Question

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Python: How do I pass a variable by reference?

I’m trying to write a function that modifies one of the passed parameters. Here’s the current code:

def improve_guess(guess, num): 
  return (guess + (num/guess)) / 2

x = 4.0
guess = 2.0
guess = improve_guess(guess, x)

However, I want to write the code in such a way that I don’t have to do the final assignment. That way, I can just call:

improve_guess(guess,x) 

and get the new value in guess.

(I intentionally didn’t mention passing-by-reference because during my net-searching, I found a lot of academic discussion about the topic but no clean way of doing this. I don’t really want to use globals or encapsulation in a list for this.)

Answer 1

You can’t do this directly since integer and floating-point types are immutable in Python.

You could wrap guess into a mutable structure of some sort (e.g. a list or a custom class), but that would get very ugly very quickly.

P.S. I personally really like the explicit nature of guess = improve_guess(guess, x) since it leaves no doubt as to what exactly is being modified. I don’t even need to know anything about improve_guess() to figure that out.