Spawning an alien to chase the player
Long multiple choice decisions in python work well as dictionaries, rather than elifs.  

def handle_event(event):
    OnKeyDown = {K_w:(0, 4), K_s:(0,-4), K_a:(-4, 0), K_d:(4,0)}
    OnKeyUp = {K_w:(False, True), K_s:(False, True), K_a:(True, False), K_d:(True, False)}

    if event.type == KEYUP:
         delta = OnKeyDown[event]
         movex += delta[0]
         movey += delta[1]
    elif event.type == KEYDOWN:
         delta = OnKeyUp[event]
         if delta[0]: movex = 0
         if delta[1]: movey = 0

   x += movex
   y += movey