Modify 3rd index of array in json using python

There are a couple of problems with your program as it is.

The first issue is you're not looking up the last element of your 'allowed' arrays:

a_dict = {row['allowed'][1]:"L"}

Remember, array indicies start at zero. eg:

['Index 0', 'Index 1', 'Index 2']

But the main problem is when you walk over each row, you fetch the contents of that row, but then don't do anything with it.

import json 
with open('test.json') as f: 
   data = json.load(f) 
   for row in data['rows']: 
       a_dict = {row['allowed'][1]:"L"} 

       # a_dict is twiddling its thumbs here...
       # until it's replaced by the next row's contents
...

It just gets replaced by the next row of the for loop, until you're left with the final row all by itself in "a_dict", since the last one of course isn't overwritten by anything. Which in you sample, would be:

{'E': 'L'}

Next you load the original json data again (though, you don't need to -- it's still in your data variable, unmodified), and add a_dict to it:

with open('test.json') as f: 
    data = json.load(f) 
    data.update(a_dict) 

This leaves you with this:

{
  "rows": [
    {
      "Company": "google", 
      "allowed": ["A", "B", "C"]
    }, 
    {
      "Company": "Yahoo", 
      "allowed": ["D", "E", "F"]
    }
  ], 
  "E": "L"
}

So, to fix this, you need to:

1. Point at the correct 'allowed' index (in your case, that'll be [2]), and

2. Modify the rows, instead of copying them out and merging them back into data.

   In your for loop, each row in data['rows'] is pointing at the value in data, so you can update the contents of row, and your work is done.

One thing I wasn't clear on was whether you meant to update all rows (implied by your looping over all rows), or just update the first row (as shown in your example desired output).

So here's a sample fix which works in either case:

import json

modify_first_row_only = True

with open('test.json', 'r') as f:
    data = json.load(f)
    rows = data['rows']

    if modify_first_row_only:
        rows[0]['allowed'][2] = 'LOOK'
    else:
        for row in rows:
            row['allowed'][2] = 'LOOK'

with open('test.json', 'w') as f:
    json.dump(data, f, indent=2)