dictionary value changes without performing any update

Question

Why are the values getting updated for every key?

d = {0: [0], 1: [0, 1]}
for i in range(2, 4):
    d[i] = d[i - 1]
    d[i].append(i)
print(d)

Output:

{0: [0], 1: [0, 1, 2, 3], 2: [0, 1, 2, 3], 3: [0, 1, 2, 3]}

I want it like this in the output:

{0: [0], 1: [0, 1], 2: [0, 1, 2], 3: [0, 1, 2, 3]}

Can someone please help me understand why this is happening and the possible solutions? It would be great if you could attach the related Python documentation!

Thank you. I can do that but the problem was why the values are getting updated for every key — Shambhav Agrawal, Jul 17 '21 at 16:46
Lists are references. You set all your dictionary keys to the same reference and slowly add to this reference. All lists above 0 are _the same list_ - not copys. because you re-added the same reference over and over. — Patrick Artner, Jul 17 '21 at 16:46
`for i in d: print(i, d[i], id(d[i]) )` will show you the same id for all the values you copied — Patrick Artner, Jul 17 '21 at 16:47
Lists are *objects*; the values in the dictionary are references to those lists. — chepner, Jul 17 '21 at 16:50

score 4 · Answer 1 · edited Aug 16 '22 at 00:24

4

Assigning a list will copy the elements by reference. Copy the values of the list using .copy() method.

From Python documentation:

Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other.

d = {0: [0], 1: [0, 1]}
for i in range(2, 4):
    d[i] = d[i - 1].copy()
    d[i].append(i)
print(d)

Output:

{0: [0], 1: [0, 1], 2: [0, 1, 2], 3: [0, 1, 2, 3]}

Explanation:

When we use d[i] = d[i-1] in d[i] stores the reference of the list stored in d[i-1]. Then when we are updating d[i] with the d[i].append(i) it updates the list of d[i-1] index too.

edited Aug 16 '22 at 00:24

wjandrea

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answered Jul 17 '21 at 16:39

arshovon

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Thank you so much! but I wish to know what's wrong in my code "d[i] = d[i-1]" why it's updating everywhere ?? – Shambhav Agrawal Jul 17 '21 at 16:44
@ShambhavAgrawal it's because you're appending values to the same reference inside the loop. You can python's `id(obj)` (gives hash-value of obj) to check the reference of an object. – Rohit Babu Jul 17 '21 at 16:52

dictionary value changes without performing any update

1 Answers1