Game States — Start → Play → Game Over

By the end of this lesson you can:

• Define a state variable with three possible values: "start", "play", and "over".
• Branch draw() and update() on state to show the correct screen.
• Advance through all three states with SPACE and reset back to the start screen.

Python can branch on a string just like a number. Predict the output:

state = "play"

if state == "start":
    print("Show title screen")
elif state == "play":
    print("Run the game")
elif state == "over":
    print("Show game over")
else:
    print("Unknown state")

Run the game

Think of a game like a traffic light. It can be red, amber, or green — but only one at a time. A state machine is exactly that: one variable holding the current mode, and code that behaves differently in each mode.

The state variable

state = "start"   # can be "start", "play", or "over"

Using plain strings instead of numbers (0, 1, 2) makes the code easy to read. You can see at a glance what each branch handles.

Branching draw() on state

def draw():
    screen.fill("black")
    if state == "start":
        screen.draw.text("Press SPACE to play",
                         center=(WIDTH // 2, HEIGHT // 2),
                         fontsize=30, color="white")
    elif state == "play":
        # draw the game world here
        pass
    elif state == "over":
        screen.draw.text("Game Over — SPACE to restart",
                         center=(WIDTH // 2, HEIGHT // 2),
                         fontsize=30, color="red")

Advancing the state

def on_key_down(key):
    global state
    if key == keys.SPACE:
        if state == "start":
            state = "play"
        elif state == "over":
            state = "start"   # restart the loop

The game moves from "start" to "play" on the first SPACE. When the game ends, code inside update() sets state = "over". A second SPACE press loops back to "start".

Aiman builds the full three-screen skeleton with a falling counter game as the play state. Save as game_states.py:

# game_states.py — start / play / over skeleton
import pgzrun
import random

WIDTH = 480
HEIGHT = 360
TITLE = "State Machine Demo"

state = "start"
score = 0
timer = 10.0       # seconds of play time

ball_x = 240
ball_y = 0
ball_speed = 3

def reset():
    global score, timer, ball_x, ball_y
    score = 0
    timer = 10.0
    ball_x = random.randint(20, WIDTH - 20)
    ball_y = 0

def update(dt):
    global state, score, timer, ball_x, ball_y
    if state != "play":
        return
    timer -= dt
    if timer <= 0:
        state = "over"
        return
    ball_y += ball_speed
    if ball_y > HEIGHT:
        ball_y = 0
        ball_x = random.randint(20, WIDTH - 20)
        score += 10

def on_key_down(key):
    global state
    if key == keys.SPACE:
        if state == "start":
            reset()
            state = "play"
        elif state == "over":
            state = "start"

def draw():
    screen.fill("black")
    if state == "start":
        screen.draw.text("FALLING BALL", center=(WIDTH // 2, 140),
                         fontsize=48, color="gold")
        screen.draw.text("Press SPACE to start",
                         center=(WIDTH // 2, 210), fontsize=26, color="white")
    elif state == "play":
        screen.draw.filled_circle((ball_x, int(ball_y)), 14, "dodgerblue")
        screen.draw.text(f"Score: {score}", topleft=(8, 8),
                         fontsize=22, color="white")
        screen.draw.text(f"Time: {timer:.1f}", topleft=(8, 34),
                         fontsize=22, color="white")
    elif state == "over":
        screen.draw.text("Time's Up!", center=(WIDTH // 2, 140),
                         fontsize=48, color="red")
        screen.draw.text(f"Final Score: {score}",
                         center=(WIDTH // 2, 200), fontsize=30, color="white")
        screen.draw.text("SPACE to play again",
                         center=(WIDTH // 2, 250), fontsize=22, color="grey")

pgzrun.go()

A state machine is one string variable (state) and branching code.draw() and update() both check state and behave differently in each mode. Pressing SPACE (or meeting a condition in update()) changes the state. Always reset counters and positions when moving back to "start".