Is it possible to change an object in an array while looping through the array?

Question

I am trying to change the structure of an object inside of an array that I am looping though with a for loop. Nothing seems to happen to the object unless I append it to a new array.

I just can't help but think there is a better way to do this...

I have replaced all the real data and naming with placeholder data just for structure.

Essentially though I just want to restructure obj in the original_array rather than appending to a new one.

    rebuilt_array = []

    for obj in original_array:
        rebuilt_obj = {
            "inner_obj_1": {
                "item_1": obj["old_item_1"],
                "item_2": obj["old_item_2"],
                "item_3": obj["old_item_3"],
            },
            "another_item": obj["old_another_item"],
            "inner_obj_2": {
                "item_1": obj["old_item_1"],
                "item_2": obj["old_item_2"],
                "item_3": obj["old_item_3"],
            },
        }

        rebuilt_array.append(rebuilt_obj)

Looks reasonable to me. If you don't plan on adding obj_3 or item_4, then this is probably clearer than generic code would be. — Mateen Ulhaq
– Mateen Ulhaq, Commented Dec 21, 2022 at 5:33
What is the type of the obj? Is it a dict or some custom object? — Jay
– Jay, Commented Dec 21, 2022 at 5:33
I would have personally used a comprehension, but for loop + append is OK too. — Mateen Ulhaq
– Mateen Ulhaq, Commented Dec 21, 2022 at 5:34
obj is a local variable, so even if you modify it, it won't be reflected in the original list. In order to modify objects in original_array, you will have to use something like for i, obj in enumerate(original_array) — Jay
– Jay, Commented Dec 21, 2022 at 5:44

Jay · Accepted Answer · 2022-12-21 06:05:55Z

When you do for obj in original_array, obj is a local variable and a copy of the original object in the list, so modifying it won't reflect it in the list, here's a way you can do the same thing without creating a new list and updating the list in-place -

original_array = [{"old_item_1": 1, "old_item_2": 2, "old_item_3": 3, "old_another_item": -1}]

for i, obj in enumerate(original_array):
    new_obj = {
        "inner_obj_1": {
            "item_1": obj["old_item_1"],
            "item_2": obj["old_item_2"],
            "item_3": obj["old_item_3"],
        },
        "another_item": obj["old_another_item"],
        "inner_obj_2": {
            "item_1": obj["old_item_1"],
            "item_2": obj["old_item_2"],
            "item_3": obj["old_item_3"],
        },
    }
    original_array[i] = new_obj

print(original_array)

Output:

[{'inner_obj_1': {'item_1': 1, 'item_2': 2, 'item_3': 3}, 'another_item': -1, 'inner_obj_2': {'item_1': 1, 'item_2': 2, 'item_3': 3}}]

You can also further modify it so that we don't need the new_obj creation and you can update the older object in place directly by updating original_array[i] dict as you would a normal dict. e.g. original_array[i]['another_item'] = original_array[i].pop("old_another_item") which would rename old_another_item key to another_item in your original object, but this might become dirtier compared to above approach.

Here's the corresponding code for it -

for i in range(len(original_array)):
    original_array[i] = {
        "inner_obj_1": {
            "item_1": original_array[i]["old_item_1"],
            "item_2": original_array[i]["old_item_2"],
            "item_3": original_array[i]["old_item_3"],
        },
        "another_item": original_array[i].pop("old_another_item"),
        "inner_obj_2": {
            "item_1": original_array[i].pop("old_item_1"),
            "item_2": original_array[i].pop("old_item_2"),
            "item_3": original_array[i].pop("old_item_3"),
        }
    }

Note the use of pop() to rename dict keys. If you want to keep original keys in your dict as well, then you can skip using pop().

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