[python] How do I write JSON data to a file?

I have JSON data stored in the variable data.

I want to write this to a text file for testing so I don't have to grab the data from the server each time.

Currently, I am trying this:

obj = open('data.txt', 'wb')
obj.write(data)
obj.close

And I am receiving this error:

TypeError: must be string or buffer, not dict

How to fix this?

if you are trying to write a pandas dataframe into a file using a json format i'd recommend this

destination='filepath'
saveFile = open(destination, 'w')
saveFile.write(df.to_json())
saveFile.close()

All previous answers are correct here is a very simple example:

#! /usr/bin/env python
import json

def write_json():
    # create a dictionary  
    student_data = {"students":[]}
    #create a list
    data_holder = student_data["students"]
    # just a counter
    counter = 0
    #loop through if you have multiple items..         
    while counter < 3:
        data_holder.append({'id':counter})
        data_holder.append({'room':counter})
        counter += 1    
    #write the file        
    file_path='/tmp/student_data.json'
    with open(file_path, 'w') as outfile:
        print("writing file to: ",file_path)
        # HERE IS WHERE THE MAGIC HAPPENS 
        json.dump(student_data, outfile)
    outfile.close()     
    print("done")

write_json()

To write the JSON with indentation, "pretty print":

import json

outfile = open('data.json')
json.dump(data, outfile, indent=4)

Also, if you need to debug improperly formatted JSON, and want a helpful error message, use import simplejson library, instead of import json (functions should be the same)

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)

The JSON data can be written to a file as follows

hist1 = [{'val_loss': [0.5139984398465246],
'val_acc': [0.8002029867684085],
'loss': [0.593220705309384],
'acc': [0.7687131817929321]},
{'val_loss': [0.46456472964199463],
'val_acc': [0.8173602046780344],
'loss': [0.4932038113037539],
'acc': [0.8063946213802453]}]

Write to a file:

with open('text1.json', 'w') as f:
     json.dump(hist1, f)

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'?????': 1}
with open('data.txt', 'w') as outfile:
    json.dump(data, outfile)

It however works fine in Python 3.

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

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({'?????': 1}))
37
>>> len(json.dumps({'?????': 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.

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.

Writing JSON to a File

import json

data = {}
data['people'] = []
data['people'].append({
    'name': 'Scott',
    'website': 'stackabuse.com',
    'from': 'Nebraska'
})
data['people'].append({
    'name': 'Larry',
    'website': 'google.com',
    'from': 'Michigan'
})
data['people'].append({
    'name': 'Tim',
    'website': 'apple.com',
    'from': 'Alabama'
})

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

Reading JSON from a File

import json

with open('data.txt') as json_file:
    data = json.load(json_file)
    for p in data['people']:
        print('Name: ' + p['name'])
        print('Website: ' + p['website'])
        print('From: ' + p['from'])
        print('')

The accepted answer is fine. However, I ran into "is not json serializable" error using that.

Here's how I fixed it with open("file-name.json", 'w') as output:

output.write(str(response))

Although it is not a good fix as the json file it creates will not have double quotes, however it is great if you are looking for quick and dirty.

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))

Read and write JSON files with Python 2+3; works with unicode

# -*- coding: utf-8 -*-
import json

# Make it work for Python 2+3 and with Unicode
import io
try:
    to_unicode = unicode
except NameError:
    to_unicode = str

# Define data
data = {'a list': [1, 42, 3.141, 1337, 'help', u'€'],
        'a string': 'bla',
        'another dict': {'foo': 'bar',
                         'key': 'value',
                         'the answer': 42}}

# Write JSON file
with io.open('data.json', 'w', encoding='utf8') as outfile:
    str_ = json.dumps(data,
                      indent=4, sort_keys=True,
                      separators=(',', ': '), ensure_ascii=False)
    outfile.write(to_unicode(str_))

# Read JSON file
with open('data.json') as data_file:
    data_loaded = json.load(data_file)

print(data == data_loaded)

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. This is useful if you want to compare json files with a diff tool / put them under version control.
• separators: To prevent Python from adding trailing whitespaces

With a package

Have a look at my utility package mpu for a super simple and easy to remember one:

import mpu.io
data = mpu.io.read('example.json')
mpu.io.write('example.json', data)

Created JSON file

{
    "a list":[
        1,
        42,
        3.141,
        1337,
        "help",
        "€"
    ],
    "a string":"bla",
    "another dict":{
        "foo":"bar",
        "key":"value",
        "the answer":42
    }
}

Common file endings

.json

Alternatives

• CSV: Super simple format (read & write)
• JSON: Nice for writing human-readable data; VERY commonly used (read & write)
• YAML: YAML is a superset of JSON, but easier to read (read & write, comparison of JSON and YAML)
• pickle: A Python serialization format (read & write)
• MessagePack (Python package): More compact representation (read & write)
• HDF5 (Python package): Nice for matrices (read & write)
• XML: exists too *sigh* (read & write)

For your application, the following might be important:

• Support by other programming languages
• Reading / writing performance
• Compactness (file size)

See also: Comparison of data serialization formats

In case you are rather looking for a way to make configuration files, you might want to read my short article Configuration files in Python