set calling argument to None inside function

Question

How can I make a python function act on the original object that is given as an argument,

def f(x):
    x = None

y = 1
f(y)
y  # Returns 1, I want y to return None

I know that y == 1 is the correct behaviour. My question is: is there a trick that makes it possible to have a python function behave as if the argument were passed by reference?

You cannot rebind names outside the scope of the function, not without dirty trickery that only works in limited cases. — Martijn Pieters, Jul 23 '14 at 14:25
That's what I really wanted to know: is there some dirty trickery that allows me to obtain a similar effect? — Quant Metropolis, Jul 23 '14 at 15:34

score 1 · Answer 1 · answered Jul 23 '14 at 14:25

1

def f(x):
    return None

y = 1
y = f(y)
y  # Returns 1, I want y to return None

You need to return from a function and assign the return to a variable

answered Jul 23 '14 at 14:25

gkusner

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score 0 · Answer 2 · answered Jul 23 '14 at 14:27

Your function f(x) doesn't actually return anything. When you set x = None, all you're doing is reassigning the local x that's inside f; you're not actually changing the variable that you passed in. That's not something you can do very easily in Python, and you usually don't want to. You'd want something like:

def f(x):
  x = None
  return x

And then assign y to f(x) in your main code.

set calling argument to None inside function

2 Answers2