input: 
  def print_checkerboard():
      """Prints a 7x7 checkerboard pattern.
      >>> print_checkerboard()
      ## ## ##
       ## ## ##
      ## ## ##
       ## ## ##
      ## ## ##
       ## ## ##
      ## ## ##
      """
  
output:     for i in range(7):
          if i % 2 == 0:
              print("## ## ##")
          else:
              print(" ## ## ##")