Why can't Python parse this JSON data?

Your data is not valid JSON format. You have [] when you should have {}:

• [] are for JSON arrays, which are called list in Python
• {} are for JSON objects, which are called dict in Python

Here's how your JSON file should look:

{
    "maps": [
        {
            "id": "blabla",
            "iscategorical": "0"
        },
        {
            "id": "blabla",
            "iscategorical": "0"
        }
    ],
    "masks": {
        "id": "valore"
    },
    "om_points": "value",
    "parameters": {
        "id": "valore"
    }
}

Then you can use your code:

import json
from pprint import pprint

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

pprint(data)

With data, you can now also find values like so:

data["maps"][0]["id"]
data["masks"]["id"]
data["om_points"]

Try those out and see if it starts to make sense.

Your data.json should look like this:

{
 "maps":[
         {"id":"blabla","iscategorical":"0"},
         {"id":"blabla","iscategorical":"0"}
        ],
"masks":
         {"id":"valore"},
"om_points":"value",
"parameters":
         {"id":"valore"}
}

Your code should be:

import json
from pprint import pprint

with open('data.json') as data_file:    
    data = json.load(data_file)
pprint(data)

Note that this only works in Python 2.6 and up, as it depends upon the with-statement. In Python 2.5 use from __future__ import with_statement, in Python <= 2.4, see Justin Peel's answer, which this answer is based upon.

You can now also access single values like this:

data["maps"][0]["id"]  # will return 'blabla'
data["masks"]["id"]    # will return 'valore'
data["om_points"]      # will return 'value'

Justin Peel's answer is really helpful, but if you are using Python 3 reading JSON should be done like this:

with open('data.json', encoding='utf-8') as data_file:
    data = json.loads(data_file.read())

Note: use json.loads instead of json.load. In Python 3, json.loads takes a string parameter. json.load takes a file-like object parameter. data_file.read() returns a string object.

To be honest, I don't think it's a problem to load all json data into memory in most cases. I see this in JS, Java, Kotlin, cpp, rust almost every language I use. Consider memory issue like a joke to me :)

On the other hand, I don't think you can parse json without reading all of it.