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Randomness: let the computer surprise you

Most code does the same thing every time. Randomness breaks that rule on purpose. It's how a program picks a surprise, rolls a dice, or paints a sky that's never the same twice.

The big idea

The random tools let the computer choose for you: one item from a list, or a whole number between two ends.

See it in code

1Start simple

random.choice() picks one item from a list. Every item has the same chance:

python
import random

flavors = ["snickerdoodle", "oatmeal", "sugar cookie", "cinnamon swirl"]
print("Surprise cookie:", random.choice(flavors))
One run printed this. Yours may differ:
Surprise cookie: sugar cookie

Press the 'Run Code' button a few times. The same code gives different answers, and that's the whole point.

2A step further

random.randint(1, 3) picks a whole number from 1 to 3. Both ends count, so it can pick 1, 2, or 3:

python
import random

for customer in range(8):
    discount = random.randint(1, 3)
    print("Discount: $", discount)
Eight picks in a row, from one run:
Discount: $ 2
Discount: $ 1
Discount: $ 2
Discount: $ 3
Discount: $ 1
Discount: $ 1
Discount: $ 3
Discount: $ 1

The pick sits inside the loop, so it happens fresh on each trip. Put it above the loop and all eight customers would get the same number.

3In the paint studio

The paint studio has its own random tools: random_number(low, high) and pick(list). Here each spark flies to a random spot in a random color:

python
spark_count = 24
spread = 110
burst_x = 200
burst_y = 150
burst_colors = ["gold", "orange red", "white"]

paper("midnight blue")
for count in range(spark_count):
    end_x = burst_x + random_number(-spread, spread)
    end_y = burst_y + random_number(-spread, spread)
    line(x1=burst_x, y1=burst_y, x2=end_x, y2=end_y, color=pick(burst_colors), width=2)
    dot(x=end_x, y=end_y, color="white")
rect(x=200, y=375, width=400, height=50, color="black")
A burst that changes every Run:
A fireworks burst of thin gold, orange-red, and white lines flying out from one point in a midnight-blue sky over a dark harbor.

end_x is random, and it's stored in a jar. That lets the line and the dot share the same spot, so each spark has a bright tip.

The same idea, everywhere

Games lean on chance all the time. A cupcake drops from a random spot. A power-up appears one time in five. Without randomness, every game would play out the same.

Try it yourself

Change spark_count to 60 and spread to 160. Run it three times and compare. Then put your own three colors in burst_colors.

The common mistake

Expecting randint to work like range. range(1, 3) stops before 3. But random.randint(1, 3) includes 3. Both ends count when the computer is picking.

What it unlocks

Random tools arrive through an import. Put a random pick inside a for loop and you're making generative art, art that's different every time.

Older, or want more depth? Read the Teens version →

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