Python Identical data is not identical and functions differently

Question

I'm trying to add a unique ID to every item in a list and struggling with some strange behaviour I don't understand from Python.

I have this function:

    def add_IDs(d):

        for x in range(len(d)):
            var = d.pop(x)
            var['list_id'] = x
            d.insert(x, var)
        return d

Into which I input this data:

[{'db_number': 1, 'quantity': 15, 'quality': 1},
 {'db_number': 1, 'quantity': 20, 'quality': 0},
 {'db_number': 1, 'quantity': 20, 'quality': 0},
 {'db_number': 1, 'quantity': 80, 'quality': 0},
 {'db_number': 2, 'quantity': 4, 'quality': 0}]

I expect this output:

[{'db_number': 1, 'quantity': 15, 'quality': 1, 'list_id': 0},
 {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 1},
 {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2},
 {'db_number': 1, 'quantity': 80, 'quality': 0, 'list_id': 3},
 {'db_number': 2, 'quantity': 4, 'quality': 0, 'list_id': 4}]

But instead the second dict in the list, gets 'list_id': 2 instead of 'list_id': 1

[{'db_number': 1, 'quantity': 15, 'quality': 1, 'list_id': 0},
 {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2},
 {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2},
 {'db_number': 1, 'quantity': 80, 'quality': 0, 'list_id': 3},
 {'db_number': 2, 'quantity': 4, 'quality': 0, 'list_id': 4}]

As a test I wrote this:

    def add_IDs(d):

        for x in range(len(d)):
            var = d.pop(x)
            var['list_id'] = x
            d.insert(x, var)
        return d

    data2 = [{'db_number': 1, 'quantity': 15, 'quality': 1},
             {'db_number': 1, 'quantity': 20, 'quality': 0},
             {'db_number': 1, 'quantity': 20, 'quality': 0},
             {'db_number': 1, 'quantity': 80, 'quality': 0},
             {'db_number': 2, 'quantity': 4, 'quality': 0}]

    print(data)
    print(data2)

    l1 = add_IDs(data)
    l2 = add_IDs(data2)
    print(l1)
    print(l2)

    print("")
    print('Does data = data2?')
    print(data == data2)
    print('Does l1 = l2?')
    print(l1 == l2)

Which gives this output:

[{'db_number': 1, 'quantity': 15, 'quality': 1}, {'db_number': 1, 'quantity': 20, 'quality': 0}, {'db_number': 1, 'quantity': 20, 'quality': 0}, {'db_number': 1, 'quantity': 80, 'quality': 0}, {'db_number': 2, 'quantity': 4, 'quality': 0}] 
[{'db_number': 1, 'quantity': 15, 'quality': 1}, {'db_number': 1, 'quantity': 20, 'quality': 0}, {'db_number': 1, 'quantity': 20, 'quality': 0}, {'db_number': 1, 'quantity': 80, 'quality': 0}, {'db_number': 2, 'quantity': 4, 'quality': 0}] 
[{'db_number': 1, 'quantity': 15, 'quality': 1, 'list_id': 0}, {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2}, {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2}, {'db_number': 1, 'quantity': 80, 'quality': 0, 'list_id': 3}, {'db_number': 2, 'quantity': 4, 'quality': 0, 'list_id': 4}] 
[{'db_number': 1, 'quantity': 15, 'quality': 1, 'list_id': 0}, {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 1}, {'db_number': 1, 'quantity': 20, 'quality': 0, 'list_id': 2}, {'db_number': 1, 'quantity': 80, 'quality': 0, 'list_id': 3}, {'db_number': 2, 'quantity': 4, 'quality': 0, 'list_id': 4}]

Does data = data2? 
False 
Does l1 = l2? 
False

Process finished with exit code 0

As far as I can see, the input data is identical for both, the inbuilt comparison tool tells me the printed values are identical, but the output is still different and the checks say they are different. Can someone shed some light on what I'm missing?

Answer 1

First off, you can simplify your logic substantially:

def add_ids(items):
    for index, item in enumerate(items):
        item['list_id'] = index

I was unable to reproduce your issue in python 2 or python3.

Note that the len issue mentioned in a comment will not come into play, since you calculate it once, when the range is created, not at each point in the list.

---

With the additional information from your comment that the approach above gave the same behavior, I know your issue - you are using the same object for two entries in your list.

x, y, z = {}, {}, {}

items = [x, y, y, z]
for index, item in enumerate(items):
    print(index, item, id(item))

Note that index 1 and 2 have the same id

0 {} 4446764960
1 {} 4446764960
2 {} 4446790512
3 {} 4430894656

Then running

add_ids(items)

sets the index for y twice, once for index 1 and then for index 2.

assert items == [{'list_id': 0}, {'list_id': 2}, {'list_id': 2}, {'list_id': 3}]

Any change to y will show up in both items[1] and items[2], since they are the same object.