How do I write JSON data to a file in Python?

To get utf8-encoded file (for smaller size) in Python 2.x:

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

For example for data = {u'абвгд': 1} I get 17 bytes for utf8 vs 26 for ascii version.

The code is simpler in Python 3.x:

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

I would answer with slight modification with aforementioned answers and that is to write a prettified JSON file which human eyes can read better. For this, pass sort_keys as True and indent with 4 space characters and you are good to go. Also take care of ensuring that the ascii codes will not be written in your JSON file:

with open('data.txt', 'w') as outfile:
     json.dump(jsonData, outfile, sort_keys = True, indent = 4,
ensure_ascii=False)

For those of you who are trying to dump greek or other "exotic" languages such as me but are also having problems (unicode errors) with weird characters such as the peace symbol (\u262E) or others which are often contained in json formated data such as Twitter's, the solution could be as follows (sort_keys is obviously optional):

import codecs, json
with codecs.open('data.json', 'w', 'utf8') as f:
     f.write(json.dumps(data, sort_keys = True, ensure_ascii=False))

I don't have enough reputation to add in comments, so I just write some of my findings of this annoying TypeError here:

Basically, I think it's a bug in the json.dump() function in Python 2 only - It can't dump a Python (dictionary / list) data containing non-ASCII characters, even you open the file with the encoding = 'utf-8' parameter. (i.e. No matter what you do). But, json.dumps() works on both Python 2 and 3.

To illustrate this, following up phihag's answer: the code in his answer breaks in Python 2 with exception TypeError: must be unicode, not str, if data contains non-ASCII characters. (Python 2.7.6, Debian):

import json
data = {u'\u0430\u0431\u0432\u0433\u0434': 1} #{u'абвгд': 1}
with open('data.txt', 'w') as outfile:
    json.dump(data, outfile)

It however works fine in Python 3.

Antony Hatchkins's first solution works in Python 2, but as a follow up to his answer: The unicode part is not necessary (and IMHO, it's wrong), the following works on my system (ensure_ascii=Fasle is also not strictly needed for the code to work, but more related to the output format):

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

In fact, the above code work in both Python 2 and 3 (the original code with unicode() call won't work in Python 3 because unicode() function no longer exists in Python 3). I guess the reason both code (with / without unicode() call) work in Python 2 is because Python 2 uses bytes while to write on the file handler, and unicode is converted to bytes implicitly before writing.

json.dump(data, open('data.txt', 'wb'))

Write a data in file using JSON use json.dump() or json.dumps() used. write like this to store data in file.

import json
data = [1,2,3,4,5]
with open('no.txt', 'w') as txtfile:
    json.dump(data, txtfile)

this example in list is store to a file.

Just like phiphags answer, but with nice formatting:

import json
with open('data.txt', 'w') as outfile:
    json.dump(data, outfile, indent=4, sort_keys=True, separators=(',', ':'))

Explanation of the parameters of json.dump:

• indent: Use 4 spaces to indent each entry, e.g. when a new dict is started (otherwise all will be in one line),
• sort_keys: sort the keys of dictionaries
• separators: To prevent Python from adding trailing whitespaces