Python - Appending JSON object to exisiting JSON object

Question

I am trying to append JSON objects to existing JSON object in text file. My first set of data look likes this.

data = [
        {
          "username": "Mike",
          "code": "12345",
          "city": "NYC"
        }
      ]

Then I need to append another set of JSON objects to existing file to look like this:

data = [
        {
          "username": "Mike",
          "code": "12345",
          "city": "NYC"
        },
        {
          "username": "Kelly",
          "code": "56789",
          "city": "NYC"
        }
      ]

When I try to run:

with open('data2.txt', 'a') as outfile:
    json.dump(data, outfile)

my data is not in correct JSON format. Can you please advise how to append to text file correctly?

You can't just append to a text file and expect it to somehow know how to be in json format. You have to read the file, convert to json, then add your values and re-write it — Falmarri
– Falmarri, Commented Feb 6, 2017 at 22:12

koalo · Accepted Answer · 2017-02-06 22:12:21Z

5

First read the data from the file.

with open('data2.txt') as data_file:    
    old_data = json.load(data_file)

Then append your data to the old data

data = old_data + data

Then rewrite the whole file.

with open('data2.txt', 'w') as outfile:
    json.dump(data, outfile)

answered Feb 6, 2017 at 22:12

koalo

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Comments

Ollie · Accepted Answer · 2017-05-26 08:50:14Z

This is probably not the most Pythonic way to handle your request but I hope it will help with some issues you might encounter. I wrapped the loading and dumping into try-except bracelets to make the code more robust. The biggest surprise for myself was that you don't use 'a' when opening the file as output file but 'w' instead. However,this makes perfect sense if you consider that you append already in line "data.append(data1)", so there is no need to append twice when dumping to the file.

data = [{"username": "Mike", "code": "12345", "city": "NYC"}]
data1 = {"username": "Kelly", "code": "56789", "city": "NYC"}
data2 = {"username": "Bob", "code": "12222", "city": "NYC"}

try:
    with open('append.txt', 'r') as fin:
        data = json.load(fin)
except FileNotFoundError as exc:
    pass

try:
    if data:
        data.append(data1)
        with open('append.txt', 'w') as fout:
            json.dump(data, fout)
except UnboundLocalError as exc:
    with open('append.txt', 'w') as fout:
        json.dump(data, fout)

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