Why is my dictionary item updated when I modify a different dictionary?

Question

I have a dictionary of lists.

I have a function to combine 2x of these into a single new object, like this:

def combine(d_a, d_b):
    new_dict = {}

    dict_a_copy = d_a.copy()
    dict_b_copy = d_b.copy()
 
    for entry_key in dict_a_copy.keys():
        if entry_key in dict_b_copy:
            # Entry exists - append values
            for list_item in dict_a_copy[entry_key]:
                dict_b_copy[entry_key].append(list_item)
        else:
            # New entry
            dict_b_copy[entry_key] = dict_a_copy[entry_key]

    new_dict = dict_b_copy
    return new_dict


if __name__ == "__main__":
    dict_a = {"Item1": [1,2,3], "Item2":[4,5,6]}
    dict_b = {"Item1": [7,8,9]}
    print("dict_a = " + str(dict_a))
    print("dict_b = " + str(dict_b))

    print("\nCombining")
    dict_ab = combine(dict_a, dict_b)
    
    print("dict_a = " + str(dict_a))
    print("dict_b = " + str(dict_b))
    print("dict_ab = " + str(dict_ab))

When I run it, I expect to see this:

dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item1': [7, 8, 9]}

Combining
dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item1': [7, 8, 9]}
dict_ab = {'Item1': [7, 8, 9, 1, 2, 3], 'Item2': [4, 5, 6]}

However, dict_b gets modified as well and the result is this:

dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item1': [7, 8, 9]}

Combining
dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item1': [7, 8, 9, 1, 2, 3]}
dict_ab = {'Item1': [7, 8, 9, 1, 2, 3], 'Item2': [4, 5, 6]}

If I change dict_b.items to use a unique dictionary name, then it works:

    dict_b.items = {"Item3": [7,8,9]}

This gives:

dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item3': [7, 8, 9]}

Combining
dict_a = {'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}
dict_b = {'Item3': [7, 8, 9]}
dict_ab = {'Item3': [7, 8, 9], 'Item1': [1, 2, 3], 'Item2': [4, 5, 6]}

Can someone explain what is happening?

You only copy the outer dict, the inner list is still the same. — MisterMiyagi, Jun 19 '22 at 08:07
Does this answer your question? [What is the difference between shallow copy, deepcopy and normal assignment operation?](https://stackoverflow.com/questions/17246693/what-is-the-difference-between-shallow-copy-deepcopy-and-normal-assignment-oper) — MisterMiyagi, Jun 19 '22 at 08:09
As an aside, `list_a_copy = l_a.items.copy()` is a confusing name for a dictionary. — juanpa.arrivillaga, Jun 19 '22 at 08:09
Thanks juanpa.arrivillaga, that does answer my question, and "yes" the name was not great; it was a left-over from some code simplifications to post a shorter question. — Iain Waugh, Jun 19 '22 at 09:12

Why is my dictionary item updated when I modify a different dictionary?

0 Answers0