Move the aliens – to the right only:
alien_start_time = time.time()
ALIENS_STEP_TIME = 0.8
...
while True:
...
# ########################################
# Change the World
# ########################################
# Move alien
if now - alien_start_time > ALIENS_STEP_TIME:
alien_columns = [(column + 1) % fb.width for column in alien_columns]
alien_start_time = now
# Check for collision
if missile_y == alien_row and missile_x in alien_columns:
alien_columns.remove(missile_x)
missile_x, missile_y = -1, -1
if not alien_columns:
break
...
if alien_columns:
print("Ouch!")
else:
print("You win!")
Full project: