input: 
  def print_checkerboard(n: int) -> None:
      """Prints an n by n checkerboard pattern.
      >>> print_checkerboard(7)
      # # # # #
       # # # # #
      # # # # #
       # # # # #
      # # # # #
       # # # # #
      # # # # #
      """
      for i in range(n):
          for j in range(n):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()
  
output:     for i in range(n):
          for j in range(n):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()