How to assign a new list to a list passed as an argument?

Question

I'm fulfilling an API for a function that needs to take in a list, make a copy of it with some modifications and then set the out parameter as the copy. i.e. the function returns nothing, but must change the list object that was passed in as an argument.

Something like this:

l = [1,2,3]

def f(passed_in_list):
   different_list = [4,5,6]
   passed_in_list = different_list[:]

print(l)   # Prints [1,2,3]. How to make it [4,5,6] ?

How can I achieve this?

@khachik Oh haha, I tried that but as rahlf23 pointed out I did not call the function. Thank you very much. If you write up an answer I will accept (if question doesn't get closed before that) — DBedrenko, Sep 26 '18 at 16:48
https://stackoverflow.com/questions/986006/how-do-i-pass-a-variable-by-reference — Alex Germann, Sep 26 '18 at 16:48

score 2 · Answer 1 · answered Sep 26 '18 at 16:49

2

This strikes me as a dangerous design pattern, creating a function with intentional side effects on global scope. I would be careful doing things like this. That being said it doesn't look like you've called your function which may be why you aren't seeing what you expect.

answered Sep 26 '18 at 16:49

Sven Harris

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I agree, i would prefer that the function returns a new list and you do l = f(l) to reassign the new list to the variable l – Corentin Limier Sep 26 '18 at 17:08

score 2 · Accepted Answer · answered Sep 26 '18 at 16:51

2

You need to modify the list, not the variable that points to it. And as mentioned by others, you need to call that function:

l = [1,2,3]


def f(passed_in_list):
   different_list = [4,5,6]
   passed_in_list[:] = different_list[:]


f(l)
print(l)

answered Sep 26 '18 at 16:51

khachik

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@vash_the_stampede yes as passed_in_list[:] = updates the passed_in_list parameter – Corentin Limier Sep 26 '18 at 17:07

score 1 · Answer 3 · answered Sep 26 '18 at 16:50

1

You need to actually call your function:

l = [1,2,3]

def f(passed_in_list):

   passed_in_list[:] = [4,5,6]

f(l)

print(l)

Yields:

[4, 5, 6]

answered Sep 26 '18 at 16:50

rahlf23

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How to assign a new list to a list passed as an argument?

3 Answers3