when do we initialise a function call within the function vs as an argument?

Question

I have a question about arguments in functions, in particular initialising an array or other data structure within the function call, like the following:

def helper(root, result = []):
    ...

My question is, what is the difference between the above vs. doing:

def helper(root): 
    result = []

I can see why this would be necessary if we were to run recursions, i.e. we would need to use the first case in some instances.

But are there any other instances, and am I right in saying it is necessary in some cases for recursion, or can we always use the latter instead?

Thanks

You should never use the first form: https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument — rdas, May 15 '21 at 10:40
Okay thanks, it's just that I have seen it in many solutions in Leetcode — heio fhow, May 15 '21 at 10:45
rdas, how about def helper(root, counter = 0) ; would I use this ever? or should I always organise it in the second way above? — heio fhow, May 15 '21 at 10:48
And noted haha. Anywhere other suggestions for websites for good questions/code? — heio fhow, May 15 '21 at 10:48
Having 0 as a default value for an argument is fine. It's using mutable objects like `list`, `set` that's problematic if you don't know the exact behavior. — rdas, May 15 '21 at 10:49

score 0 · Accepted Answer · answered May 15 '21 at 11:05

Python uses pointers for lists, so initializing a list or any other mutable objects in function definition is a bad idea. The best way of doing it is like this:

def helper(root, result=None):
    if isinstance(result, type(None)):
        result = []

Now if you only pass one argument to the function, the "result" will be an empty list. If you initiate the list within the function definition, by calling the function multiple times, "result" won't reset and it will keep the values from previous calls.

when do we initialise a function call within the function vs as an argument?

1 Answers1