error in json python parsing

Question

I've loaded a JSON file but I'm not able to parse it in order to update or insert values.

The JSON structure is similar to this one:

{
    "nodes": [
        {
            "id": "node1",
            "x": 21.0,
            "y": 8.0
        },
        {
            "id": "node5",
            "x": 3.0,
            "y": 5.0
        }
    ]
}

While my python code to retrieve a node is similar to this:

jsonData = defaultdict(list)
with open('data.json', 'r') as f:
    jsonData = json.load(f)
print jsonData['nodes']['id'] == 'node5'

The error I get is "TypeError: list indices must be integers, not str".

How can I retrieve a node and how can I update it?

By the way, the JSON you have is broken, the third from last line has a , after the } which shouldn't be there. — Joseph
– Joseph, Commented Mar 25, 2015 at 22:09
Thank you, it's my fault but fortunally it's only a bad copy and paste, the original JSON is OK. — Chris Cris
– Chris Cris, Commented Mar 25, 2015 at 22:15

Joseph · Accepted Answer · 2015-03-25 22:21:27Z

1

In your JSON, nodes is a list of objects, so you can't try to access elements inside it using a string like you are doing with 'id'.

Instead, you can iterate over it:

with open('data.json', 'r') as f:
    jsonData = json.load(f)

for item in jsonData['nodes']:
    print item['id'], item['x'], item['y']

[Edit] To address your comment:

with open('data.json', 'r') as f:
    jsonData = json.load(f)

jsonData['nodes'] = {e['id']: e for e in jsonData['nodes']}
jsonData['nodes']['node5']['z'] = 12

edited Mar 25, 2015 at 22:21

answered Mar 25, 2015 at 22:05

Joseph

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2 Comments

Chris Cris Over a year ago

thank you Joseph but in this way how can I add a new value to an old node? (Say... I need to add "z"= 12 in the node with id = 'node5'.)

Joseph Over a year ago

If you don't have any control over the JSON you're reading in, you have two options: one, iterate over all of json['nodes'] and inside the loop, check if the current element is the one you want to modify. Or, better yet, just turn jsonData['nodes'] into a dict itself based on its contents, e.g jsonData['nodes'] = {e['id']: e for e in jsonData['nodes']}

Alex · Accepted Answer · 2015-03-25 22:39:31Z

0

This snippet adds a a new value (z=12) to an old node and updates the existing node y

import json
from collections import defaultdict

jsonData = defaultdict(list)
with open('c:/temp/data.json', 'r') as f:
    jsonData = json.load(f)
for item in jsonData['nodes']:
    if  item['id']=='node5':
       item['y'] = 5
       item['z'] = 12

answered Mar 25, 2015 at 22:39

Alex

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