How can I get the concatenation of two lists in Python without modifying either one?

Yes: list1 + list2. This gives a new list that is the concatenation of list1 and list2.

The simplest method is just to use the + operator, which returns the concatenation of the lists:

concat = first_list + second_list

One disadvantage of this method is that twice the memory is now being used . For very large lists, depending on how you're going to use it once it's created, itertools.chain might be your best bet:

>>> import itertools
>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = itertools.chain(a, b)

This creates a generator for the items in the combined list, which has the advantage that no new list needs to be created, but you can still use c as though it were the concatenation of the two lists:

>>> for i in c:
...     print i
1
2
3
4
5
6

If your lists are large and efficiency is a concern then this and other methods from the itertools module are very handy to know.

Note that this example uses up the items in c, so you'd need to reinitialise it before you can reuse it. Of course you can just use list(c) to create the full list, but that will create a new list in memory.

concatenated_list = list_1 + list_2

you could always create a new list which is a result of adding two lists.

>>> k = [1,2,3] + [4,7,9]
>>> k
[1, 2, 3, 4, 7, 9]

Lists are mutable sequences so I guess it makes sense to modify the original lists by extend or append.