> g = ((i+2)**2 for i in range(10))
> list(g)
[4, 9, 16, 25, 36, 49, 64, 81, 100, 121]
> list(g)  #便利结束后再次访问时内部元素已经清空
[]

> g = ((i+2)**2 for i in range(10))  #重新建立一个生成器对象
> next(g)
4
> next(g)
9
> g.__next__()
16
> g.__next__()
25
> g.__next__()
36
> next(g)
49
> next(g)
64
> next(g)
81
> next(g)
100
> next(g)  #当next的下一个超出生成的范围时，抛出异常
121
> next(g)
Traceback (most recent call last):
 File "<pyshell#41>", line 1, in <module>
  next(g)
StopIteration