input: Write a function that prints a 7x7 grid where each cell is either a '#' or a space, based on whether the sum of its row and column indices is even or odd. If the sum is even, print '#'; otherwise, print a space.
  
  def print_grid():
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()
  
  assert print_grid()=="""
  #######
   ##### 
  #######
   ##### 
  #######
   ##### 
  #######
  """
  assert print_grid()=="""
  #######
   ##### 
  #######
   ##### 
  #######
   ##### 
  #######
  """
  assert print_grid()=="""
  #######
   ##### 
  #######
   ##### 
  #######
   ##### 
  #######
  """
  assert print_grid()=="""
  #######
   ##### 
  #######
   ##### 
  #######
   ##### 
  #######
  """
  assert print_grid()=="""
  #######
   ##### 
  #######
   ##### 
  #######
   ##### 
  #######
  """
output: def print_grid():
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()