How to package a sequence functions that act on parameter in order in Python

Question

Imagine there are three functions, all them accept and return the same type args.

Normally, we can write it as fun3(fun2(fun1(args)), this can be say that a sequence function act on parameter in order, which likes one variety Higher-order functions "map".

You know in Mathematica, we can write this as fun3@fun2@fun1@args.

Now the question is that can we integrate fun3@fun2@fun1 as another fun without modifying their definition, so fun(args) can replace fun3(fun2(fun1(args)), this looks more elegant and concise.

woody · Accepted Answer · 2020-06-18 11:40:00Z

2

def merge_steps(*fun_list):
    def fun(arg):
        result = arg
        for f in fun_list:
            result = f(result)
        return result

    return fun


def plus_one(arg):
    return arg + 1


def double_it(arg):
    return arg ** 2


def power_ten(arg):
    return arg ** 10


combine1 = merge_steps(power_ten, plus_one, double_it)
combine2 = merge_steps(plus_one, power_ten, double_it)

combine1(3) 
> 3486902500

or use lambda:

steps = [power_ten, plus_one, double_it]

reduce(lambda a, f: f(a), steps, 3)
> 3486902500

answered Jun 18, 2020 at 11:40

woody

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1 Comment

eason Over a year ago

really cool , it is my ideal ,especially that the combine of lambda and reduce suits me, it is a chip off the mathematica " f@@expr or Apply[f,expr] " like f2@f1@t gives f2[f1[t]] , without define a function . Thanks

EdKenbers · Accepted Answer · 2020-06-18 10:44:22Z

1

I think you can use Function Recursion in python to do this.

def function(args, times):
    print(f"{times} Times - {args}")
    if times > 0 : 
        function(args,times - 1)

function("test", 2)

Note: I just add times argument to not generate infinite loop.

answered Jun 18, 2020 at 10:44

EdKenbers

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1 Comment

eason Over a year ago

it gives me that it goes far.

FMc · Accepted Answer · 2020-06-18 11:33:22Z

1

I'm not certain I understand your question, but are you talking about function composition along these lines?

# Some single-argument functions to experiment with.

def double(x):
    return 2 * x

def reciprocal(x):
    return 1 / x

# Returns a new function that will execute multiple single-argument functions in order.

def compose(*funcs):
    def g(x):
        for f in funcs:
            x = f(x)
        return x
    return g

# Demo.

double_recip_abs = compose(double, reciprocal, abs)
print(double_recip_abs(-2))   # 0.25
print(double_recip_abs(.1))   # 5.0

edited Jun 18, 2020 at 11:33

answered Jun 18, 2020 at 10:58

FMc

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3 Comments

eason Over a year ago

it works also, but it doesn't seperate the new function generation and its parameter call , 'compose' returns the final result , not the new function。

FMc Over a year ago

@eason OK, now I think I understand your goal. See edited answer.

eason Over a year ago

absolutely, the use of closures, it hit me, but I am an introductory python er.

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