code.py

import json


def check_nick(id_member=319990365):
    with open('nicks.json') as json_file:
        data = json.load(json_file)
    for x in data['players']:
        for y in x['user_{0}'.format(id_member)]:
            yield y['nick']


print(*check_nick(), sep='\n')

nicks.json

{
    "players": [{
        "name": "users",
        "user_319990365": [
            {
                "id": 319990365,
                "nick": "developer"
            }
        ]
    }]
}