生成器

在Python中，這種一邊循環(huán)一邊計(jì)算的機(jī)制，稱為生成器：generator。

創(chuàng)建生成器方法1

要?jiǎng)?chuàng)建一個(gè)生成器，有很多種方法。第一種方法很簡(jiǎn)單，只要把一個(gè)列表生成式的 [ ] 改成 ( )
可以用next(list)打印出來(lái)

In [18]: l = [x for x in range(5)]

In [19]: l
Out[19]: [0, 1, 2, 3, 4]

In [20]: l = (x for x in range(5))

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)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-27-101c36968c6d> in <module>()
----> 1 next(l)

創(chuàng)建生成器方法2

著名的斐波拉契數(shù)列（Fibonacci），除第一個(gè)和第二個(gè)數(shù)外，任意一個(gè)數(shù)都可由前兩個(gè)數(shù)相加得到：

In [28]: def fib(times):
   ....:     n = 0
   ....:     a,b = 0,1
   ....:     while n<times:
   ....:         print(b)
   ....:         a,b = b,a+b
   ....:         n+=1
   ....:     return 'done'
   ....: 

In [29]: fib(5)
1
1
2
3
5
Out[29]: 'done'

In [30]: def fib(times):
   ....:     n = 0
   ....:     a,b = 0,1
   ....:     while n<times:
   ....:         yield b
   ....:         a,b = b,a+b
   ....:         n+=1
   ....:     return 'done'
   ....: 

In [31]: F = fib(5)

In [32]: next(F)
Out[32]: 1

In [33]: next(F)
Out[33]: 1

In [34]: next(F)
Out[34]: 2

In [35]: next(F)
Out[35]: 3

In [36]: next(F)
Out[36]: 5

In [37]: next(F)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-37-8c2b02b4361a> in <module>()
----> 1 next(F)