Python：为什么在打印列表时在末尾出现“无”？

Question

我可以在打印此列表时摆脱“无”一词吗？

from random import randint
board = []
for i in range(0,5):
  board.append(["O"] * 5)

def print_board(board_in):
  for x in board_in:
print (" ".join(x))

def random_row(board_in):
  row = randint(0, len(board_in)-1)
  return row

def random_col(board_in):
  col = randint(0, len(board_in)-1)
  return col

random_row(board)
random_col(board)

print (print_board(board))

现在，出现的内容是：

O O O O O
O O O O O
O O O O O
O O O O O
O O O O O
None

我如何摆脱“无”？为什么还在那里？我不明白...