为什么Python3要把map函数改成了map class?费解

[lee Alexander]

我做了个测试，2.6中map函数遍历数组的速度比for循环快，为啥Python3中要去掉map函数，改成map类，经过测试改成map类后实现相同的功能比map函数满得多。

而且经过测试，Python3.1的for循环也比2.6来得慢，真是郁闷，怎么越整越回去了呢？

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[victor lee]

now, in 3.1, map is lazy

2011/1/12 lee Alexander <superp...@gmail.com>

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[lee Alexander]

lazy了有什么好处呢，为了Lazy一下效率降低太多了，很显然划不来，Python的函数既然已经是对象可以当参数传递了，那么传递函数对象好了，干嘛非要把map干掉呢？

[zhao shichen]

我也不理解,本来很好用的东西,给阉割了,就好像java对c++的阉割一样.

呆痴木讷，君子四德

[victor lee]

• First of all, Lazy is great when you wont use all the items at once or you are going to produce the thing one by one later

For Example

l=map(lambda x:do_some_func,list_)

the map wont do anything till you visit it, for example

for i in l :

or maybe

x=list(l)

 

• Secondly, map(func,iterable) is now similar to ( func(i) for i in iterable ), which is a generator instead of a list.

in py2, there, map is equals to [f(i) for i in x] which in py3 has the same effects.
Noticing that map is a bit slower than the [ f(i) for i in x ] as it is using funcs, and also in the below case , [ f(I) for i in x ] is much faster

map( lambda x:x+1,l)  # slower than [i+1 for i in x]

that is because python will first create a lambda

• Last but not least, in my opinion not everyone know the syntax of ( f(i) for i in x if condition(i) ), so there is a Map class to avoid the speed wasting
• PS : if you want just "faster" py2 style map, use [f(i) for i in l] instead.

sorry for bothering you using my poor english as my Input Method is broken
Best Regards
VIC
100.1.13

2011/1/13 lee Alexander <superp...@gmail.com>

[lee Alexander]

我重新测试了一次，又发现了一点奇怪的现象，按照 lambda x:x+1的方式测试map确实慢了很多，但是我测试的时候图方便，直接使用了str这个buildin函数，结果map就比for迭代和列表解析快，为了方便表达我直接贴代码了

ls=range(100000)

def loop1():

    for i in ls:

        x=i+1

def loop2():

    map(lambda x:x+1,ls)

def loop3():

    [i+1 for i in ls]

结果是

[0.29097795486450195, 0.30311298370361328, 0.28920412063598633]

[0.96499300003051758, 0.94327902793884277, 0.97041106224060059]

[0.34845209121704102, 0.33587312698364258, 0.34033107757568359]

map完败

但是我换成昨天实验的代码

ls=range(100000)

def loop1():

    for i in ls:

        str(i)

def loop2():

    map(str,ls)

def loop3():

    [str(i) for i in ls]

结果：

[1.5978419780731201, 1.5836057662963867, 1.6044631004333496]

[1.5450370311737061, 1.5522379875183105, 1.3477730751037598]

[2.1679210662841797, 1.9605419635772705, 2.2622189521789551]

str和map有什么不可告人的秘密？

[victor lee]

dont use time to have the timer
try timeit.
i think the loop you run is too small?

2011/1/13 lee Alexander <superp...@gmail.com>

[victor lee]

import timeit;
help(timeit) for documents

2011/1/13 victor lee <victor...@gmail.com>

[lee Alexander]

我就是timeit测试的

[shhgs]

lazy这都是跟FP学的。

至于你的这个所谓的性能测试，没有任何意义。这点性能的差别，不是因为map，是因为int和str的区别。

lazy有lazy的好处。

import itertools
seed = itertools.count()
f = map(lambda x : pow(x, 2), seed)

for i in f:
if i >= 100:
break
print(i)

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[pansz]

2011/1/13 victor lee <victor...@gmail.com>:

> sorry for bothering you using my poor english as my Input Method is broken
> Best Regards
> VIC
> 100.1.13

qq and sogou all have cloud input written in AJAX, you got input
method provide that you have a decent web-browser.