create a json file from a list in python

Question

I have this code that allows me to create a JSON file from a list, creates the list, checks that the elements don't repeat, and then creates the JSON file.

import json

res = []
seen = set()

def add_entry(res, id, tf1, tf2, tf3):

    # check if in seen set
    if (id, tf1, tf2, tf3) in seen:
        return res

    # add to seen set
    seen.add(tuple([id, tf1, tf2, tf3]))

    # append to results list
    res.append({'metrica': id, 'ft1': tf1, 'tf2': tf2, 'tf3': tf3})

    return res

args = ['ext:tech.DB2', '40,023', '168,481', '15,55']
res = add_entry(res, *args)

args = ['ext:bim.DB2', '25,02', '18,81', '5,35']
res = add_entry(res, *args)

def write_to_json(lst, fn):
    with open(fn, 'a', encoding='utf-8') as file:
        for item in lst:
            x = json.dumps(item, indent=4)
            file.write(x + '\n')

#export to JSON
write_to_json(res, 'elements.json')

Only the JSON file is not a real JSON file:

{
    "metrica": "ext:bim.DB2",
    "ft1": "40,023",
    "tf2": "168,481",
    "tf3": "15,55"
}
{
    "metrica": "ext:bim.DB2",
    "ft1": "25,02",
    "tf2": "18,81",
    "tf3": "5,35"
}

How can I make it like this:

{"data":[{
    "metrica": "ext:bim.DB2",
    "ft1": "40,023",
    "tf2": "168,481",
    "tf3": "15,55"
},
{
    "metrica": "ext:bim.DB2",
    "ft1": "25,02",
    "tf2": "18,81",
    "tf3": "5,35"
}]}

the json file I generate it in the code and then dynamically I need to pass it to another function, even if I wanted to, I could not change the values manually, also because I would have more than 900 elements to change, how can I do it dynamically?

Does this answer your question? How to dynamically build a JSON object? You already have the list, for your expected output json.dump({"data": lst}, file, indent=4) should ideally be enough. — Abdul Aziz Barkat
– Abdul Aziz Barkat, Commented Dec 5, 2023 at 14:43

matszwecja · Accepted Answer · 2023-12-06 08:11:59Z

4

Instead of saving each element of list separately, save the entire list.

def write_to_json(lst, fn):
    out = {}
    out["data"] = lst
    with open(fn, 'w', encoding='utf-8') as file:
        x = json.dumps(out, indent=4)
        file.write(x + '\n')

To simplify further,

x = json.dumps(out, indent=4)
file.write(x + '\n')

is more or less the same as json.dump(out, file, indent=4) (dump writes directly to file, dumps returns a string - same is true for analogous load and loads methods that read from file or parse a string). We can then get rid of explicit out object and just create it inside the dump function. That gives us much cleaner version that Abdul suggested in comment.

def write_to_json(lst, fn):
    with open(fn, 'w', encoding='utf-8') as file:
        json.dump({'data': lst}, file, indent=4)

edited Dec 6, 2023 at 8:11

answered Dec 5, 2023 at 14:43

matszwecja

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3 Comments

quamrana Over a year ago

(Also, don't use 'a'. Use 'w' to ensure a valid file each time)

matszwecja Over a year ago

@quamrana Fair, just copied what was in OPs original code

quamrana Over a year ago

Yes, I realised that. Anyway +1 for a working answer.

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