I am new to Python and I am playing with JSON data. I would like to retrieve the JSON data from a file and add to that data a JSON key-value "on the fly".

That is, my json_file contains JSON data as-like the following:

{"key1": {"key1A": ["value1", "value2"], "key1B": {"key1B1": "value3"}}}

I would like to add the "ADDED_KEY": "ADDED_VALUE" key-value part to the above data so to use the following JSON in my script:

{"ADDED_KEY": "ADDED_VALUE", "key1": {"key1A": ["value1", "value2"], "key1B": {"key1B1": "value3"}}}

I am trying to write something as-like the following in order to accomplish the above:

import json

json_data = open(json_file)
json_decoded = json.load(json_data)

# What I have to make here?!

json_data.close()

Your json_decoded object is a Python dictionary; you can simply add your key to that, then re-encode and rewrite the file:

import json

with open(json_file) as json_file:
    json_decoded = json.load(json_file)

json_decoded['ADDED_KEY'] = 'ADDED_VALUE'

with open(json_file, 'w') as json_file:
    json.dump(json_decoded, json_file)

I used the open file objects as context managers here (with the with statement) so Python automatically closes the file when done.