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python之yield表达式学习

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Release： 2016-06-16 08:42:11
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python中有一个略微奇怪的表达式叫yield expression，本文就来探究一下这是个什么东西。一步一步来。

iterable

复制代码 代码如下:

mylist = [1,2,3]
for item in mylist:
    print str(item)

mylist是一个列表（list），我们可以逐条取出每一个item，这个过程叫做iteration。像list这样可以用”for…in…”依次遍历的对象被称为iterable，其他的iterable还有string、tuple、dict等。iterable的一个特点是所有的item会存储到内存中，这样会产生一些不便和不利的地方，于是催生了generator（后面讲到）。

list comprehension(列表推导式)

复制代码 代码如下:

mylist = [x*x for x in range(3)]

表达式右边是一个for循环的简写形式，用[]包裹起来（称为list comprehension），表达式的值是一个list，我们可以像普通list那样使用”for…in…”遍历其元素，如：
复制代码 代码如下:

for item in mylist:
    print str(item)
generator

generator

对上面的list comprehension稍作修改：

复制代码 代码如下:

mygenerator = (x*x for x in range(3))
for item in mygenerator:
    print item

可以看到只是把[]换成了()，这时表达式的值不再是list，而是一个generator。

generator也属于iterable，但是其调用方式非常特别。

yield

复制代码 代码如下:

def creatGenerator():
    mylist = range(3)
    for x in mylist:
        yield x*x
       
mygenerator = creatGenerator()

for x in mygenerator:
    print(x)


yield的使用方法和return是一样的。但是（重点来了）：
Related labels：
python yield
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