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The game loop: a game's heartbeat

A game looks alive because it redraws itself many times a second. That repeating cycle — clear, update, draw — is the game loop. It's the heartbeat of every game.

The big idea

The game loop repeats three steps every frame: clear the stage, update where things are, then draw them.

See it in code

1Start simple

A loop's job is to repeat. Here the same three steps — clear, update, draw — run three times in a row. That repeat is the whole idea:

python
for frame in range(3):
    print("clear")
    print("update")
    print("draw")
Run it — the same three steps, over and over:
clear
update
draw
clear
update
draw
clear
update
draw

Three trips around the loop, three sets of steps. A game does this too — the same steps, again and again.

2A step further

A game loop also counts how many times it runs. Each trip we add 1 to frames, and the loop keeps going until frames reaches 5:

python
frames = 0
while frames < 5:
    frames = frames + 1
    print("Frame", frames)

print("The loop ran", frames, "frames")
Run it — the loop counts each frame as it goes:
Frame 1
Frame 2
Frame 3
Frame 4
Frame 5
The loop ran 5 frames

The loop ran until frames reached 5, then stopped. A real game loop runs like this — just way more times, and it moves things too.

3In our world

Now the real thing. Each time around the loop we do three things: Stage.clear wipes the last picture, move_down updates the position a little, and draw paints the new one. Over and over, that slides the enemy down the screen:

python
from game import Stage, Enemy

screen = Stage.create()
enemy = Enemy(240, 60)

frames = 0
while not enemy.at_bottom():
    Stage.clear(screen)     # 1. clear the stage
    enemy.move_down()       # 2. update positions
    enemy.draw(screen)      # 3. draw everything
    frames += 1

print("The loop ran", frames, "frames")
Run it — the loop runs hundreds of times to move the enemy down:
The loop ran 500 frames
A single purple enemy sprite resting near the bottom of a dark play area.

It took 500 trips around the loop to bring the enemy down. In a real game the loop keeps going with while running:. It does clear, update, draw many times a second. That's why everything looks so smooth.

The same idea, everywhere

Every game has this beating heart: clear, update, draw, repeat. It's how a phone game and your game both keep the action going. Change what happens in update and draw, and you change the whole game.

Try it yourself

Give the enemy enemy.speed = 3 before the loop, and watch the frame count drop — bigger steps mean fewer frames. Then add a second enemy and move and draw it inside the same loop.

The common mistake

Forgetting to Stage.clear each frame. Skip it, and the old pictures stay on screen, so the enemy leaves a smeary trail instead of moving cleanly. Clear first, then draw fresh.

What it unlocks

The loop is powered by while loops and drives frames and motion, sprites, and game state.

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