Dojo Defender: Session 2

Your ship has a weapon now. Enemies are coming. Shoot them down before they reach the bottom. Every 10 enemies means a new wave — and it gets harder and harder.

What you’ll learn today

Shooting with the space bar and a cooldown
Lists for bullets and enemies
Collision between bullets and enemies
Waves that get harder and harder
Enemies that make sine movements

Step 0: Open the starter

Open Thonny.
Download the Dojo Defender starter for today, unpack the ZIP and open main.py.
Were you not there last time? Download the solution from session 1 and start with that.
Click Run. You see the same game as last time: ship, asteroids, explosions.
Problems? Take a look at Installing Thonny.

Step 1: Shooting with SPACE

Your ship has a laser cannon. Add a list for bullets at the top:

bullets = []

Use on_key_down — it is called only once when you press a key (unlike update, which keeps repeating):

def on_key_down(key):
    if key == keys.SPACE:
        bullet = Actor('bullet')
        bullet.x = ship.x
        bullet.y = ship.y - 20
        bullets.append(bullet)

Make the bullets move up in update() and remove them when they are off screen:

for bullet in bullets[:]:
    bullet.y -= 8
    if bullet.y < -10:
        bullets.remove(bullet)

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

for bullet in bullets:
    bullet.draw()

Step 2: ✅ Basic: shooting cooldown

Without a cooldown you fire dozens of bullets per second. Add a timer:

shoot_cooldown = 0

In update():

if shoot_cooldown > 0:
    shoot_cooldown -= 1

Adjust on_key_down so that you can only shoot when the cooldown is 0:

def on_key_down(key):
    global shoot_cooldown
    if key == keys.SPACE and shoot_cooldown == 0:
        bullet = Actor('bullet')
        bullet.x = ship.x
        bullet.y = ship.y - 20
        bullets.append(bullet)
        shoot_cooldown = 15

Step 3: ⭐ Stretch: adding enemies

Enemies spawn at the top and fall down. Add a list:

enemies = []

A spawn function:

def spawn_enemy():
    enemy = Actor('enemy_drone')
    enemy.x = random.randint(30, WIDTH - 30)
    enemy.y = -30
    enemy.speed = random.uniform(1, 3)
    enemies.append(enemy)

Call spawn_enemy() in update() with a spawn interval:

# at the top:
spawn_counter = 0
spawn_interval = 120

# in update():
spawn_counter += 1
if spawn_counter >= spawn_interval:
    spawn_counter = 0
    spawn_enemy()

Draw enemies in draw():

for enemy in enemies:
    enemy.draw()

Step 4: 🔥 Expert: collision + waves

Bullet hits enemy: remove both, +10 points:

for bullet in bullets[:]:
    bullet.y -= 8
    if bullet.y < -10:
        bullets.remove(bullet)
        continue
    for enemy in enemies[:]:
        if bullet.colliderect(enemy):
            bullets.remove(bullet)
            enemies.remove(enemy)
            global score
            score += 10
            break

Enemy hits the bottom or the ship: lose a life (same as an asteroid).

Waves: a new wave every 10 enemies. Enemies spawn faster:

# at the top:
wave = 1
enemies_destroyed = 0

# on destroy:
enemies_destroyed += 1
if enemies_destroyed % 10 == 0:
    wave += 1
    spawn_interval = max(60, spawn_interval - 10)

Show the current wave in draw():

screen.draw.text(f"Wave: {wave}", (10, 70), color="yellow", fontsize=24)

Show “WAVE N!” in big letters on a new wave (use a timer).

Sine movement: let enemies veer left and right:

import math  # at the top

enemy.x = enemy.start_x + math.sin(timer * 0.03 + enemy.strafe_offset) * 50

Showcase

Show your Dojo Defender to a coach. How many waves do you reach? Do you have the sine movement working? Compare scores with a buddy.

See you next time!

“Next time: POWER-UPS. Shields, speed boosts and spread shots. Your ship gets stronger the longer you survive.”

Take it home

Try one of these extensions at home:

Enemy variation — Use enemy_charger.png for a faster enemy that doesn’t do a sine, but comes straight at you.
Sound — Add sounds/laser.wav and sounds/explosion.wav.
Hard mode — Enemies spawn faster and faster, up to every 30 frames at most.
Score multiplier — The higher the wave, the more points per kill: score += 10 * wave.