生成器（generator），迭代器（yield）

提示：扫一扫查出行
【扫一扫了解最新限行尾号】
复制提示

g=(i for i in range(10)) #小括号表示生成一个迭代生成器。[]是列表生成器

g.__next__()

yield将一个函数变成生成器

import time

def f():

    for i in range(10):

        yield i  #如果换成return i，则最终只能返回0。

        print('aa')

m=f()    #m是一个generator

print(m)

print(dir(m))

print(m.__next__())

print(m.__next__())

time.sleep(2)

print(m.__next__())

按字节读取文本：

def rb():

    f=open('e:\\a.txt','rb')

    while True:

        size=6 #每次读取6个字节

        block=f.read(size)

        if block:

            yield block

        else:

            break

    f.close()

rr=rb()

for r in rr:

    print r

yield中的next()和send()：

def yed():

    print "enter fun",

    yield 10

yy = yed()

yy.next() #返回"enter fun"
print yy.next() #返回 enter fun 10

def yed():

    print "enter fun",

    yield 10

    print "next1"

    yield 20

yy = yed()

yy.next()

yy.next()

# #返回enter fun next1

def yed():

    print "enter fun",

    yield 10

    print "next1"

    yield 20

yy = yed()

yy.next()

yy.send('hello') #此处send的参数hello被yield 10接收了，但是并没有进行打印，所以不会被打印

#返回enter fun next1

def yed():

    print "enter fun",

    y1 = yield 10

    print "next1",

    print y1

    yield 20

yy = yed()

yy.next()

yy.send('hello') #send发送的值会成为当前yield的结果，即yield 10，即y1，并且send的返回结果是下一个yield的结果（相当于了next()）

#返回 enter fun next1 hello

def yed():

    print "enter fun",

    y1 = yield 10

    print "next1",

    print y1

    y2 = yield 20

yy = yed()

mn = yy.next() #结果为yield 10的参数，即10

ms = yy.send('hello') #结果为yield 20的参数，即20。hello被y1接收到了

print 'mn: %s, ms: %s' %(mn,ms)

#返回enter fun next1 hello

#mn: 10, ms: 20

第一次调用时，要使用next()语句或是send(None)，不能使用send发送一个非None的值，否则会出错的，因为没有yield语句来接收这个值。所以第一次时next() == send(None)
send(msg) 和 next()是有返回值的，它们的返回值很特殊，返回的是下一个yield表达式的参数。比如yield 10，则返回10 。

协同处理数据：http://www.tuicool.com/articles/VBfAzaQ

我们现在利用yield关键字会自动冻结函数堆栈的特性，想象一下，假如现在有两个函数f1()和f2(),各自包含yield语句，见下例。主线程先启动f1(), 当f1()执行到yield的时候，暂时返回。这时主线程可以将执行权交给f2()，执行到f2()的yield后，可以再将执行权交给f1()，从而实现了在同一线程中交错执行f1()和f2()。f1()与f2()就是协同执行的程序，故名协程。

import random

#消费生成的数据

def consume():

    print 'waiting to consume'

    while True:

        data = yield

        sum_data = sum(data)

        print '%s was consumed,the sum is %d ' %(str(data),sum_data)

#生成数据

def produce():

    print 'begin to produce'

    consumer = consume()

    consumer.next()

    while True:

        data = random.sample(range(10),3)

        print '%s produced' %(str(data))

        consumer.send(data)

        yield

producer = produce()

for i in range(5):

    producer.next()    #直接执行produce()里面的代码：在produce里面先生成data，然后再将其发送给consume

可以优化为如下：

import random

#消费生成的数据

def consume():

    print 'waiting to consume'

    while True:

        data = yield

        sum_data = sum(data)

        print '%s was consumed,the sum is %d ' %(str(data),sum_data)

#生成数据

def produce(consumer):

    print 'begin to produce'

    while True:

        data = random.sample(range(10),3)

        print '%s produced' %(str(data))

        consumer.send(data)

        yield

consumer = consume()

producer = produce(consumer)

consumer.next() #打印waiting to consum

for i in range(5):

    producer.next()    #直接执行produce()里面的代码：在produce里面先生成data，然后再将其发送给consume

返回结果如下：

waiting to consume

begin to produce

[2, 9, 3] produced

[2, 9, 3] was consumed,the sum is 14

[1, 5, 2] produced

[1, 5, 2] was consumed,the sum is 8

[8, 5, 4] produced

[8, 5, 4] was consumed,the sum is 17

[6, 1, 2] produced

[6, 1, 2] was consumed,the sum is 9

[8, 6, 3] produced

[8, 6, 3] was consumed,the sum is 17

异步并发？？？？

from time import sleep,ctime

def consumer(name):

    print '%s want to consume' % name

    while True:

        bb = yield

        print '%s was consumed by %s' %(bb,name)

def producer(pname):

    c1=consumer('A')

    c2=consumer('B')

    c1.next()

    c2.next()

    print '%s begin to produce' % pname

    for i in range(5):

        sleep(1)

        print '%s produce 2' % pname

        c1.send(i)

        c2.send(i)

producer('zhangsan')

返回结果如下：

A want to consume

B want to consume

zhangsan begin to produce

zhangsan produce 2

0 was consumed by A

0 was consumed by B

zhangsan produce 2

1 was consumed by A

1 was consumed by B

zhangsan produce 2

2 was consumed by A

2 was consumed by B

zhangsan produce 2

3 was consumed by A

3 was consumed by B

zhangsan produce 2

4 was consumed by A

4 was consumed by B