How do I append/prepend list in function without modifying source?

Question

Given this source list:

source_list = [2, 3, 4]

and this function:

def function(list_in):
    list_in.append(5)
    list_in.insert(0, 1)
    return list_in

As expected, I get:

>>> function(source_list)
[1, 2, 3, 4, 5]

But if I call the variable source_list outside of the function I still get:

>>> source_list
[1, 2, 3, 4, 5]

Is there an alternative way of modifying (specifically appending/prepending to) a list within a function such that the original list is not changed?

Answer 1

If you have access to the function, you can copy the list passed:

like this:

def function(list_in_):     # notice the underscore suffix
    list_in = list_in_[:]   # copy the arg into a new list
    list_in.append(5)
    list_in.insert(0, 1)
    return list_in          # return the new list

otherwise you can call the function with a copy of your source_list, and decide if you want a new list, or if you prefer the source_list mutated, as demonstrated by @tdelaney