Why Does Modifying a List Passed as a Parameter in a Function Not Affect the Original List?

Modifying Lists Within Functions

In programming, it's common to work with lists, especially in functions that manipulate data. However, an issue arises when attempting to modify lists passed as parameters within a function.

Consider the following code snippet:

def function1(list_arg):
    a = function2()  # returns an array of numbers
    list_arg = list(a)

list1 = [0] * 5
function1(list1)

You might expect list1 to be modified after calling function1, but it remains unchanged. This is because when assigning something to the list_arg variable, it points to a new value. However, the original list isn't affected.

To overcome this, you can modify the elements of the original list instead:

def function1(list_arg):
    a = function2()  # returns an array of numbers
    list_arg[:] = list(a)

Using list_arg[:] effectively replaces all elements of the original list. However, it's worth noting that in-place modifications like this can be confusing and may not always be the best approach.

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