Sequential function mapping in python

问题

I have a bunch of functions in a list:

funcs = [f1, f2, f3, f4, f5]

and all of the functions take in return a single argument, eg.

f1 = lambda x: x*2

I'd like to map all these functions together

result = lambda x: f5(f4(f3(f2(f1(x)))))

or, iterating over funcs

def dispatch(x):
    for f in funcs:
        x = f(x)
    return x

dispatch works fine, but I couldn't figure out a clean way to do this using iterools. Is it possible? Does this sequential function mapping idiom have a name?

回答1:

There is no point in using itertools here; you are producing one output, and you could not apply this to an infinite iterable. You have to have a finite number of functions in the input iterable for this to work at all.

Use the reduce() function:

from functools import reduce

x = reduce(lambda res, func: func(res), funcs, x)

The functools.reduce() import helps the above work in both Python 2 and 3.

reduce(), together with map(), filter() and, yes, itertools, is an often used tool in functional programming.

回答2:

Another (less efficient, alas) way of looking at Martijn's answer is to realize that you want to compose the list of functions.

# function composition: compose(f,g)(x) = f(g(x))
def compose(f, g):
    return lambda x: f(g(x))

# Identity for function composition
# compose(f, identity)(x) = f(x)
identity = lambda x: x

# result(x) = f1(f2(...fn(x)...))
result = reduce(compose, funcs, identity)

来源：https://stackoverflow.com/questions/35398472/sequential-function-mapping-in-python

标签

python

itertools