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\

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

What you are looking for is called a closure.

def make_h(c):
   def h(x):
       return f(x, c)
   return h

Now if you assign h = make_h(c), then h(x) equals f(x, c), and you can pass your h to G.

If you wish, the functools library has support for closures (functools.partial)

Answer 3

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...

Answer 4

Try this:

def h():
    return lambda x: f(x,c)

No need to supply the x to h - you could pass a function to wrap eventually if it wouldn't be in the scope. In fact, h is obsolete unless you want it to work with any 2-argument function:

def h(functionToWrap, fixedSecondArgument):
     return lambda x: functionToWrap(x, fixedSecondArgument)