Question

I am absolutely dumbfounded by why the following is happening:

Here is my code:

---

def add_one(array):

    new_array = array
    length = len(array)

    for i in range(length):

        new_array[i] = new_array[i]+1


    return new_array

x = [1,2,3,4,5];
y = add_one(x)

print x

print y

---

Here are the results:

[2, 3, 4, 5, 6]

[2, 3, 4, 5, 6]

---

I do not understand why x is changed.

My speculations:
Somehow x is given as a global variable to the function add_one. I included ‘new_array = array’ so that if array was somehow the global variable x, x would not be changed. However, somehow new_array also became the global variable x when ‘new_array = array’ was executed. I wrote an alternate version of the function add_one which did not give me issues:

---

def add_one(array):

    new_array = []
    length = len(array)

    for i in range(length):
        new_array.append(array[i]+1)

---

It seems that if a local variable (that is a array) is edited by it’s index within a function, it turns global with respect to the global variable that was taken as an input for that function?

I have no idea what is going on. Any explanations would be highly appreciated.

Answer 1

Nothing is made global for obscure readons. array is still a perfectly normal local variable. But it’s a reference, as everything in Python (well, every name at least) is a reference:

• add_one(x) passes a reference (not like a reference in e.g. C++, more like a pointer – if either of that means anything to you) to x to add_one
• new_array = array will just copy that reference
• No copying happening, all these names point to the same object!

That means that calling a method on array in add_one modified the very same object x of the caller refers to – re-assigning array (array = []) will just overwrite the local copy of the reference and not affect other variables referring to the same object (after all, it’s quite possible that there is no such variable!), but member or item assignment (e.g. new_array[i] = ...) is basically a method call and will modify the object.

Generally, the easiest way to generate a new list from an existing one with some pattern of modification applied to each element and optionally some filtering is using a list comprehension. [x + 1 for x array] in this example.