如何在Python 3中使用过滤，映射和归约

迈不过友情╰ 2022-11-24 11:21 114阅读 0赞

本文翻译自：[How to use filter, map, and reduce in Python 3][How to use filter_ map_ and reduce in Python 3]

*`filter` , `map` , and `reduce` work perfectly in Python 2. Here is an example:* **`filter` ， `map`和`reduce`可以在Python 2中完美地工作。这是一个示例：**

>>> 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

*But in Python 3, I receive the following outputs:* **但是在Python 3中，我收到以下输出：**

>>> 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

*I would appreciate if someone could explain to me why this is.* **如果有人可以向我解释为什么，我将不胜感激。**

*Screenshot of code for further clarity:* **代码的屏幕截图，以进一步阐明：**

[![Python 2和3的IDLE会话并排][Python 2_3_IDLE]][Python 2_3_IDLE]

--------------------

### \#1楼 ###

参考：[https://stackoom.com/question/Ve6e/如何在Python-中使用过滤-映射和归约][https_stackoom.com_question_Ve6e_Python-_-]

--------------------

### \#2楼 ###

*The functionality of `map` and `filter` was intentionally changed to return iterators, and reduce was removed from being a built-in and placed in `functools.reduce` .* **`map`和`filter`的功能被有意地更改为返回迭代器，并且reduce不再是内置的，而是放置在`functools.reduce` 。**

*So, for `filter` and `map` , you can wrap them with `list()` to see the results like you did before.* **因此，对于`filter`和`map` ，可以用`list()`将它们包装起来，以像以前一样查看结果。**

>>> def f(x): return x % 2 != 0 and x % 3 != 0
    ...
    >>> list(filter(f, range(2, 25)))
    [5, 7, 11, 13, 17, 19, 23]
    >>> def cube(x): return x*x*x
    ...
    >>> list(map(cube, range(1, 11)))
    [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
    >>> import functools
    >>> def add(x,y): return x+y
    ...
    >>> functools.reduce(add, range(1, 11))
    55
    >>>

*The recommendation now is that you replace your usage of map and filter with generators expressions or list comprehensions.* **现在的建议是，用生成器表达式或列表推导替换map和filter的用法。** *Example:* **例：**

>>> def f(x): return x % 2 != 0 and x % 3 != 0
    ...
    >>> [i for i in range(2, 25) if f(i)]
    [5, 7, 11, 13, 17, 19, 23]
    >>> def cube(x): return x*x*x
    ...
    >>> [cube(i) for i in range(1, 11)]
    [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
    >>>

*They say that for loops are 99 percent of the time easier to read than reduce, but I'd just stick with `functools.reduce` .* **他们说for循环在99％的时间里都比减少容易阅读，但是我只是坚持使用`functools.reduce` 。**

***Edit** : The 99 percent figure is pulled directly from the [What's New In Python 3.0][What_s New In Python 3.0] page authored by Guido van Rossum.* **编辑 ：99％的数字直接从Guido van Rossum编写[的“ Python 3.0的新功能”][What_s New In Python 3.0]页面中提取。**

--------------------

### \#3楼 ###

*You can read about the changes in [What's New In Python 3.0][What_s New In Python 3.0 1] .* **您可以阅读[Python 3.0的新增功能中][What_s New In Python 3.0 1]的更改。** *You should read it thoroughly when you move from 2.x to 3.x since a lot has been changed.* **从2.x升级到3.x时，您应该仔细阅读该文档，因为已经做了很多更改。**

*The whole answer here are quotes from the documentation.* **此处的完整答案是文档中的引号。**

***[Views And Iterators Instead Of Lists][]*** **[视图和迭代器而不是列表][Views And Iterators Instead Of Lists]**

> *Some well-known APIs no longer return lists:* **一些著名的API不再返回列表：**
> 
>  *  *\[...\]* **\[...\]**
>  *  *[`map()`][map] and [`filter()`][filter] return iterators.* **[`map()`][map]和[`filter()`][filter]返回迭代器。** *If you really need a list, a quick fix is eg `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.* **如果您确实需要列表，则快速修复方法是例如`list(map(...))` ，但是更好的解决方法通常是使用列表理解（尤其是当原始代码使用lambda时），或者重写代码以使其不根本不需要列表。** *Particularly tricky is `map()` invoked for the side effects of the function;* **为函数的副作用调用`map()`尤其棘手；** *the correct transformation is to use a regular `for` loop (since creating a list would just be wasteful).* **正确的转换是使用常规的`for`循环（因为创建列表将很浪费）。**
>  *  *\[...\]* **\[...\]**

