• 在Python中，这种一边循环一边计算的机制，称为生成器：generator。

创建生成器方法(1)

• 要创建一个生成器，有很多种方法。第一种方法很简单，只要把一个列表生成式的 [ ] 改成 ( )
  可以用next(list)打印出来

In [18]: l = [x for x in range(10)]

In [19]: l
Out[19]: [0, 1, 2, 3, 4,5,6,7,8,9]

In [20]: l = (x for x in range(10))

In [21]: l
Out[21]: <generator object <genexpr> at 0x7f92fadaed58>
In [22]: next(l)
Out[22]: 0

In [23]: next(l)
Out[23]: 1

In [24]: next(l)
Out[24]: 2

In [25]: next(l)
Out[25]: 3

In [26]: next(l)
Out[26]: 4

In [27]: next(l)
Out[28]: 5

In [29]: next(l)
Out[30]: 6

In [31]: next(l)
Out[32]: 7

In [33]: next(l)
Out[34]: 8

In [35]: next(l)
Out[36]: 9
In [37]: next(l)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-37-101c36968c6d> in <module>()
----> 1 next(l)