How do we change the indexes of a list without changing the original list on python?

Question

I'm trying to add 2 to the elements of a list without changing the elements of the original list in python. I used the append method, but when I try to return the original is modified too. It should work like this:

>> list1 = [1, 3, 5]
>> add_two(list1)
[3, 5, 7]
>> list1 == [1, 3, 5]
True

However this is what I got:

>> list1 = [1, 3, 5]
>> add_two(list1)
[3, 5, 7]
>> list1
[3, 5, 7]

Can anyone help me please?

Answer 1

List is a mutable data type. If you want to manipulate it without changing the original, you should pass the copy of that list to the function add_two().

In [1]: def add_two(lst):
   ...:     lst[2] += 2
   ...:     return lst
   ...:

In [2]: a = [ 1,2,3]

In [3]: add_two(a)
Out[3]: [1, 2, 5]

In [4]: a
Out[4]: [1, 2, 5]

In [5]: add_two(a.copy())
Out[5]: [1, 2, 7]

In [6]: a
Out[6]: [1, 2, 5]

EDIT

I see you want to increase each list element by two. Then it is easiest to use list comprehension - creates a new list, no need to pass a copy...

def add_two(lst):
     return [v+2 for v in lst]