drawing.
draw(event): Draws lines on the canvas as the mouse is moved. It creates a line object, stores it in the lines list, and updates the last_x and last_y values.
stop_draw(event): Sets the drawing flag to False when the mouse button is released.
undo(): Removes the last drawn line by deleting it from the canvas and removing its reference from the lines list.
erase(): Clears the entire canvas by deleting all elements with the tag "all."