alien.py

import pygame
from pygame.sprite import Sprite
class Alien(Sprite):
	"""class which is single alien"""
	def __init__(self,ai_settings,screen):
		"""init alien and set its init position"""
		super(Alien,self).__init__()
		self.screen = screen
		self.ai_settings = ai_settings
		#load alien image, and set its rect attributes
		self.image = pygame.image.load('images/alien.bmp')
		self.rect = self.image.get_rect()
	
		#every alien is right-left nearby
		self.rect.x = self.rect.width
		self.rect.y = self.rect.height
	
		#store alien position
		self.x = float(self.rect.x)
	
	def blitme(self):
		"""draw aliens in given position"""
		self.screen.blit(self.image, self.rect)
	
	def check_edges(self):
		"""if alien is on the edges,return True"""
		screen_rect = self.screen.get_rect()
		if self.rect.right >= screen_rect.right:
			return True
		elif self.rect.left <= 0:
			return True
			
	def update(self):
		"""right move aliens"""
		self.x += (self.ai_settings.alien_speed_factor*
						self.ai_settings.fleet_direction)
		self.rect.x = self.x