Asteroids

A ship with nothing to dodge isn't much of a game. This lesson adds the asteroids themselves, and a screen-wrapping trick that both the ship and asteroids will share.

An Asteroid class

import random
import pygame

class Asteroid:
    def __init__(self, x, y, size=40):
        self.x = x
        self.y = y
        self.size = size
        self.dx = random.uniform(-2, 2)
        self.dy = random.uniform(-2, 2)

    def update(self, width, height):
        self.x = (self.x + self.dx) % width   # wrap around the left/right edges
        self.y = (self.y + self.dy) % height  # wrap around the top/bottom edges

    def draw(self, screen):
        pygame.draw.circle(screen, (160, 160, 160), (int(self.x), int(self.y)), self.size, width=2)

self.x % width is the key trick: Python's modulo operator wraps a value that goes past width back around to 0, and a negative value back around to just under width. Applying it to both x and y every frame gives you the classic Asteroids screen-wrap for free, drift off the right edge, and you reappear on the left.

Spawning a field of them

def spawn_asteroids(count, width, height):
    asteroids = []
    for _ in range(count):
        x = random.uniform(0, width)
        y = random.uniform(0, height)
        asteroids.append(Asteroid(x, y))
    return asteroids

asteroids = spawn_asteroids(5, 800, 600)

Updating and drawing all of them

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # ... player input handling from the previous lesson ...
    player.update()

    for asteroid in asteroids:
        asteroid.update(800, 600)

    screen.fill((10, 10, 30))
    player.draw(screen)
    for asteroid in asteroids:
        asteroid.draw(screen)
    pygame.display.flip()
    clock.tick(60)

Looping over asteroids twice, once to update() and once to draw(), is deliberate: every asteroid's position should be fully updated before any of them are drawn, otherwise you'd occasionally draw a half-updated frame (some asteroids moved, some not yet), causing visible stutter.