Python Dictionary Update is Updating every Dictionary in a List of Dictionaries

Asked May 08 '23 at 18:44

Active May 08 '23 at 18:44

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I have a list of dictionaries that I am updating through a for loop. However, it seems calling update() on a dictionary within the list is resulting in every dictionary in the list being update with the most current loop's values.

While the original code is more complex, I stripped away all the unrelated stuff. Below is a test snippet I wrote just to see if it's the issue of the other pieces in my code or just the update().

test = [{}] * 3

for i in range(0, 3):
        test[i].update({'test': i, 'val': i+1})
print(test)

The expected result is:

[{'test': 0, 'val': 1}, {'test': 1, 'val': 2}, {'test': 2, 'val': 3}]

But instead I get:

[{'test': 2, 'val': 3}, {'test': 2, 'val': 3}, {'test': 2, 'val': 3}]

Not entirely sure why this is the case. If anyone could help I would really appreciate it. Thank you!

asked May 08 '23 at 18:44

Mark Wong

`[{}] * 3` does not do what you think it does. – Scott Hunter May 08 '23 at 18:45
1

`test = [{}] * 3` creates a list of three references to the same dictionary. You're looking for `[{} for _ in range(3)]`. – B Remmelzwaal May 08 '23 at 18:45
The duplicate is about list of lists, but applies all the same to a list of dicts created in this way. – wim May 08 '23 at 18:46
Ah, thanks for the quick responses. That does fix the issue. Thank you all. – Mark Wong May 08 '23 at 19:02
1

To be clear, it applies to **anything**. For any ``, `[]*n` creates a `list` with `n` references to whatever object the `` evaluates to. – juanpa.arrivillaga May 08 '23 at 19:32

Python Dictionary Update is Updating every Dictionary in a List of Dictionaries

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