def lines(file):for line in file:yield lineyield '\n'def blocks(file):block=[]for line in lines(file):if line.strip():block.append(line)elif block:yield ''.join(block).strip()block=[]請問下大蝦，上面的elif block中block列表為判斷條件是什么意思，什么情況下才執行這個語句？還有下面的yield 代碼是啥意思呢？