Add element to a JSON file?

One possible issue I see is you set your JSON unconventionally within an array/list object. I would recommend using JSON in its most accepted form, i.e.:

test_json = { "a": 1, "b": 2}

Once you do this, adding a json element only involves the following line:

test_json["c"] = 3

This will result in:

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

Afterwards, you can add that json back into an array or a list of that is desired.

alternatively you can do

iter(data).next()['f'] = var

You can do this.

data[0]['f'] = var