Python convert JSON to CSV

i have a JSON file which contains:

{
    "leaderboard": {
        "$": [
            {
                "userId": 1432024286216,
                "userName": "Joe Bloggs",
                "score": 111111,
                "gameType": "standard",
                "dateCreated": 1432024397833,
                "_id": 1432024397833
            },
            {
                "userId": 1432024626556,
                "userName": "Jane Bloggs",
                "score": 222222,
                "gameType": "demo",
                "dateCreated": 1432024730861,
                "_id": 1432024730861
            }
        ]
    },
    "users": {
        "$": [
            {
                "userId_U": 1432024286000,
                "userName_U": "Paul Bloggs",
                "score_U": 333333,
                "gameType_U": "standard",
                "dateCreated_U": 1432024397833,
                "_id_U": 1432024397833
            },
            {
                "userId_U": 1432024626777,
                "userName_U": "John Bloggs",
                "score_U": 444444,
                "gameType_U": "demo",
                "dateCreated_U": 1432024730861,
                "_id_U": 1432024730861
            }
        ]
    }
}

i am trying to create a CSV from this in Python. The CSV creates the headers: userId, userName etc. only from the 'leaderboard' data object only and populate the corresponding data for it. so create a column each for: userId, userName etc.

i started coding this but i am getting the 'leaderboard' and 'users' headers created and their data in 1 cell beneath them. my code:

import json, csv

x = open('test/dbTest.json')

rows = json.load(x)
with open('test.csv', 'wb+') as f:
    dict_writer = csv.DictWriter(f, fieldnames=['leaderboard', 'users'])
    dict_writer.writeheader()
    dict_writer.writerow(rows)

i have tried to change the field name to 'userId' , 'userName' etc but it then gives error: ValueError: dict contains fields not in fieldnames: u'users', u'leaderboard'

can someone please help how to get extract the data i need? explanation why above code is incorrect and kindly point me in the right direction please? apologies if not clear and if missing anything.

also, the CSV should look like:

userId,userName,score,gameType,dateCreated,_id,
1432024286216,Joe Bloggs,111111,standard,1432024397833,1432024397833
1432024626556,Jane Bloggs,222222,demo,1432024730861,1432024730861

and to clarify, 'users' and 'leaderboard' are different with different field names.

# json_data being the literal file data, in this example

import json
import csv

data = json.loads(json_data)['leaderboard']['$']

with open('/tmp/test.csv', 'w') as outf:
    dw = csv.DictWriter(outf, data[0].keys())
    dw.writeheader()
    for row in data:
        dw.writerow(row)