yield：生成器

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def count(n):
    while n > 0:
        yield n   #生成值：n
        n -= 1
if __name__ == '__main__':
    c=count(2)
    print c.next()
    print c.next()
    print c.next()

./iter.py
2
1
Traceback (most recent call last):
  File "./iter.py", line 26, in <module>
    print c.next()
StopIteration

###################################################3333

 

18 def count(n):
 19     print 'counting'
 20     while n > 0:
 21         print 'n :%d'%n
 22         print ' before yield'
 23         yield n   #生成值：n
 24         n -= 1
 25         print 'after yield'
 26 if __name__ == '__main__':
 27     c=count(2)
 28     print 'call next'
 29     print c.next()
 30     print c.next()
"iter.py" 46L, 1073C written                                                                               
common> ./iter.py
call next
counting
n :2
 before yield
2
after yield
n :1
 before yield
1
after yield
Traceback (most recent call last):
  File "./iter.py", line 31, in <module>
    print c.next()
StopIteration

 

在调用count函数时：c=count(5)，并不会打印"counting"只有等到调用c.next()时才真正执行里面的语句。每次调用next()方法时，count函数会运行到语句yield n处为止，next()的返回值就是生成值n，再次调用next()方法时，函数继续执行yield之后的语句

 

for i in count(5):
    print i,

 

 

 

 

 

 

 

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