The Game Loop

Every real-time game (not just Asteroids) is built around one core idea: the game loop, an endless cycle that processes input, updates state, and draws the next frame, over and over, many times per second.

The three phases

import pygame

pygame.init()
screen = pygame.display.set_mode((800, 600))
clock = pygame.time.Clock()

running = True
while running:
    # 1. Handle input/events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # 2. Update game state
    # (nothing to update yet)

    # 3. Draw the frame
    screen.fill((10, 10, 30))  # dark space background
    pygame.display.flip()

    clock.tick(60)  # cap the loop at 60 frames per second

pygame.quit()

• Handle events: pygame.event.get() drains the queue of everything that happened since the last frame (key presses, window close, etc). Without checking for pygame.QUIT, clicking the window's close button would do nothing.
• Update: where you'll move the player, move asteroids, check collisions, this lesson's loop doesn't update anything yet, but the slot is there.
• Draw: screen.fill(...) clears the previous frame to a solid color, then pygame.display.flip() actually shows everything you've drawn since the last flip, nothing is visible on screen until you call it.

Why clock.tick(60)?

clock = pygame.time.Clock()
# ... inside the loop:
clock.tick(60)

clock.tick(60) pauses just long enough to cap the loop at 60 iterations per second. Without it, the loop would run as fast as the CPU allows, hundreds or thousands of times per second, burning a full CPU core and making movement speed depend on how fast the computer happens to be. Capping the frame rate keeps the game's speed consistent across different machines.