Session 1: Catch the Stars

In this session you build a mini game: a star falls down and you catch it with a basket. By the end of the session everything moves, the game keeps track of how many stars you catch, and you have your first game of your own.

What you’ll learn today

• Using variables to position objects
• The draw() function: what you draw every frame
• The update() function: what changes every frame
• Keyboard input: moving with the arrow keys

Step 0: Setup

1. Open Thonny.
2. Download the starter via this link, unpack the ZIP and open main.py in Thonny.
3. Click the green Run button. You see a dark blue window with a basket at the bottom and a star at the top, but nothing moves yet.
4. Problems? Take a look at Installing Thonny.

Step 1: Let the star fall

The update() function runs 60 times per second. So everything you put in it runs 60 times per second. That is how movement works in a game.

Right now there is only pass in update(). That means “do nothing”. Remove pass and add this:

def update():
    star.y = star.y + 3

Click Run. The star now falls from top to bottom. When it falls off the screen, it disappears. We’ll fix that in a moment.

What happens here? star.y is the vertical position of the star. The bigger y, the lower on the screen. By adding + 3 every frame, the star drops a little each time.

The coordinate system: (0, 0) is at the top left, not at the bottom left like in maths. x goes to the right, y goes down. The star starts at y = 30 (almost at the top), the basket sits at y = 370 (almost at the bottom).

Move your mouse over the screen below to see the coordinates:

Want to know more about how the screen works? → Coordinates explained

Adjust the sliders and watch how actor.x and actor.y change as the actor moves:

Step 2: ✅ Basic: move the basket

Time to steer the basket. Pygame Zero has a handy keyboard variable that keeps track of which keys are pressed.

Add this to update(), after the line about the star:

def update():
    star.y = star.y + 3

    if keyboard.left:
        basket.x = basket.x - 5
    if keyboard.right:
        basket.x = basket.x + 5

Click Run and use the arrow keys. The basket moves along now!

Tip (optional, counts as Stretch): If you move the basket too far left or right, it disappears off the screen. Can you add a check that stops that? Hint: use WIDTH and compare it with basket.x.

Step 3: ⭐ Stretch: catch the star

Now you want something to happen when the basket touches the star. For that you use colliderect. It checks whether two rectangles overlap.

Add this to update():

    if star.colliderect(basket):
        star.y = 0

The star jumps back to the top when you catch it. But what if you miss it? Add this too:

    if star.y > HEIGHT:
        star.y = 0

Bonus: Make the star come back at a random spot every time. Add this at the top of the file:

import random

Then replace star.y = 0 (in both places) with:

        star.x = random.randint(20, WIDTH - 20)
        star.y = 0

Now the star falls at a different spot every time. Much harder to catch!

Step 4: 🔥 Expert: add a score

A real game keeps track of what you score. For that you need a variable that lives outside the functions: a global variable.

Click the button below to see how score changes every time you catch a star:

Add this at the top, outside all functions (after the lines about basket and star):

score = 0

In draw(), add:

    screen.draw.text(f"Score: {score}", topleft=(10, 10), color="white")

In update(), change the catch code so the score goes up:

    if star.colliderect(basket):
        global score
        score = score + 1
        star.x = random.randint(20, WIDTH - 20)
        star.y = 0

global score tells Python: “I mean the score from outside this function, not a new local variable.” Without that line you get an error message.

Click Run. The score sits at the top left and goes up every time you catch a star!

Showcase

Show your game to a coach and a buddy. Tell them what you made: what works, and what extras you added.

See you next time!

“Next time: things that FIGHT BACK. And they go BOOM, with real sound.”

Take it home

Try one of these extensions at home. Each one comes with a code hint to get you going.

1. Limit lives — Add levens = 3. On a miss (star.y > HEIGHT): levens -= 1. Stop the game when levens == 0 with game_over = True and show “Game Over” in draw().

2. Random star colour — On every catch: give the star a new colour with random.randint(50, 255) for red, green and blue. Start at 50, not at 0 — otherwise the star is sometimes black and invisible.

3. Background — Draw fixed stars in the background in draw() before the score. Use screen.draw.filled_circle(x, y, 2, "white") at 20 random positions. Seed random.seed(42) so they don’t flicker.

4. Second star — Make sterren = [star, Actor("star")]. Give every star its own x. Update both of them in update() and draw them in draw().

5. High score — Keep track of high_score. If score > high_score: high_score = score. Show f"Best: {high_score}" next to the score.

6. Smaller basket — After every 5 points: basket.width = max(30, basket.width - 5). Reset basket.width on a restart.

7. Bomb — Add a red circle that falls now and then. Use a timer bom_timer and random.randint to decide when it shows up. If the basket hits the bomb: score -= 2.

8. Difficulty — After every 5 stars: raise the falling speed by 1. Build in a maximum: max(3, star_snelheid).

Stuck? Ask a coach at the next dojo, or just try something else. Programming is doing.

View the cheatsheet for this session