How to get values for certain keys from a list of dictionaries

Question

My Input JSON data:

{
    "data": [
        {
            "config": "current",
            "id": "0"
        },
        {
            "config": "current",
            "id": "1"
        },
        {
            "config": "current",
            "id": "2"
        },
        {
            "config": "current",
            "id": "3"
        },
        {
            "config": "previous",
            "id": "4",
        },
        {
            "config": "previous",
            "id": "5"
        },
        {
            "config": "current",
            "id": "6"
        }
    ]
}

I want to form a dictionary of lists out of above input data based on common key/value pair:

{
    "current": ["0", "1", "2", "3", "6"],
    "previous": ["4", "5"]
}

How can this be achieved using python?

This is actually an invalid JSON, `"id": "4",` should be `"id": "4"`. — PApostol, May 23 '21 at 17:51

wjandrea · Accepted Answer · 2021-05-23T18:10:42.627

Assuming you already know how to parse JSON, you can do this:

d = {
    "data": [
        {"config": "current", "id": "0"},
        {"config": "current", "id": "1"},
        {"config": "current", "id": "2"},
        {"config": "current", "id": "3"},
        {"config": "previous", "id": "4"},
        {"config": "previous", "id": "5"},
        {"config": "current", "id": "6"}]
    }

result = {}
for d0 in d['data']:
    ids = result.setdefault(d0['config'], [])
    ids.append(d0['id'])

print(result)
# -> {'current': ['0', '1', '2', '3', '6'], 'previous': ['4', '5']}

dict.setdefault() is used to get the id list if it exists, or if not, set it to a default, which is an empty list here. It's functionally the same as this:
```
config = d0['config']
if config not in result:
    result[config] = []
result[config].append(d0['id'])
```
You could also use collections.defaultdict(list) to do the same thing even more easily.

[This explanation is taken from my answer here.]

Thank you so much @wjandrea. I was trying to do it in a much complex way. Using dict.setdefault() is much easier which i was not aware of. — Vishwanath S Manvi, May 23 '21 at 18:55

Yuri Khristich · Answer 2 · 2021-05-23T18:38:29.530

jsn = {
    "data": [
        {"config": "current", "id": "0"},
        {"config": "current", "id": "1"},
        {"config": "current", "id": "2"},
        {"config": "current", "id": "3"},
        {"config": "previous", "id": "4",},
        {"config": "previous", "id": "5"},
        {"config": "current", "id": "6"}
    ]
}

current = [x["id"] for x in jsn["data"] if x["config"] == "current"]
previous = [x["id"] for x in jsn["data"] if x["config"] == "previous"]

res = {"current": current, "previous": previous}

print(res) # {'current': ['0', '1', '2', '3', '6'], 'previous': ['4', '5']}

Or the same algorigthm with a function:

def get_ids(json_string, key):
    return [x["id"] for x in json_string["data"] if x["config"] == key]

res = { 
    "current": get_ids(jsn, "current"), 
    "previous": get_ids(jsn, "previous") 
    }

print(res) # {'current': ['0', '1', '2', '3', '6'], 'previous': ['4', '5']}

You might want to avoid calling it `json` cause that's the name of the [stdlib JSON library](https://docs.python.org/3/library/json.html). — wjandrea, May 23 '21 at 17:55

How to get values for certain keys from a list of dictionaries

2 Answers2