Dojo Defender: Session 1

Your spaceship has landed in dark space. Time to bring it to life. In this session you add a ship you can steer and you make asteroids fall down. Dodge them or lose a life. How long do you survive?

What you’ll learn today

Using lists to keep track of multiple asteroids
Spawning and removing objects
Detecting collisions with colliderect
Keeping track of lives and showing an explosion effect
Forcing game over when your lives run out

Step 0: Setup

Open Thonny.
Download the Dojo Defender starter, unpack the ZIP and open main.py.
Click Run. You see a dark blue screen with your spaceship at the bottom. You can already move it with the arrow keys.
Problems? Take a look at Installing Thonny.

Step 1: Let asteroids spawn

A game without danger is boring. Add a list in which you keep track of all the asteroids, and a function that creates new asteroids:

# at the top, after WIDTH and HEIGHT:
asteroids = []

def spawn_asteroid():
    rock = Actor('asteroid_big')
    rock.x = random.randint(30, WIDTH - 30)
    rock.y = -30
    rock.speed = random.uniform(3, 5)
    asteroids.append(rock)

You also need import random at the top of the file. Add that line.

Call the function in update() with a counter so that an asteroid does not appear every single frame:

# at the top, outside the functions:
timer = 0

def update():
    global timer
    timer += 1
    if timer % 60 == 0:   # every second (60 frames)
        spawn_asteroid()

Put this before the movement code of the ship.

Step 2: ✅ Basic: dropping and drawing asteroids

Asteroids have to fall down and disappear when they are off the screen:

def update():
    global timer
    timer += 1
    if timer % 60 == 0:
        spawn_asteroid()

    for rock in asteroids[:]:      # [:] = copy so you can remove safely
        rock.y += rock.speed
        if rock.y > HEIGHT + 30:
            asteroids.remove(rock)

Don’t forget to draw the asteroids in draw():

def draw():
    screen.fill((10, 10, 30))
    for rock in asteroids:
        rock.draw()
    ship.draw()

Step 3: ⭐ Stretch: collision + lives

When an asteroid hits your ship, you lose a life. Add lives and game_over at the top:

lives = 3
game_over = False

Add collision detection in the for loop:

    for rock in asteroids[:]:
        rock.y += rock.speed

        if rock.y > HEIGHT + 30:
            asteroids.remove(rock)
        elif rock.colliderect(ship):
            asteroids.remove(rock)
            global lives, game_over
            lives -= 1
            if lives <= 0:
                game_over = True

Show the lives and the score in draw():

    screen.draw.text(f"Score: {score}", (10, 10), color="white", fontsize=24)
    screen.draw.text(f"Lives: {lives}", (10, 40), color="red", fontsize=24)

Stop the game when game_over is True:

def update():
    if game_over:
        return
    ...

Show a game-over screen in draw():

    if game_over:
        screen.draw.text(
            "GAME OVER",
            center=(WIDTH / 2, HEIGHT / 2),
            color="red",
            fontsize=60,
        )
        screen.draw.text(
            f"Final Score: {score}",
            center=(WIDTH / 2, HEIGHT / 2 + 60),
            color="white",
            fontsize=30,
        )

Step 4: 🔥 Expert: explosions + invisibility

When an asteroid hits you, you want to see an explosion instead of the ship just disappearing. Use the four explosion frames (explosion_1 to explosion_4):

explosion = Actor('explosion_1')
explosion_active = False
explosion_frame = 0
explosion_timer = 0

On a collision: hide the ship, start the explosion:

        elif rock.colliderect(ship):
            asteroids.remove(rock)
            global lives, game_over, explosion_active, explosion_frame, explosion_timer
            lives -= 1

            explosion.pos = (ship.x, ship.y)
            explosion.image = 'explosion_1'
            explosion_frame = 1
            explosion_timer = 5
            explosion_active = True
            ship.visible = False

            if lives <= 0:
                game_over = True

Play the explosion in update():

    if explosion_active:
        explosion_timer -= 1
        if explosion_timer <= 0:
            explosion_frame += 1
            if explosion_frame > 4:
                explosion_active = False
                ship.visible = True
                ship.pos = (WIDTH / 2, HEIGHT - 50)
            else:
                explosion.image = f'explosion_{explosion_frame}'
                explosion_timer = 5

Then add an invisibility period so that you don’t get hit again straight away:

invincible = False
invincible_timer = 0

In update():

    if invincible:
        invincible_timer -= 1
        if invincible_timer <= 0:
            invincible = False
            ship.visible = True

Adjust the collision check:

        elif rock.colliderect(ship) and not invincible and ship.visible:

Draw the explosion in draw():

    if explosion_active:
        explosion.draw()

Showcase

Show your Dojo Defender to a coach and a buddy. How long do you survive? Do you have explosions? Does it look cool?

See you next time!

“Next time: your spaceship SHOOTS BACK. Enemies in formation, and you blast them away with lasers. This is going to be epic.”

Take it home

Try one of these extensions at home:

Sound — Add sounds/explosion.wav when an asteroid explodes (sounds.explosion.play()).
Harder — Make asteroids fall faster as the score rises: rock.speed = random.uniform(3 + score // 10, 5 + score // 10).
Power-up — Add a rare green power-up that gives an extra life.
Smaller — Use asteroid_med or asteroid_small for variation in size and speed.