def loop only change list in local scope

Question

In Python 3, I want to define a function that takes in a list a, does some operations and then returns some property of a. The problem is that I want a to stay unchanged.

Take for example the following code

def test(a):
    a.remove(1)
    return max(a)

When I run this, I get:

>>> b = [0,1,2,3]
>>> test(b)
3
>>> b
[0,2,3]

whereas I want b to stay unchanged

>>> b
[0,1,2,3]

I find this rather confusing, as Python does leave floats, ints, ... unchanged when being passed to a function.
Can anyone please explain what's going on? And what should I do to avoid this issue?

Thanks in advance

The difference is that floats and ints are immutable whereas list is not — smac89, Oct 08 '19 at 20:43
Assignment is not mutation; `b = [1]` is very different from `b.append(1)`. — chepner, Oct 08 '19 at 20:46
Another duplicate: https://stackoverflow.com/questions/17686596/function-changes-list-values-and-not-variable-values-in-python — smac89, Oct 08 '19 at 20:47

score 0 · Accepted Answer · answered Oct 08 '19 at 20:46

0

The list.remove method alters the list in-place. If you don't want the given list to change you can make a copy of the list first instead:

def test(a):
    a = a[:]
    a.remove(1)
    return max(a)

answered Oct 08 '19 at 20:46

blhsing

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def loop only change list in local scope

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