Python中的generator学习

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本文深入探讨了Python中的生成器(generator)概念，包括其与迭代器(iteration)的区别、如何使用yield关键字创建生成器、以及如何利用生成器提高代码效率。通过实例展示了生成器的基本用法及其优势。

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generator

1. Python中的generator学习

1 Python中的generator学习

Iteration
generator
Iteration和generator的区别
多线程支持情况

1.1 Iteration

For语句使用Iteration在Python中for语句迭代的数据都是支持Iteration协议的如:
```
for x in [1,2,3,4]:
    print x,
....
1 2 3 4
```
Iteration的其它使用情况
- 函数入参为Iterable类型sum(s), min(s), max(s)
- 构造函数list(s), tuple(s), set(s), dict(s)
- in操作符item in s

Iteration协议

拥有__iter__方法
拥有next方法

如:

In [10]: items = [1,3,7]

In [11]: it = iter(items)

In [12]: it.next()
Out[12]: 1

In [13]: it.next()
Out[13]: 3

In [14]: it.next()
Out[14]: 7

In [15]: it.next()
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
/home/rcy/lib/python2.7/site-packages/django/core/management/commands/shell.pyc in <module>()
----> 1 it.next()

StopIteration:

翻议for语句

for x in obj:
     # statements
<==>
_iter = iter(obj)
while 1:
    try:
        x = _iter.next()
    except StopIteration:
        break
    # statements

自定议的iteration类型

class Range(object):
    def __init__(self, num, start = 0, step = 1):
        self.index = start
        self.step  = step
        self.end   = num

    def __iter__(self):
        return self

    def next(self):
        if self.index >= self.end:
            raise StopIteration
        r = self.index
        self.index += self.step
        return r

for x in Range(10):
    print x,

1.2 Generator

Generator是返回值为序列的函数如:

def Range(num, start=0,step=1):
    while start < num:
        yield start
        start += step

for x in Range(10):
    print x

Generator函数在调用时并不会运行如:

def Range(num, start=0,step=1):
    print "Hello generation"
    while start < num:
        yield start
        start += step
x = Range(10)

Generator函数返回对象拥有next函数,在调用next函数时开始运行,并且到第一个yield语句返回值,并暂停运行,在下一次调用next时再次运行.如:

x = Range(10) # 这里并没有print "Hello generation"
x.next()
"Hello generation"
0
x.next()

当generator函数返回时,迭代结束

x.next()
9
x.next()
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
/home/robin/Env/mezproj/lib/python2.7/site-packages/django/core/management/commands/shell.pyc in <module>()
----> 1 x.next()

StopIteration:

generator除了可以使用yield函数实现,也可以使用下面方法

a = [1,2,3,4,5]
b = (x*x for x in a) # 这里x为generator

(expression for i in s if cond1
            for j in t if cond2
            ...
            if condfinal)
<==>
for i in s:
    if cond1:
        for j in t:
            if cond2:
                ...
                if condfinal:
                    yield expression

generator的中断方法调用throw GeneratorExit

def close_generator(gen):
    try:
        gen.throw(GeneratorExit)
    except (GeneratorExit, StopIteration):
        pass
    else:
        raise RuntimeError("generator ignored GeneratorExit")

1.3 Iteration和generator的区别

generator是一次性运算,并不能反复使用;Iteration生成对象可以多次使用generator还可以同过调用send函数往object内部传值,而iterable对象是没有send函数的

def hello():
    print "Hello"
    x = yield 1
    print " ", x
    y = yield 2
    print " over with ",y
h = hello()
m = h.next() # or h.send(None), not h.send(msg)send传值时必序在yield中断时
print m
n = h.send('robin')
print n
h.send('success')

Created: 2014-01-05 Sun 20:57

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