Alien shuffle for pygame zero

game.py

import random

alien = Actor('alien')
alien.topright = 0, 10
# these pieces of state needed to be attached to an actor to be updated...
alien.score = 0
alien.screen_color = 128, 80, 0

bad_alien = Actor('alien')
bad_alien.topright = 400, 10

WIDTH = 500
HEIGHT = alien.height + 20

def draw():
    screen.fill(alien.screen_color)
    alien.draw()
    bad_alien.draw()
    screen.draw.text("score %d" % (alien.score), topleft=(10, 10))
    
def update():
    if keyboard.left:
        if alien.right > 0:
            alien.left -= 10
    elif keyboard.right:
        alien.left += 10

    alien.left += 2
    if alien.left > WIDTH:
        alien.score += 5
        alien.right = 0
    
    if alien.colliderect(bad_alien):
        set_alien_hurt()
        
    bad_alien.top += 1
    if bad_alien.top > HEIGHT:
        bad_alien.bottom = -100    

def on_mouse_down(pos):
    if alien.collidepoint(pos):
        set_alien_hurt()

def set_alien_hurt():
    if alien.image == 'alien':
        alien.score -= 20
        sounds.eep.play()
    alien.image = 'alien_hurt'
    clock.schedule_unique(reset_alien, 1.0)
    change_background()

def reset_alien():
    alien.image = 'alien'
    
def change_background():
    alien.screen_color = (random.randint(0, 255), random.randint(0, 255), random.randint(0, 255))
    
clock.schedule_interval(change_background, 10.0)