Question

I want to duplicate a list and modify the duplicate, but when I try this, I end up modifying the original as well.

A highly simplified version of my problem:

list = [2,3,6,8,9,6,7,4,56,8,9,6,7,8]

new_list = list

new_list[2] = 'n'

print list
print new_list

I only want to change item 2 of the new list, but when I try this, both the new list and the old list change to:

[2,3,'n',8,9,6,7,4,56,8,9,6,7,8]

Is there something quite fundamental I’m missing here?

I can make it do what I want if I add each item of the list to the next list one by one:

list = [2,3,6,8,9,6,7,4,56,8,9,6,7,8]

new_list = []

for item in list:
    new_list.append(item)

new_list[2] = 'n'

print list
print new_list

If I do that, only the new_list changes, but is there a more straight-forward way?

Answer 1

To make a new copy of your list, try:

newList = list(oldList)

Or more cryptic concise via slicing:

newlist = oldList[:]

Just assigning oldList to newList will result in two names pointing to the same object, like so:

Generic object copying functions are provided by the copy module. (Image taken from: http://henry.precheur.org/python/copy_list).