yield, async

yield实现斐波那契序列：

import sys, time

def fib():
    a,b,c = 0,1,0
    while True:
        yield c
        a,b = b, c
        c = a + b

if __name__ == '__main__':
    fib_iter = fib()
    for i in range(int(sys.argv[1])):
        print(fib_iter.__next__())

Send:

send函数用来向fib_iter发送数据，这样数据就可以双向流动。我们尝试使用send来模拟一个比较慢的生成器，我们让它一秒钟生成一个数：

import sys, time

def fib():
    a,b,c = 0,1,0
    while True:
        sleep_sec = yield c
        time.sleep(sleep_sec)
        a,b = b, c
        c = a + b

def fib1():
    index = 1
    a = 0
    b = 1
    while index:
        yield b
        a, b = b, a + b
        index += 1

if __name__ == '__main__':
    fib_iter = fib()
    print(fib_iter.__next__())    #先执行一下，让它停留在yield
    for i in range(int(sys.argv[1])):
        result = fib_iter.send(1)   #
        print(result)

yield from 是什么？

在上面的yield中，我们通过for循环使用__next__()方法来获取下一个值，也就是说想要获取下一个值就要重新yield一下；yield from 它可以简化这个过程，看看实例：

import sys, time

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

def gener(n):
    yield from fib(n)

if __name__ == '__main__':
    print(list(gener(5000)))

执行结果：

D:\>python fib.py
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4
181]

yield from ， send传递信息：

import sys, time

def fib(n):
    a,b,c = 0,1,0
    while c < n:
        sleep_sec = yield c
        time.sleep(sleep_sec)
        a,b = b, c
        c = a + b

def gener(n):
    yield from fib(n)

if __name__ == '__main__':
    gen = gener(10)
    print(gen.send(None))
    print(gen.send(1))
    print(gen.send(1))

执行结果：

D:\>python fib.py
0
1
1

asyncio 和 yield from:

asyncio是一个基于事件循环的实现异步的I/O模块。通过yield from, 我们可以将协程asynico.sleep的控制权交给事件循环，然后挂起当前协程；之后，由事件循环决定何时唤醒asyncio.sleep，然后接着执行后面的代码; 实例中我们用sleep来模拟阻塞：

import sys, time, asyncio

def fib(n):
    a,b,c = 0,1,0
    while c < n:
        yield from asyncio.sleep(2)
        print('-->', c)
        a,b = b, c
        c = a + b

def stupid_fib(n):
    a,b,c = 0,1,0
    while c < n:
        yield from asyncio.sleep(2)
        print('==>', c)
        a,b = b,c
        c = a + b

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    tasks = [asyncio.async(fib(10)), asyncio.async(stupid_fib(10))]
    loop.run_until_complete(asyncio.wait(tasks))
    print('all task finished')
    loop.close()

执行结果：

D:\>python fib.py
--> 0
==> 0
--> 1
==> 1
--> 1
==> 1
--> 2
==> 2
--> 3
==> 3