title: error with first simple game with Pygame
body: <pre><code>import os, sys
  import pygame
  from pygame.locals import *
  
  pygame.init()
  screen = pygame.display.set_mode((800, 600))
  pygame.mouse.set_visible(False)
  
  def loadimage(name, colorkey=None):
      try:
          pygame.image.load(os.path.join(name))
      except pygame.error, message:
          print "Failed to load image ",name
  
  
  class Userplane(pygame.sprite.Sprite):
      def __init__(self):
          pygame.sprite.Sprite.__init__(self)
          self.image, self.rect = loadimage("userplane.bmp")
  
      def update(self):
          pos = pygame.mouse.get_pos()
          self.rect.midtop = pos
  
  def main():
  
      pygame.init()
      screen = pygame.display.set_mode((800, 600))
      pygame.mouse.set_visible(False)
  
      background = pygame.Surface(screen.get_size())
      background = background.convert()
      background.fill((255,255,255))
  
  
      screen.blit(background, (0,0))
      pygame.display.flip()
  
      user = Userplane()
      allsprites = pygame.sprite.RenderPlain((user))
      clock = pygame.time.Clock()
  
      while 1:
          clock.tick(60)
          for event in pygame.event.get():
              if event.type == QUIT:
                  return
              elif event.type == KEYDOWN and event.key == K_ESCAPE:
                  return
  
          allsprites.update()
  
      screen.blit(background(0,0))
      pygame.display.flip()
  
  if __name__ == '__main__': main()
  </code></pre>
  
  <p>hi im very new, ive just finished the codeacademy course and the chimppunch game in the pygame tutorial. Im using almost an exact copy of that game but instead im trying to display a plane at the bottom of the screen instead of a fist. my Userplane class keeps throwing the following error and i dont know why, 'typeerror: 'nonetype' object is not iterable</p>