如何在 Python 3 中使用过滤器、映射和缩减 [英] How to use filter, map, and reduce in Python 3
问题描述
filter
、map
和 reduce
在 Python 2 中完美运行.这是一个例子:
但在 Python 3 中,我收到以下输出:
<预><代码>>>>过滤器(f,范围(2, 25))<过滤器对象在 0x0000000002C14908>>>>地图(立方体,范围(1, 11))<0x0000000002C82B70处的映射对象>>>>减少(添加,范围(1, 11))回溯(最近一次调用最后一次):文件<pyshell#8>",第 1 行,在 <module> 中减少(添加,范围(1, 11))NameError:未定义名称reduce"如果有人能向我解释为什么会这样,我将不胜感激.
为了更清晰的代码截图:
您可以阅读 Python 3.0 中的新功能.当您从 2.x 迁移到 3.x 时,您应该仔细阅读它,因为发生了很多变化.
此处的全部答案均来自文档.
<块引用>一些著名的 API 不再返回列表:
- [...]
map()
和filter()
返回迭代器.如果你真的需要一个列表,一个快速的解决方法是例如list(map(...))
,但更好的解决方法通常是使用列表推导式(尤其是当原始代码使用 lambda 时),或者重写代码使其不需要列表根本.特别棘手的是map()
为函数的副作用而调用;正确的转换是使用常规的for
循环(因为创建列表只会浪费).- [...]
- [...]
- 删除了
reduce()
.使用functools.reduce()
如果你真的需要它;然而,在 99% 的情况下,显式for
循环更具可读性. - [...]
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
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
I would appreciate if someone could explain to me why this is.
Screenshot of code for further clarity:
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.
The whole answer here are quotes from the documentation.
Views And Iterators Instead Of Lists
Some well-known APIs no longer return lists:
- [...]
map()
andfilter()
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 ismap()
invoked for the side effects of the function; the correct transformation is to use a regularfor
loop (since creating a list would just be wasteful).- [...]
- [...]
- Removed
reduce()
. Usefunctools.reduce()
if you really need it; however, 99 percent of the time an explicitfor
loop is more readable.- [...]
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