Python 列表为判断条件?

Question

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=[]请问下大虾，上面的elif block中block列表为判断条件是什么意思，什么情况下才执行这个语句？还有下面的yield 代码是啥意思呢？