def countdown(n):
    print "conunting down form" ,n
    while n>=0:
        print "first n:",n 
        newvalue=(yield n)
        print "second n:",n 
        print "first newvalue",newvalue

        if newvalue is not None:
            n=newvalue
        else:
            n-=1
        print "second newvalue",newvalue
c=countdown(5)
for n in c:
    # print n 
    if n==5:
        c.send(3)

输出结果：
conunting down form 5
first n: 5
second n: 5
first newvalue 3
second newvalue 3
first n: 3
second n: 3
first newvalue None
second newvalue None
first n: 2
second n: 2
first newvalue None
second newvalue None
first n: 1
second n: 1
first newvalue None
second newvalue None
first n: 0
second n: 0
first newvalue None
second newvalue None

这个期间发送的send值是不是改变了n的值，是不是会继续进行n=3的迭代？