This is how i did it. I hope it helps. edit, yeey it looks like a code now :D

import json
import os

def where_json(file_name):
    return os.path.exists(file_name)


if where_json('data.json'):
    pass

else:

    data = {  
        'user': input('User input: '),
        'pass': input('Pass input: ')
    }


    with open('data.json', 'w') as outfile:  
        json.dump(data, outfile)

You cannot append to a json-file as such. See the json file as one (serialized) python object, in your case a dictionary.

You could load the json file (if it's there), and if not initialise it as en empty dict.

Then add the data that you wish and save it again (in one piece).

import json
import time

fname = 'test.json'

loaded = {}
try:
    with open(fname, "r") as f:
        loaded = json.load(f)
except IOError:
    # may complain to user as well
    pass

loaded['appended'] = time.time()

with open(fname, "w") as f:
    json.dump(loaded, f)

Your logic isn't quite correct.

I'd suggest handling the non existence first, then load the file regardless (because it must exist then)

import simplejson as json
import os

filename = 'ages.json'
f = None 
# checks if the file doesn't exists or if the file is empty
if not os.path.isfile(filename) or os.stat(filename).st_size == 0:
    f = open(filename, "w")
    data = {"name": "Helio", "age": 88}
    print("No file Found, setting default age to", data["age"])
    json.dump(data, f)
if not f:  # open the file that exists now 
    f = open(filename) 
    data = json.load(f) 
f.close()  # close the file that was opened in either case 

# Print the data from the file
print("Current age is", data["age"], "-- adding a year.")
data["age"] = data["age"] + 1
print("New age is", data["age"])

To get utf8-encoded file as opposed to ascii-encoded in the accepted answer for Python 2 use:

import io, json
with io.open('data.txt', 'w', encoding='utf-8') as f:
  f.write(json.dumps(data, ensure_ascii=False))

The code is simpler in Python 3:

import json
with open('data.txt', 'w') as f:
  json.dump(data, f, ensure_ascii=False)

On Windows, the encoding='utf-8' argument to open is still necessary.

To avoid storing an encoded copy of the data in memory (result of dumps) and to output utf8-encoded bytestrings in both Python 2 and 3, use:

import json, codecs
with open('data.txt', 'wb') as f:
    json.dump(data, codecs.getwriter('utf-8')(f), ensure_ascii=False)

The codecs.getwriter call is redundant in Python 3 but required for Python 2

Readability and size:

The use of ensure_ascii=False gives better readability and smaller size:

>>> json.dumps({'price': '€10'})
'{"price": "\\u20ac10"}'
>>> json.dumps({'price': '€10'}, ensure_ascii=False)
'{"price": "€10"}'

>>> len(json.dumps({'абвгд': 1}))
37
>>> len(json.dumps({'абвгд': 1}, ensure_ascii=False).encode('utf8'))
17

Further improve readability by adding flags indent=4, sort_keys=True (as suggested by dinos66) to arguments of dump or dumps. This way you'll get a nicely indented sorted structure in the json file at the cost of a slightly larger file size.

First of all, instead of doing everything in same place have a separate function to create folder (if already not present) and dump json data as below:

def write_json(target_path, target_file, data):
    if not os.path.exists(target_path):
        try:
            os.makedirs(target_path)
        except Exception as e:
            print(e)
            raise
    with open(os.path.join(target_path, target_file), 'w') as f:
        json.dump(data, f)

Then call your function like :

write_json('/usr/home/target', 'my_json.json', my_json_data)