Return function with function

Question

I would like to do something like the following:

def getFunction(params):
   f= lambda x:
       do stuff with params and x
   return f

I get invalid syntax on this. What is the Pythonic/correct way to do it?

This way I can call f(x) without having to call f(x,params) which is a little more messy IMO.

Answer 1

A lambda expression is a very limited way of creating a function, you can't have multiple lines/expressions (per the tutorial, "They are syntactically restricted to a single expression"). However, you can nest standard function definitions:

def getFunction(params):
   def to_return(x):
       # do stuff with params and x
   return to_return

Functions are first-class objects in Python, so once defined you can pass to_return around exactly as you can with a function created using lambda, and either way they get access to the "closure" variables (see e.g. Why aren't python nested functions called closures?).

Answer 2

It looks like what you're actually trying to do is partial function application, for which functools provides a solution. For example, if you have a function multiply():

def multiply(a, b):
    return a * b

... then you can create a double() function¹ with one of the arguments pre-filled like this:

from functools import partial

double = partial(multiply, 2)

... which works as expected:

>>> double(7)
14

---

¹ Technically a partial object, not a function, but it behaves in the same way.

Answer 3

You can't have a multiline lambda expression in Python, but you can return a lambda or a full function:

def get_function1(x):
    f = lambda y: x + y
    return f

def get_function2(x):
    def f(y):
        return x + y
    return f