Python Turtle Graphics — Core Concepts

What turtle does

Python’s turtle module provides a canvas and a programmable cursor (the turtle) that draws lines as it moves. It is part of the standard library — no installation needed. Turtle is built on top of tkinter, Python’s default GUI toolkit.

import turtle

t = turtle.Turtle()
t.forward(100)
t.left(90)
t.forward(100)
turtle.done()

This opens a window with a turtle that draws two lines forming an L-shape.

Coordinate system

The turtle starts at (0, 0) — the center of the window — facing right (east). The canvas uses standard Cartesian coordinates: positive x is right, positive y is up. You can reposition the turtle with:

t.goto(100, 50)     # move to (100, 50), drawing a line
t.setx(200)         # change x only
t.sety(-100)        # change y only
t.home()            # return to (0, 0) facing east

Query position and heading:

x, y = t.position()
angle = t.heading()  # degrees, 0 = east, 90 = north

Movement commands

Core movement methods:

• forward(distance) / fd(distance) — move in the current direction
• backward(distance) / bk(distance) — move in reverse
• left(angle) / lt(angle) — turn counterclockwise
• right(angle) / rt(angle) — turn clockwise
• circle(radius, extent=360) — draw a circle or arc
• speed(n) — set drawing speed: 1 (slowest) to 10, or 0 (instant)

Pen control

Control whether the turtle draws and how:

t.penup()            # lift pen (move without drawing)
t.pendown()          # lower pen (draw while moving)
t.pensize(3)         # line width in pixels
t.pencolor("red")    # line color (name, hex, or RGB tuple)
t.pen(pencolor="blue", pensize=2, speed=5)  # set multiple at once

Color and filling

Fill closed shapes with color:

t.fillcolor("yellow")
t.begin_fill()
for _ in range(4):
    t.forward(100)
    t.left(90)
t.end_fill()

Set both pen and fill color simultaneously:

t.color("red", "orange")  # pen=red, fill=orange

Use RGB tuples with turtle.colormode(255):

turtle.colormode(255)
t.pencolor(65, 105, 225)  # royal blue

Drawing patterns with loops

Turtle shines when combined with loops. A simple star:

for _ in range(5):
    t.forward(150)
    t.right(144)

A spiral:

for i in range(100):
    t.forward(i * 2)
    t.left(91)

Nested loops create complex patterns:

for _ in range(36):
    for _ in range(4):
        t.forward(80)
        t.left(90)
    t.left(10)

This draws 36 squares, each rotated 10 degrees — producing a circular pattern.

Stamps and shapes

The turtle cursor can take different shapes:

t.shape("turtle")    # arrow, turtle, circle, square, triangle, classic

Stamp the current shape onto the canvas:

for _ in range(12):
    t.stamp()
    t.forward(50)
    t.left(30)

Register custom shapes from images:

turtle.register_shape("rocket.gif")
t.shape("rocket.gif")

Event handling

Make programs interactive with keyboard and mouse events:

def move_forward():
    t.forward(20)

def turn_left():
    t.left(15)

screen = turtle.Screen()
screen.onkey(move_forward, "Up")
screen.onkey(turn_left, "Left")
screen.listen()
turtle.done()

Mouse click events:

def draw_dot(x, y):
    t.penup()
    t.goto(x, y)
    t.dot(10, "blue")

screen.onclick(draw_dot)

Screen control

Configure the window:

screen = turtle.Screen()
screen.setup(width=800, height=600)
screen.bgcolor("black")
screen.title("My Drawing")

Save the drawing:

canvas = screen.getcanvas()
canvas.postscript(file="drawing.eps")

For PNG output, convert the EPS with Pillow or Ghostscript.

Writing text

Draw text on the canvas:

t.penup()
t.goto(-100, 100)
t.write("Hello, Turtle!", font=("Arial", 24, "bold"), align="center")

The align parameter accepts "left", "center", or "right".

The one thing to remember: Turtle translates Python control flow — loops, functions, conditionals — into visible movement on a canvas, making abstract programming concepts concrete and immediate.