def countdown(n): 
  print "counting down" 
  while n>0: 
    yield n #生成一个n值 
    n -=1 
>>> c = countdown(5) 
>>> c.next() 
counting down 
5 
>>> c.next() 
4 
>>> c.next() 
3 

>>> for i in countdown(5): 
...   print i 
...   
counting down 
5 
4 
3 
2 
1 

def h(n): 
  while n>0: 
    m = (yield n) 
    print "m is "+str(m) 
    n-=1 
    print "n is "+str(n) 
>>> p= h(5) 
>>> p.next() 
5 
>>> p.next() 
m is None 
n is 4 
4 
>>> p.send("test") 
m is test 
n is 3 
3