In python, is a function return a shallow or deep copy?

Question

In python, if I have

x = y

any modification to x will also modify y, and I can do

x = deepcopy(y)

if I want to avoid modifying y while working on x

Say, instead, that I have:

myFunc(): return y def main(): x = myFunc()

Is it still the case that modifying x will modify y, or since it is a return from another function it will be like a deepcopy?

Can you be more specific. In your case it's not clear where does 'y' come from. The code you posted will not run because 'y' is not known — Vlad
– Vlad, Commented Feb 3, 2017 at 20:22
Functions don't return any sort of copies. In other words, it is neither. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Feb 3, 2017 at 20:27

shx2 · Accepted Answer · 2017-02-03 20:22:16Z

21

In python everything is a reference. Nothing gets copied unless you explicitly copy it.

In your example, x and y reference the same object.

answered Feb 3, 2017 at 20:22

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3 Comments

I think this is a really important principle to learn in Python: assignment never makes a copy. You can extend that to function returns as well.

I knew that about assignment in a function, I was not sure about return because I think it is different in Java

Why does extending a list of ravel()ed ndarrays does not create a shallow copy of the original ndarrays?

Aeroblop · Accepted Answer · 2017-02-03 20:28:55Z

It will be a shallow copy, as nothing has been explicitly copied.

def foo(list): list[1] = 5 return list

For example:

>>> listOne = [1, 2] >>> listTwo = [3, 4] >>> listTwo = listOne >>> foo(listTwo) [1, 5] >>> listOne [1, 5]