def draw():
"""Your draw func.
0 is fully transparent, 255 is fully opaque
screen = screen
surface_start, surface_end are your two image surfaces to fade.
"""
surface_start.set_alpha(start)
surface_end.set_alpha(end)
screen.blit(surface_start)
screen.blit(surface_end)
# note: you probably want to add a timer, ie: only increment
# every Xms instead of every frame, unless you have a fixed FPS.
if start > 0: start -= 1
if end < 255: end += 1