优雅地按顺序应用 Python 函数列表

问题描述

我有一个输入值 val 和一个按顺序应用的函数列表:

I have an input value val and a list of functions to be applied in the order:

funcs = [f1, f2, f3, ..., fn]

如何优雅地应用而不是写作

How to apply elegantly and not writing

fn( ... (f3(f2(f1(val))) ... )

并且也不使用 for 循环:

and also not using for loop:

tmp = val
for f in funcs:
    tmp = f(tmp)

感谢 Martijn 的精彩回答.我发现了一些阅读:https://mathieularose.com/function-composition-in-python/ .

Thanks Martijn for the awesome answer. There's some reading I found: https://mathieularose.com/function-composition-in-python/ .

推荐答案

使用 reduce() 函数:

Use the reduce() function:

# forward-compatible import
from functools import reduce

result = reduce(lambda res, f: f(res), funcs, val)

reduce() 将第一个参数，一个可调用的，应用于从第二个参数中取出的每个元素，加上到目前为止的累积结果(如 (result, element)).第三个参数是一个起始值(否则将使用 funcs 中的第一个元素).

reduce() applies the first argument, a callable, to each element taken from the second argument, plus the accumulated result so far (as (result, element)). The third argument is a starting value (the first element from funcs would be used otherwise).

在 Python 3 中，内置函数被移至 functools.reduce() 位置;为了向前兼容，Python 2.6 及更高版本中提供了相同的参考.

In Python 3, the built-in function was moved to the functools.reduce() location; for forward compatibility that same reference is available in Python 2.6 and up.

其他语言可能将此称为折叠.

Other languages may call this folding.

如果您还需要每个函数的中间结果，请使用 itertools.accumulate()(仅从 Python 3.3 开始，对于采用函数参数的版本):

If you need intermediate results for each function too, use itertools.accumulate() (only from Python 3.3 onwards for a version that takes a function argument):

from itertools import accumulate, chain
running_results = accumulate(chain(val, funcs), lambda res, f: f(res))

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