alien.py

import pygame
from pygame.sprite import Sprite

class Alien(Sprite):
    """A class to represent a single alien in the fleet."""
    def __init__(self,ai_settings,screen):
        """Initialize the alien and set its starting position."""
        super(Alien,self).__init__()
        self.screen=screen
        self.ai_settings=ai_settings

        #Load the alien image and set its rect attribute.
        self.image=pygame.image.load('alien.bmp')
        self.rect=self.image.get_rect()

        #start each new alien near the top left of the screen.
        self.rect.x=self.rect.width
        self.rect.y=self.rect.height

        #store the alien's exact position.
        self.x=float(self.rect.x)


    def blitme(self):
        """Draw the alien at its current location."""
        self.screen.blit(self.image,self.rect)

    def update(self):
        self.x+=(self.ai_settings.alien_speed_factor*self.ai_settings.fleet_direction)
        self.rect.x=self.x

    def check_edges(self):
        screen_rect=self.screen.get_rect()
        if self.rect.right >=screen_rect.right:
            return True
        elif self.rect.left<=0:
            return True