How do I write a function that returns another function?

In Python, I'd like to write a function make_cylinder_volume(r) which returns another function. That returned function should be callable with a parameter h, and return the volume of a cylinder with height h and radius r.

I know how to return values from functions in Python, but how do I return another function?

5 Answers

Try this, using Python:

import math def make_cylinder_volume_func(r): def volume(h): return math.pi * r * r * h return volume 

Use it like this, for example with radius=10 and height=5:

volume_radius_10 = make_cylinder_volume_func(10) volume_radius_10(5) => 1570.7963267948967 

Notice that returning a function was a simple matter of defining a new function inside the function, and returning it at the end - being careful to pass the appropriate parameters for each function. FYI, the technique of returning a function from another function is known as currying.

Using lambdas, also known as anonymous functions, you can abstract out the volume function inside the make_cylinder_volume_func to a single line. In no way different from Óscar López's answer, the solution using lambda is still in a sense 'more functional'.

This is how you can write the accepted answer using a lambda expression:

import math def make_cylinder_volume_fun(r): return lambda h: math.pi * r * r * h 

And then call as you'd any other curried function:

volume_radius_1 = make_cylinder_volume_fun(1) volume_radius_1(1) => 3.141592653589793 

Just want to point out that you can do this with pymonad

 import pymonad @pymonad.curry def add(a, b): return a + b add5 = add(5) add5(4) 9 

I know I am too late to the party, but I think you might find this solution interesting.

from math import pi from functools import partial def cylinder_volume(r, h): return pi * r * r * h make_cylinder_with_radius_2 = partial(cylinder_volume, 2) make_cylinder_with_height_3 = partial(cylinder_volume, h=3) print(cylinder_volume(2, 3)) # 37.6991118431 print(make_cylinder_with_radius_2(3)) # 37.6991118431 print(make_cylinder_with_height_3(2)) # 37.6991118431 

Here is documentation about how partial works.

Here is one massive example which covers many of single multiple argument cases in one function

def maths(var='NA'): if var.lower() == 'add': def add(*args): return "Sum is : "+str(sum(args)) return add elif var.lower() == 'substract': def substract(a,b): if a>b: return "Difference is : "+str(a-b) else: return "Difference is : "+str(b-a) return substract elif var.lower() == 'multiply': def multiply(*args): temp = 1 for x in args: temp = temp*x return "multiplication is : "+str(temp) return multiply elif var.lower() == 'divide': def divide(a,b): return "Division is : "+str(a/b) return divide else: print("Please choose one of given operations: 'add','substract','multiply','divide'") 

Here first call the maths function with the required operation and then use the returned function for the actual calculation