Python3异步编程：yield与yield from

本是古典何须时尚 2022-08-28 11:48 192阅读 0赞

## yield ##

yield把函数变为generator  
gennerator需要收到一次send(None)或者next才开始执行函数  
函数执行了yield语句后，会挂起，直到外部调用send传递一个对象作为yield语句返回值，则继续函数  
generator中如果执行return语句也会向常规函数一样终止  
yield会执行完下一条语句后，挂起这个方法，保存该语句结果  
直到外部调用next或者send才继续方法的执行  
比如下面例子

def Test3():
        print("Test3 in")
        for i in range(3):
            s = yield print("do yield test3 %s" % i)
            print("Test3 get %s" % s)
        print("Test3 end")
    
    print("start3")
    t = Test3()
    print("start3 send")
    t.send(None)
    print("start3 send end")
    for i in range(2):
        print("start3 range %s" % i)
        t.send(i)
    print("start3 range end")
    """
    --------------------------OUTPUT---------------------
    start3
    start3 send
    Test3 in
    do yield test3 0
    start3 send end
    start3 range 0
    Test3 get 0
    do yield test3 1
    start3 range 1
    Test3 get 1
    do yield test3 2
    start3 range end
    """

未终止的generator(还有yield挂起在等待send)也可以通过调用close方法强行终止

## yield from ##

yield from 迭代器等价于

for xx in 迭代器:
    	yield xx

因为next(g)等价于g.send(None)

class Generator(Iterator):
        __slots__ = ()
        def __next__(self):
            return self.send(None)

所以可以这样写

def Test():
        r = yield from [1,2,3,4,5]
        print("t %s" % r)
    g = Test()
    for i in range(5):
        print(next(g))

## 嵌套使用 ##

再看看嵌套使用的例子

def InnerYieldPrint(i):
        print("inner yield %s" % i)
        return i
    
    def InnerYield():
        print("InnerYield")
        for i in range(5):#因为是5所以算上启动要外部发送6次
            s = yield InnerYieldPrint(i)
            print("inner get %s" % i)
            print("Test2 give inner %s" % s)
        print("InnerYield end")
        yield 99#因为send每次都要执行到下一个yield语句，才算结束，不然会抛出异常
    
    def Test():
        yield from InnerYield()
        print("yield from end")
    
    def Test2():
        """等价于Test"""
        print("Test2")
        x = InnerYield()
        print("go die")
        iCnt = 0
        for r in x:#等价于 r = next(x)，for in会捕捉stopiteration异常退出循环，那么第一次就是启动InnerYield
            iCnt += 1
            print("cnt %s" % iCnt)
            print("prepare Test2 %s" % r)
            if iCnt >= 6:
                print("break")
                break
            s = yield r#这里返回作为外面的send返回值了
            print("Test2 %s" % s)
        print("yield from end")
        yield 999#因为send每次都要执行到下一个yield语句，才算结束，不然会抛出异常
    
    g = Test2()
    print("start")
    print(g.send(None))  # 启动Test2
    for i in [-1, -2, -3, -4, -5,]:
        print("pro %s" % i)
        print("pro send res %s" % g.send(i))
     
     """
    --------------------------OUTPUT---------------------
    start
    Test2
    go die
    InnerYield
    inner yield 0
    cnt 1
    prepare Test2 0
    0
    pro -1
    Test2 -1
    inner get 0
    Test2 give inner None
    inner yield 1
    cnt 2
    prepare Test2 1
    pro send res 1
    pro -2
    Test2 -2
    inner get 1
    Test2 give inner None
    inner yield 2
    cnt 3
    prepare Test2 2
    pro send res 2
    pro -3
    Test2 -3
    inner get 2
    Test2 give inner None
    inner yield 3
    cnt 4
    prepare Test2 3
    pro send res 3
    pro -4
    Test2 -4
    inner get 3
    Test2 give inner None
    inner yield 4
    cnt 5
    prepare Test2 4
    pro send res 4
    pro -5
    Test2 -5
    inner get 4
    Test2 give inner None
    InnerYield end
    cnt 6
    prepare Test2 99
    break
    yield from end
    pro send res 999
    
     """

> 这里比较奇特的是，因为for in会捕捉stopiteration异常退出循环，所以所以InnerYield中的yield  
> 99可以改为return 99而Test2中的yield 999不能改为return，除非外部改为

g = Test2()
    for i in g:
    	pass

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