> alienList = []
for z0 in range( COLUMNS ):
for z1 in range (ROWS ):
alien = Alien((LEFT + z0 * DISTANCE, TOP + z1 * DISTANCE))
alienList.append(alien)
... should read:
alienList = []
for z0 in range( COLUMNS ):
for z1 in range (ROWS ):
alien = Alien((LEFT + z0 * DISTANCE, TOP + z1 * DISTANCE))
alienList.append(alien)
of course.