Pass a function as a variable with one input fixed

Question

Say I have a two dimensional function f(x,y) and another function G(function) that takes a function as an input. BUT, G only takes one dimensional functions as input and I'm wanting to pass f to G with the second variable as a fixed parameter.

Right now, I am just declaring a third function h that sets y to a set value. This is what it looks like in some form:

def f(x,y):
   something something something
   return z;

def G(f):
    something something something

def h(x):
   c= something
   return f(x,c);
G(h)

At some point I was also making y a default parameter that I would change each time.

Neither of these are as readable as if I was somehow able to call

G(f(x,c))

that particular syntax doesn't work. What is the best way to do this?

Answer 1

An ideal solution would use partial application, but the quickest and easiest way to accomplish this would be to wrap f inside a lambda statement like this:

G(lambda x: F(x, C))

In this example, the lambda syntax creates an anonymous function that accepts one argument, x, and calls f with that value x and the constant C. This works because the value of C is "captured" when the lambda is created and it becomes a local constant inside the lambda.

Answer 2

The functools.partial function can be used to do this (note, it's not entirely clear where c comes from in your example code, so I've assumed it's some constant).

import functools

def f(x,y):
    return x+y

c = 3

G = functools.partial(f, c)
G(4)

I think this is more explicit than the lambda approaches suggested so far.

Edit: replacing the right most argument is not possible as we are dealing with positional arguments. Depending on the level of control available, you could introduce a wrapper which handles the switching:

import functools

def f(x,y):
    return x+y

def h(c,y):
    return f(y,c)

c = 3

G = functools.partial(h, c)
G(4)

But I think you start to sacrifice readability and maintainability at this point...