[pygame] having problem swith RenderUpdate

[KKarasu <catsareninja@xxxxxxxxx>]

Hi there!

After reading a couple of tutorials i just don't get how to clear just 
the area the sprite changed.

exemple: i have an arrow, i blit it on the screen with a group.draw, and 
i dont know how to just update the area that changed by this method. i 
could blit again the entire screen, but i just want that spot, that way 
i dont have to blother bliting a hole lot of other stuff.

# this is just the method part the rest is uninportant

def start(self):
          
self.rect=(self.image.get_rect()).move(self.posicao[0],self.posicao[1]) 
# this moves the object by messing with the rect

         a=R_up.draw(screen)  #this draws the group sprites  (R_up= 
thats RenderUpdate group)
#iundestand that returns the changed areas so it says in the docs i 
should pass it to pygame.display.update
#so thats what i do
    pygame.display.update(a)


so it doesnt work.
can anyone explainme how to do this?
  
<http://www.pygame.org/docs/ref/display.html#pygame.display.update>