old_contents: #!/usr/bin/env python
  import unicornhathd
  import os
  
  try:
      unicornhathd.set_pixel(0, 0, 255, 255, 255)
      unicornhathd.set_pixel(15, 0, 255, 255, 255)
      unicornhathd.set_pixel(0, 15, 255, 255, 255)
      unicornhathd.set_pixel(15, 15, 255, 255, 255)
      unicornhathd.show()
      raw_input("Press the <ENTER> key or <CTRL+C> to exit...")
  except KeyboardInterrupt:
      pass
  unicornhathd.off()
  
new_contents: #!/usr/bin/env python
  import unicornhathd
  import os
  from time import sleep
  
  class Point:
      def __init__(self, x, y, dx, dy):
          self.x = x
          self.y = y
          self.dx = dx
          self.dy = dy
  
      def turn_on(self):
          unicornhathd.set_pixel(self.x, self.y, 255, 255, 255)
  
      def turn_off(self):
          unicornhathd.set_pixel(self.x, self.y, 0, 0, 0)
      
      def move(self):
          self.turn_off()
          self.x, self.dx = self.move_one_axis(self.x, self.dx)
          self.y, self.dy = self.move_one_axis(self.y, self.dy)
          self.turn_on()
  
      def move_one_axis(self, x_or_y, dx_or_dy):
          x_or_y += dx_or_dy
          if x_or_y < 0 or x_or_y > 15:
              dx_or_dy = dx_or_dy * -1
              x_or_y += dx_or_dy
          return x_or_y, dx_or_dy
  
  print("Press <CTRL+C> to exit...")
  
  unicornhathd.off()
  
  # Bounce backwards and forwards along each edge:
  p1 = Point(0, 0, 0, 1)
  p2 = Point(0, 15, 1, 0)
  p3 = Point(15, 0, -1, 0)
  p4 = Point(15, 15, 0, -1)
  p1.turn_on()
  p2.turn_on()
  p3.turn_on()
  p4.turn_on()
  
  try:
      while True:
          p1.move()
          p2.move()
          p3.move()
          p4.move()
          unicornhathd.show()
          sleep(0.1)
  except KeyboardInterrupt:
      pass
  
  unicornhathd.off()
  
subject: Make the 4 lights bounce backwards and forwards along each edge.