alien.py

import pygame
from pygame.sprite import Sprite

class Alien(Sprite):
    """A class representaion of an alien ship"""

    def __init__(self, ai_game):
        """Initialize alien and set position"""
        super().__init__()
        self.screen = ai_game.screen
        self.settings = ai_game.settings

        #Load alien image to rect
        self.image = pygame.image.load('images/alien.bmp')
        self.rect = self.image.get_rect()

        #Start each alien on 0,0 coordinates
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height

        #Store horizontal position of alien
        self.x = float(self.rect.x)

    def check_edges(self):
        """Return True if edge is hit by alien"""
        screen_rect = self.screen.get_rect()
        return (self.rect.right >= screen_rect.right) or (self.rect.left <= 0)

    def update(self):
        """Moving to the right or left"""
        self.x += self.settings.alien_speed *  self.settings.fleet_direction
        self.rect.x = self.x