补充生成器（yield用法）

加入yield 关键字函数变为generator，执行next遇到yield中断返回值

from functools import reduce
def odd():
    print('step1')
    yield 1
    print('step2')
    yield 2
    print('step3')
    yield 3

def fib(max):
    n, a, b = 0, 0, 1
    while n < max:
        yield(b)
        a, b = b, a + b
        n = n + 1

for i in fib(6):
    print(i)
    
o = odd()#一定要先赋给一个变量，负责直接拿odd() next是不会变的
for i in range(3):
    print(next(o))

1
1
2
3
5
8
step1
1
step2
2
step3
3