生成器输出小于max_num的fibonacci数列
代码：
def feb(max_num):
n_1 = 1
n_2 = 1
n = 0
while n < max_num:
if n == 0 or n == 1:
yield 1
n += 1
else:
yield n_1 + n_2
new_n_2 = n_1
n_1 = n_1 + n_2
n_2 = new_n_2
n += 1
g = feb(5)
for n in g:
print(n)