Random Circles

By the end of this lesson you'll be able to:

• Import the random library and generate random integers with randint()
• Use random numbers to place a turtle at an unpredictable position
• Draw a canvas full of colorful circles that look different every time you run the program

So far every program you've written produces the exact same drawing every time. The random library changes that — it generates numbers that are unpredictable, so each run of your program is a surprise.

Welcome to This Lesson!

What if you could make a program that draws something different every single time you run it? That's the magic of randomness — and today I'm going to show you how! Let's code it together!

Generating Random Numbers

To use random numbers, you first need to import the random library — just like you import turtle. Then use randint(min, max) to get a random whole number between min and max (both inclusive):

import random
x = random.randint(-150, 150)

Each time this line runs, x gets a different value somewhere between -150 and 150.

Scratch Bridge

In Scratch you used the "Pick random 1 to 10" block. In Python, random.randint(1, 10) does exactly the same thing — picks a random whole number in that range each time it runs!

Sample Code

import turtle
import random

monty = turtle.Turtle()
monty.shape('turtle')
monty.speed(10)

colors = ['red', 'orange', 'blue', 'green', 'purple', 'pink']

for i in range(10):
    x = random.randint(-150, 150)
    y = random.randint(-150, 150)
    color = colors[random.randint(0, 5)]
    monty.penup()
    monty.goto(x, y)
    monty.pendown()
    monty.color(color)
    monty.begin_fill()
    monty.circle(10)
    monty.end_fill()

monty.hideturtle()

What Do You Think Will Happen?

The loop runs 10 times, picking a new random position and color each time. How many circles will appear? Will they always be in the same spots? Make your guess — then click Run twice to see what changes!

Try It Now

Edit the code below and click Run to see the result right on this page. No account needed — everything runs in your browser.

▶ Run ↻ Reset

Were you right? Ten colorful circles scatter across the canvas in random positions — and clicking Run again produces a completely different arrangement!

How It Works

random.randint(-150, 150) returns a different integer each time — Python uses a pseudorandom algorithm that produces unpredictable-looking numbers. goto(x, y) jumps the turtle to that position, and circle(10) draws a circle with a radius of 10 steps around the current position.

speed(10) sets the turtle to maximum drawing speed so the circles appear quickly instead of crawling into place.

Explanation Table

Line	What it does
import random	Loads the random number library
random.randint(-150, 150)	Returns a random integer between -150 and 150 (inclusive)
monty.speed(10)	Sets drawing speed to maximum (10 = fastest)
colors[random.randint(0, 5)]	Picks a random index from 0 to 5 to select a color from the list
monty.circle(10)	Draws a circle with radius 10 at the current position

Index Out of Range

If your colors list has 6 items (indexes 0–5), then randint(0, 6) can produce index 6 — which doesn't exist! Always use randint(0, len(colors) - 1) or randint(0, 5) when the list has 6 items.

Learning Check

Your Turn — Make the Circles Bigger

The program below draws 10 random circles but they're all the same tiny size. Add one line inside the loop to pick a random radius between 5 and 25, store it in a variable called radius, and use it in monty.circle(radius).

▶ Run ↻ Reset

Experiments

Try these changes. Predict first, then run and compare!

1. Change range(10) to range(30). You'll know it worked when 30 circles fill the canvas instead of 10.

2. Change randint(-150, 150) to randint(-50, 50) for both x and y. You'll know it worked when all the circles cluster near the center of the canvas.

3. Add 'gold', 'cyan', and 'brown' to the colors list, and update randint(0, 5) to match the new list length. You'll know it worked when you occasionally see gold, cyan, or brown circles.

4. Change monty.circle(10) to monty.circle(random.randint(5, 25)). You'll know it worked when the circles vary in size, some tiny and some large.

5. Change the number of circles drawn by storing the count in a variable num_circles = 20 at the top and using it in range(num_circles). You'll know it worked when changing num_circles in one place controls how many appear.

Great Work!

Every run is a unique piece of art — that's the power of randomness! In the next lab you'll learn more about lists and how to loop through them directly. Let's code it together!