python - 如何在 Python 3 中使用过滤器、映射和归约

Question

filter、map 和 reduce 在 Python 2 中完美运行。下面是一个示例:

>>> 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

但在 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

如果有人能向我解释为什么会这样，我将不胜感激。

代码截图更清楚:

Answer 1

您可以阅读 What's New In Python 3.0 中的更改。 .当您从 2.x 迁移到 3.x 时，您应该仔细阅读它，因为已经发生了很多变化。

这里的全部答案是从文档中引用的。

Views And Iterators Instead Of Lists

Some well-known APIs no longer return lists:

• [...]
• 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).
• [...]

Builtins

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