Character movement bug in pygame

Question

I made a loop to keep the "enemy" character moving to left and right, but for some reason that I really can't tell why, he just goes to the right and when he hits the border limit that I defined, he "teleports" to the left border and keeps going to the right forever. I even tried to make the "enemyX_change" beeing negative, but he just goes to the left, hit the border and stops. I'm using Python 3.8.

import pygame
import random

# Initiate pygame
pygame.init()

# Display the game window
screen = pygame.display.set_mode((800, 600))

# Title and Icon
pygame.display.set_caption('Space Invaders')
icon = pygame.image.load('icon.png')
pygame.display.set_icon(icon)

# Player
playerSprite = pygame.image.load('player.png')
playerX = 370
playerY = 480
playerX_change = 0

# Enemy
enemySprite = pygame.image.load('enemy.png')
enemyX = random.randint(0, 736)
enemyY = random.randint(50, 150)
enemyX_change = 0.3
enemyY_change = 0


def player(x, y):
    screen.blit(playerSprite, (x, y))


def enemy(x, y):
    screen.blit(enemySprite, (x, y))


# Game Loop
running = True
while running:

    # Background color (RGB)
    screen.fill((0, 0, 0))

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

        # If a key is pressed, check if it's the right or left arrow key
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT:
                playerX_change = -0.3
            if event.key == pygame.K_RIGHT:
                playerX_change = 0.3
        if event.type == pygame.KEYUP:
            if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                playerX_change = 0

    # Move the spaceship to the left or right
    playerX += playerX_change

    # Prevents the player from going off the border
    if playerX <= 0:
        playerX = 0
    elif playerX >= 736:
        playerX = 736

    # Moves the enemy
    enemyX += enemyX_change

    # Prevents the enemy from going off the border
    if enemyX <= 0:
        enemyX = 0.3
    elif enemyX >= 736:
        enemyX = -0.3

    player(playerX, playerY)
    enemy(enemyX, enemyY)
    pygame.display.update()

Why should the enemy ever move to the left with your code? If enemyX is greater or equal 736 you decrease enemyX by -0.3 but in your loop you always add enemyX_change which is 0.3 too, so enemyX is unchanged once it reaches 736. — Matthias
– Matthias, Commented Apr 12, 2020 at 22:52

Rabbid76 · Accepted Answer · 2020-04-13 05:59:14Z

1

When the enemy goes out of bounds, then you have to change the movement direction (enemyX_change) rather than the position of the player (enemyX):

~~enemyX = -0.3~~

enemyX_change *= -1

Invert enemyX_change when it is less than 0 or greater than the width of the window:

# Moves the enemy
enemyX += enemyX_change

# Prevents the enemy from going off the border
if enemyX <= 0 or enemyX >= 736:
    enemyX_change *= -1

answered Apr 13, 2020 at 5:59

Rabbid76

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