invaders in new game wandering

[deanhystad]

A few problems.

Your aliens list is messed up. Each alien appears twice in the list.

for row_number in range(5):
    for column_number in range(6):
        x = alien_horizontal_spacing + column_number * (alien_width + alien_horizontal_spacing)
        y = alien_vertical_spacing + row_number * (alien_height + alien_vertical_spacing)
        alien = alien_image.get_rect(x = alien_horizontal_spacing + column_number * (alien_width + alien_horizontal_spacing), y = alien_vertical_spacing + row_number * (alien_height + alien_vertical_spacing))
        aliens.append(alien)   # Adding rectangle here
         
        a= row_number *20
        b= column_number * 20
        invader = Invader(x, y)
        aliens.append(alien)   # Adding same rectangle here

Your event loop is wrong. You only process pygame once at the start of the program. Instead of this:

running = True
if running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
            #sys.quit()
             
    while True:
        screen.fill(0)
        for alien in aliens:
            alien.x += alien_speed
            if alien.x <= 0 or alien.x >= screen_width-alien_width:
                alien_speed *= -1
                for alien in aliens:
                    alien.y += 20
        for alien in aliens:
                screen.blit(alien1,(alien))

It should look like this:

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        screen.fill(0)
        for alien in aliens:
            alien.x += alien_speed
            if alien.x <= 0 or alien.x >= screen_width-alien_width:
                alien_speed *= -1
                for alien in aliens:
                    alien.y += 20
        for alien in aliens:
                screen.blit(alien1,(alien))

That will let you quit the game. Eventually when you add key press or mouse events you'll need the event loop to continue running during the entirety of the game.

You don't synchronize when the aliens change direction. You change direction part way through moving the aliens. Let's say we have 4 aliens

1   2
3   4

The aliens are moving left to right (speed = 1) when alien 2 collides with the right side of the screen. When this happens, the aliens change direction (speed = -1) and drop 20 pixels in the Y direction. Because alien 1 and alien 2 moved right 1 pixel before the direction change, and because aliens 3 and 4 are going to move left instead of right, aliens 1 and 2 are 2 pixels further right (+x) than aliens 3 and 4.

  1   2
3   4

What you should do is reverse all the aliens at the same time. Like this:

reverse = False
        for alien in aliens:
            alien.x += alien_speed
            reverse = reverse or alien.x <= 0 or alien.x >= screen_width-alien_width
        if reverse:
            alien_speed *= -1
            for alien in aliens:
                alien.y += 20

You are not using sprites the way they are meant to be used. A sprite is an image and a rectangle. When a sprite is designed correctly you can group sprites and greatly reduce how much code you need to write. I would write your code like this:

import pygame

class Invader(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        # sprites should have an image and a rect.  This lets you draw all the sprites in a group.
        self.image = alien1
        self.rect = self.image.get_rect()
        self.rect.x = x
        self.rect.y = y

    def update(self, x=0, y=0):
        # Having an update method lets you update all the sprites in a group.
        self.rect.x += x
        self.rect.y += y

    def left(self):
        # returns x coordinate for left side of sprite
        return self.rect.left

    def right(self):
        # returns x coordinate for right side of sprite.
        return self.rect.right


pygame.init()
clock = pygame.time.Clock()  # Should use a clock to regulate the update rate of the game.
alien_image = pygame.image.load("invader_image.png")
alien1 = pygame.transform.scale(alien_image, (50, 50))
aliens = pygame.sprite.Group(
    (
        Invader(100, 50),
        Invader(50, 100),
        Invader(150, 100),
        Invader(100, 150),
    )
)
alien_speed = 8
screen_width = 1200
screen_height = 800
screen = pygame.display.set_mode((screen_width, screen_height))

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
    if any(a.left() <= 0 or a.right() >= screen_width for a in aliens):  # Check if any alien reached edge of window.
        # yes
        alien_speed = -alien_speed
        aliens.update(alien_speed, 20)  # Moves all the aliens with 1 call.
    else:
        # no
        aliens.update(alien_speed)
    screen.fill(0)
    aliens.draw(screen)  # Draws all the aliens with 1 call.
    pygame.display.update()
    clock.tick(60)  # Regulates time so this loop runs 60 times a second.

pygame.quit()

If you don't use a clock, your game will run differently on different computers.