How to update a JSON file by using Python?

python json file storage updating
user502052 picture user502052 · Jul 4, 2014 · Viewed 27.1k times · Source

I am using Python and I have a JSON file in which I would like to update a value related to a given key. That is, I have the my_file.json containing the following data

{"a": "1", "b": "2", "c": "3"}

and I would like to just change the value related to the b key from 2 to 9 so that the updated file look as like:

{"a": "1", "b": "9", "c": "3"}

How can I make that?

I tried the following but without success (the changes are not saved to the file):

with open('my_file.json', 'r+') as f:
    json_data = json.load(f)
    json_data['b'] = "9"
    f.close()

Answer

DrV picture DrV · Jul 4, 2014

You did not save the changed data at all. You have to first load, then modify, and only then save. It is not possible to modify JSON files in-place.

with open('my_file.json', 'r') as f:
    json_data = json.load(f)
    json_data['b'] = "9"

with open('my_file.json', 'w') as f:
    f.write(json.dumps(json_data))

You may also do this:

with open('my_file.json', 'r+') as f:
    json_data = json.load(f)
    json_data['b'] = "9"
    f.seek(0)
    f.write(json.dumps(json_data))
    f.truncate()

If you want to make it safe, you first write the new data into a temporary file in the same folder, and then rename the temporary file onto the original file. That way you will not lose any data even if something happens in between.

If you come to think of that, JSON data is very difficult to change in-place, as the data length is not fixed, and the changes may be quite significant.

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