Python append to list by key issue

Question

I have a dataset that, among other things, contains the creation date. I turn the date into months and generate a dict with the month number as the key and an empty list:

{8: [], 7: []}

Then I try to add the rest of the data on the key to this empty list, but all the data is added to the list, not just the desired key. This is what I get in the output:

{8: [7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8], 7: [7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
8, 8, 8, 8, 8, 8, 8]}

But, it should be like this

{8: [8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
8, 8, 8, 8, 8, 8, 8], 7: [7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7]}

What i'm doing wrong? My code is...

    group_keys = []
    for e in data:
        group_keys.append(e.get('start_date').month)
    
    group_keys = dict.fromkeys(set(group_keys), [])
    print(group_keys)
    for e in data:
        month = e.get('start_date').month
        group_keys[month].append(month)
    print(group_keys)

You define your dictionary wrong and they contain the reference to the same list. — matszwecja, Aug 29 '22 at 10:26
@matszwecja can you point, where i'm doing wrong and how to fix it? — Nataly Firstova, Aug 29 '22 at 10:27
`group_keys = dict.fromkeys(set(group_keys), [])` This line is the problem. Basically, all the items in your dictionary is the same `[]`. — ARK1375, Aug 29 '22 at 10:28
All that being said, are you sure that's how you want to store your data? Are you sure dictionary with 60 repeats of the same number is what you need? — matszwecja, Aug 29 '22 at 10:32
What I'm saying is; what additional information do you get storing data in that way that you can't get with `{7: 31, 8: 29}`? — matszwecja, Aug 29 '22 at 10:37

score 2 · Accepted Answer · answered Aug 29 '22 at 10:28

2

All of the entries in group_keys are pointing to the same list. Try using a dictionary comprehension to create group_keys

group_keys = {x:[] for x in set(group_keys)}

answered Aug 29 '22 at 10:28

Delta Charlie

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Python append to list by key issue

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