实现一个生成器，给定一个长度为y的列表每次取出x个元素；
def l_yield(lists,num):
y = len(lists)
while y >= num:
x,y = y,y - num
yield lists[y:x]
else:
if topics[0:y%num]:
yield lists[0:y%num]
def output(lst,num):
test = lst[:]
while len(test) > 0:
print test[:num]
test[:num] = []