Python 列表为判断条件?

Question

def lines(file):
for line in file:yield line
yield '\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？