input: 
  def print_checkerboard():
      """ Generates and prints a checkerboard pattern of '#' and ' ' characters for a 7x7 grid.
      The pattern alternates every two cells, starting with '#' at the top-left corner.
  
      >>> print_checkerboard()
      # # # # # # 
       # # # # # #
      # # # # # # 
       # # # # # #
      # # # # # # 
       # # # # # #
      # # # # # # 
      """
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()
  
output:     for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()