如何在Python 3中使用过滤，映射和归约 [英] How to use filter, map, and reduce in Python 3

问题描述

filter，map和reduce在Python 2中完美工作.这是一个示例:

filter, map, and reduce work perfectly in Python 2. Here is an example:

>>> def f(x):
        return x % 2 != 0 and x % 3 != 0
>>> filter(f, range(2, 25))
[5, 7, 11, 13, 17, 19, 23]

>>> def cube(x):
        return x*x*x
>>> map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

>>> def add(x,y):
        return x+y
>>> reduce(add, range(1, 11))
55

但是在Python 3中，我收到以下输出:

But in Python 3, I receive the following outputs:

>>> filter(f, range(2, 25))
<filter object at 0x0000000002C14908>

>>> map(cube, range(1, 11))
<map object at 0x0000000002C82B70>

>>> reduce(add, range(1, 11))
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    reduce(add, range(1, 11))
NameError: name 'reduce' is not defined

如果有人能向我解释为什么会这样，我将不胜感激.

I would appreciate if someone could explain to me why this is.

代码截屏以进一步清晰:

Screenshot of code for further clarity:

推荐答案

您可以在

You can read about the changes in What's New In Python 3.0. You should read it thoroughly when you move from 2.x to 3.x since a lot has been changed.

这里的完整答案是文档中的引言.

The whole answer here are quotes from the documentation.

视图和迭代器代替列表

Views And Iterators Instead Of Lists

一些著名的API不再返回列表:

Some well-known APIs no longer return lists:

• [...]
• map() 和

• [...]
• map() and filter() return iterators. If you really need a list, a quick fix is e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all. Particularly tricky is map() invoked for the side effects of the function; the correct transformation is to use a regular for loop (since creating a list would just be wasteful).
• [...]

公告

Builtins

• [...]
• 已删除reduce().如果确实需要，请使用 functools.reduce() ；但是，在99％的情况下，显式for循环更易于阅读.
• [...]

• [...]
• Removed reduce(). Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.
• [...]

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