***[Builtins][]*** **[内建][Builtins]**

>  *  *\[...\]* **\[...\]**
>  *  *Removed `reduce()` .* **删除了`reduce()` 。** *Use [`functools.reduce()`][functools.reduce] if you really need it;* **如果确实需要，请使用[`functools.reduce()`][functools.reduce] 。** *however, 99 percent of the time an explicit `for` loop is more readable.* **但是，显式`for`循环在99％的时间中更具可读性。**
>  *  *\[...\]* **\[...\]**

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### \#4楼 ###

*As an addendum to the other answers, this sounds like a fine use-case for a context manager that will re-map the names of these functions to ones which return a list and introduce `reduce` in the global namespace.* **作为其他答案的补充，对于上下文管理器来说，这听起来像是一个很好的用例，它将重新将这些函数的名称映射为返回列表的函数，并将`reduce`引入全局名称空间。**

*A quick implementation might look like this:* **快速实现可能如下所示：**

from contextlib import contextmanager    
    
    @contextmanager
    def noiters(*funcs):
        if not funcs: 
            funcs = [map, filter, zip] # etc
        from functools import reduce
        globals()[reduce.__name__] = reduce
        for func in funcs:
            globals()[func.__name__] = lambda *ar, func = func, **kwar: list(func(*ar, **kwar))
        try:
            yield
        finally:
            del globals()[reduce.__name__]
            for func in funcs: globals()[func.__name__] = func

*With a usage that looks like this:* **用法如下所示：**

with noiters(map):
        from operator import add
        print(reduce(add, range(1, 20)))
        print(map(int, ['1', '2']))

*Which prints:* **哪些打印：**

190
    [1, 2]

*Just my 2 cents :-)* **只是我的2美分:-)**

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### \#5楼 ###

*Since the `reduce` method has been removed from the built in function from Python3, don't forget to import the `functools` in your code.* **由于`reduce`方法已从Python3的内置函数中删除，因此不要忘记将`functools`导入代码中。** *Please look at the code snippet below.* **请查看下面的代码段。**

import functools
    my_list = [10,15,20,25,35]
    sum_numbers = functools.reduce(lambda x ,y : x+y , my_list)
    print(sum_numbers)

--------------------

### \#6楼 ###

*Here are the examples of Filter, map and reduce functions.* **以下是Filter，map和reduce函数的示例。**

> > > *numbers = \[10,11,12,22,34,43,54,34,67,87,88,98,99,87,44,66\]* **数字= \[10,11,12,22,34,43,54,34,67,87,88,98,99,87,44,66\]**

*//Filter* **//过滤**

> > > *oddNumbers = list(filter(lambda x: x%2 != 0, numbers))* **奇数=列表（filter（lambda x：x％2！= 0，数字））**
> > > 
> > > *print(oddNumbers)* **打印（奇数）**

*//Map* **//地图**

> > > *multiplyOf2 = list(map(lambda x: x\*2, numbers))* **multipleOf2 = list（map（lambda x：x \* 2，数字））**
> > > 
> > > *print(multiplyOf2)* **打印（multiplyOf2）**

*//Reduce* **//降低**

*The reduce function, since it is not commonly used, was removed from the built-in functions in Python 3. It is still available in the functools module, so you can do:* **由于不常用reduce函数，因此已从Python 3的内置函数中删除了它。functools模块中仍提供了reduce函数，因此您可以执行以下操作：**

> > > *from functools import reduce* **从functools进口减少**
> > > 
> > > *sumOfNumbers = reduce(lambda x,y: x+y, numbers)* **sumOfNumbers = reduce（lambda x，y：x + y，数字）**
> > > 
> > > *print(sumOfNumbers)* **打印（sumOfNumbers）**

[How to use filter_ map_ and reduce in Python 3]: https://oldbug.net/q/Ve6e/How-to-use-filter-map-and-reduce-in-Python-3
[Python 2_3_IDLE]: https://i.imgur.com/WekeD.png
[https_stackoom.com_question_Ve6e_Python-_-]: https://stackoom.com/question/Ve6e/%E5%A6%82%E4%BD%95%E5%9C%A8Python-%E4%B8%AD%E4%BD%BF%E7%94%A8%E8%BF%87%E6%BB%A4-%E6%98%A0%E5%B0%84%E5%92%8C%E5%BD%92%E7%BA%A6
[What_s New In Python 3.0]: https://docs.python.org/3.0/whatsnew/3.0.html
[What_s New In Python 3.0 1]: http://docs.python.org/3.0/whatsnew/3.0.html
[Views And Iterators Instead Of Lists]: http://docs.python.org/3.0/whatsnew/3.0.html#views-and-iterators-instead-of-lists
[map]: http://docs.python.org/3.0/library/functions.html#map
[filter]: http://docs.python.org/3.0/library/functions.html#filter
[Builtins]: http://docs.python.org/3.0/whatsnew/3.0.html#builtins
[functools.reduce]: http://docs.python.org/3.0/library/functools.html#functools.reduce