Space Invaders Game Tutorial: Build Classic Arcade with Python Pygame

Setting Up the Project

Here's the recommended folder structure for your project. All file and folder names are case-sensitive!

1. Set Up and Import Libraries

import math
import random
import pygame
from pygame import mixer
pygame.init()

• 'math' is used for collision detection using distances.
• 'random' is for placing enemies in random positions.
• 'pygame' is the core library for making 2D games.
• 'mixer' (from pygame) lets us play sounds/music.
• 'pygame.init()' starts up everything you need for a graphical game.

2. Create the Game Window

screen = pygame.display.set_mode((800, 600))

• This creates the main game window, 800 pixels wide and 600 pixels tall.
• The top-left of the window is at coordinates (0,0).

3. Load Images, Sound, and Set the Window Caption/Icon

background = pygame.image.load('Images/Background.jpg')
mixer.music.load('Sound/background.wav')
mixer.music.play(-1)  # background music loops forever
pygame.display.set_caption('Space Invaders') # window title
icon = pygame.image.load('Images/UFO.png')
pygame.display.set_icon(icon) # sets the window icon

• Loads and sets up your background, game icon, and background music.
• 'mixer.music.play(-1)' makes the music repeat continuously.

4. Set Up the Player

playerImg = pygame.image.load('Images/Player.png')
playerX = 370
playerY = 480
playerX_change = 0

• Loads the player ship image.
• Sets its starting position near the bottom center of the window.
• 'playerX_change' is used to move left/right when the player presses keys.

5. Create a Group of Enemies

enemyImg = []
enemyX = []
enemyY = []
enemyX_change = []
enemyY_change = []
no_of_enemies = 6
for i in range(no_of_enemies):
    enemyImg.append(pygame.image.load('Images/Alien.png'))
    enemyX.append(random.randint(0, 736))
    enemyY.append(random.randint(50, 150))
    enemyX_change.append(0.3)
    enemyY_change.append(40)

• Sets up 6 enemies.
• Each enemy gets a random starting position near the top of the screen.
• Each enemy moves sideways ('enemyX_change'), and drops down a little ('enemyY_change') when it hits the screen edge.

6. Add a Bullet Mechanic

bulletImg = pygame.image.load('Images/Bullet.png')
bulletX = 0
bulletY = 480
bulletX_change = 0
bulletY_change = 2
bullet_state = 'ready'

• Bullet can be fired by the player.
• 'bullet_state' can be 'ready' (player can shoot) or 'fire' (bullet is moving).

7. Fonts & Score

score_value = 0
font = pygame.font.Font('Font/bladesinger.ttf', 30)
textX = 10
textY = 10
over_font = pygame.font.Font('Font/bladesinger.ttf', 64)

• Sets up the font and display position for the current score.
• Creates a larger font for the Game Over message.

8. Helper Functions for Drawing and Display

def show_score(x, y):
    score = font.render('score:' + str(score_value), True, (255, 255, 255))
    screen.blit(score, (x, y))
def game_over_text():
    over_text = over_font.render('GAME OVER', True, (255, 255, 255))
    screen.blit(over_text, (200, 250))
def player(x, y):
    screen.blit(playerImg, (x, y))
def enemy(x, y, i):
    screen.blit(enemyImg[i], (x, y))
def fire_bullet(x, y):
    global bullet_state
    bullet_state = 'fire'
    screen.blit(bulletImg, (x+16, y+10))

• 'screen.blit(image, (x, y))' draws images at specified positions.
• These functions handle placing text, the player, bullet, and enemies on the screen.

9. Detecting Collisions

def isCollision(enemyX, enemyY, bulletX, bulletY):
    distance = math.sqrt(math.pow(enemyX - bulletX, 2) + math.pow(enemyY - bulletY, 2))
    if distance < 27:
        return True
    else:
        return False

• Uses the **distance formula** to determine if a bullet is close enough to an enemy for a collision to occur.

10. Main Game Loop

This part keeps your game running, checks for player input, moves all elements, checks for collisions, draws everything, and updates the display.

running = True
while running:
    screen.fill((0, 0, 0)) # Fills the screen with black
    screen.blit(background, (0, 0)) # Draws the background
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT:
                playerX_change = -0.4
            if event.key == pygame.K_RIGHT:
                playerX_change = 0.4
            if event.key == pygame.K_SPACE:
                if bullet_state == 'ready':
                    bullet_Sound = mixer.Sound('Sound/laser.wav')
                    bullet_Sound.play()
                    bulletX = playerX
                    fire_bullet(bulletX, bulletY)
        if event.type == pygame.KEYUP:
            if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                playerX_change = 0
    playerX += playerX_change
    if playerX <= 0:
        playerX = 0
    elif playerX >= 736:
        playerX = 736

• Handles quitting the game and keyboard input.
• Sets the bullet into motion if the spacebar is pressed.
• Makes sure the player doesn't move off screen.

Moving Enemies and Handling Game Over

for i in range(no_of_enemies):
    if enemyY[i] > 450:
        for j in range(no_of_enemies):
            enemyY[j] = 2000
        game_over_text()
        break
    enemyX[i] += enemyX_change[i]
    if enemyX[i] <= 0:
        enemyX_change[i] = 0.3
        enemyY[i] += enemyY_change[i]
    elif enemyX[i] >= 736:
        enemyX_change[i] = -0.3
        enemyY[i] += enemyY_change[i]
    collision = isCollision(enemyX[i], enemyY[i], bulletX, bulletY)
    if collision:
        explosion_Sound = mixer.Sound('Sound/explosion.wav')
        explosion_Sound.play()
        bulletY = 480
        bullet_state = 'ready'
        score_value += 1
        enemyX[i] = random.randint(0, 736)
        enemyY[i] = random.randint(50, 150)
    enemy(enemyX[i], enemyY[i], i)

• Moves enemies left and right, bounces them off the edges.
• When enemies reach the player, moves them off screen and displays GAME OVER.
• When a collision happens, plays a sound, increases the score, resets the bullet, and sends the enemy back to the top.

Bullet Movement

if bulletY <= 0:
    bulletY = 480
    bullet_state = 'ready'
if bullet_state == 'fire':
    fire_bullet(bulletX, bulletY)
    bulletY -= bulletY_change

• The bullet only moves if its state is 'fire'.
• If it moves off the top of the screen, its state is reset to 'ready', so the player can shoot again.

Drawing Everything & Updating the Screen

player(playerX, playerY)
show_score(textX, textY)
pygame.display.update()

• Draws the player and score on each iteration of the loop.
• 'pygame.display.update()' actually renders all your updated graphics to the game window.