Write pure and functional code in Python using the concept higher-order functions like in JavaScript

Question

How would one use higher-order functions (functions returning other functions) in Python?

This is my JavaScript example, whose programming concept I would like to use in Python as well. Let's say, for example, that I would like to wrap a string in an HTML element. In JavaScript, it would look like this:

var wrapInElement = function(ele) {
    return function(inp) {
        return "<" + ele + ">" + inp + "</" + ele + ">";
    };
};

And then I would use it like this:

var names = ["Mike", "Tony", "John"];
names.map(wrapInElement("p"));

How could this look like in Python? Is there a way to write functions that return customized functions like in the example above? One could also write a class in Python that accomplishes this task but wouldn't that be too much for such a basic task?

have you tried to translate your JS code into Python? have you tried anything at all? — Peter Varo
– Peter Varo, Commented Nov 19, 2013 at 9:52
Actually, I was so sure it wouldn't work that way that I didn't try what was offered as an solution. I probably should have done that, as one can really translate it one to one. — Xiphias
– Xiphias, Commented Nov 19, 2013 at 10:09

georg · Accepted Answer · 2013-11-19 10:06:44Z

4

In python, functions are first-class citizens, just like in javascript:

def adder(n):
    def add_func(x):
        return x + n
    return add_func

add3 = adder(3)
print map(add3, [1,2,3])  # [4, 5, 6]

This is not the only option, for example adder can also be written as:

def adder(n):
    return lambda x: x + n

Note that functional style is not quite pythonic, when possible, we prefer comprehensions and generators over high-order functions:

wrapped = ["<p>{}</p>".format(name) for name in names]

edited Nov 19, 2013 at 10:06

answered Nov 19, 2013 at 9:57

georg

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Comments

Matt Fenwick · Accepted Answer · 2013-11-19 17:44:44Z

2

Your JavaScript example can be rewritten (arguably more cleanly) using partial application:

// depends on `_` -- see link above
function wrapInElement(ele, inp) {
    return "<" + ele + ">" + inp + "</" + ele + ">";
};

var f = _.partial(wrapInElement, "p")

... then use f as a normal function ...

And pretty much the same way in Python:

from functools import partial

def wrap_in_element(ele, inp):
    return "<%s>%s</%s>" % (ele, inp, ele)

f = partial(wrap_in_element, "p")

... then use f as a normal function ...

While yes, one can use higher-order functions in both Python and JavaScript, if you find yourself returning functions in order to roll-your-own partial application mechanism, you should realize that this ugliness (which appears in both languages' examples) is not necessary thanks to the awesome libraries out there.

answered Nov 19, 2013 at 17:44

Matt Fenwick

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1 Comment

Xiphias Over a year ago

Let me please put emphasis on the fact that this solution is even simpler than the one that I marked as the correct answer. I didn't know partial applications, neither in JavaScript nor in Python. This is quite interesting. Thank you.

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