text: #!/usr/bin/python3.4
  
  import sys
  import turtle
  
  def border (t, screen_x, screen_y):
      
      """(Turtle, int, int)
      Draws a border around the canvas in red.
      """
      # Lift the pen and move the turtle to the center.
      t.penup ()
      t.home ()
  
      # Move to lower left corner of the screen; leaves the turtle
      # facing west.
      
      t.forward (screen_x / 2)
      t.right(90)
      t.forward(screen_y / 2)
      t.setheading(180)           # t.right(90) would also work.
      
      # Draw the border
      t.pencolor('red')
      t.pendown()
      t.pensize(10)
      for distance in (screen_x, screen_y, screen_x, screen_y):
          t.forward(distance)
          t.right(90)
  
      # Raise the pen and move the turtle home again; it's a good idea
      # to leave the turtle in a known state.
      t.penup()
      t.home()
  
  def square(t, size, color):
      """(Turtle, int, str)
  
      Draw a square of the chosen colour and size.
      """
      t.pencolor(color)
      t.pendown()
      for i in range(4):
          t.forward(size)
          t.right(90)
  
  def main():
      
      # Create screen and turtle.
      screen = turtle.Screen()
      screen.title('Square Demo')
      screen_x, screen_y = screen.screensize()
      t = turtle.Turtle()
  
      # Uncomment to draw the graphics as quickly as possible.
      ##t.speed(0)
  
      # Draw a border around the canvas
      border(t, screen_x, screen_y)
  
      # Draw a set of nested squares, varying the color.
      # The squares are 10%, 20%, etc. of half the size of the canvas.
      colors = ['red', 'orange', 'yellow', 'green', 'blue', 'violet']
      t.pensize(3)
      for i, color in enumerate(colors):
          square(t, (screen_y / 2) / 10 * (i+1), color)
  
      print('Hit any key to exit')
      dummy = input()
          
  if __name__ == '__main__':
      main()
  