How do I `jsonify` a list in Flask?

Question

Currently Flask would raise an error when jsonifying a list.

I know there could be security reasons https://github.com/mitsuhiko/flask/issues/170, but I still would like to have a way to return a JSON list like the following:

[
    {'a': 1, 'b': 2},
    {'a': 5, 'b': 10}
]

instead of

{ 'results': [
    {'a': 1, 'b': 2},
    {'a': 5, 'b': 10}
]}

on responding to a application/json request. How can I return a JSON list in Flask using Jsonify?

This is no longer a security issue, but it is still considered a best practice to always return a dict : https://softwareengineering.stackexchange.com/a/304638/369319 — Noan Cloarec, Jul 01 '20 at 07:11
This is not a duplicate because the other question does not deal with lists — G M, Mar 08 '22 at 13:31

score 88 · Answer 1 · edited Jan 27 '16 at 21:14

88

You can't but you can do it anyway like this. I needed this for jQuery-File-Upload

import json
# get this object
from flask import Response

#example data:

    js = [ { "name" : filename, "size" : st.st_size , 
        "url" : url_for('show', filename=filename)} ]
#then do this
    return Response(json.dumps(js),  mimetype='application/json')

edited Jan 27 '16 at 21:14

Roman Kagan

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answered Oct 06 '12 at 10:24

mianos

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1

There is also flask.json.dumps don't know if this is better than json.dumps – Cameron White Dec 24 '13 at 15:25
3

@CameronWhite - `flask.json` is shorthand for `try: import simplejson as json; except ImportError: import json` – Ewan Feb 03 '14 at 08:52
5

+1 for mimetype='application/json', saved me looking for the relevant header :) – Shmil The Cat Apr 03 '14 at 10:16
The funny thing is i was stuck on this in the exact same situation (jQuery File Upload) and your code helped me in another way as well. – deweydb Jun 13 '15 at 22:34
I like this answer best. From my experience, `Response()` exposes mimetype in a simpler fashion than `make_response()` and just doing `return json.dumps()` doesn't set any headers. – sofly Mar 10 '16 at 22:18

score 64 · Accepted Answer · edited Apr 29 '18 at 18:22

64

jsonify prevents you from doing this in Flask 0.10 and lower for security reasons.

To do it anyway, just use json.dumps in the Python standard library.

http://docs.python.org/library/json.html#json.dumps

edited Apr 29 '18 at 18:22

simanacci

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answered Sep 16 '12 at 01:15

FogleBird

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3

Doing this, I will need to manually set the Content-Type of header as well? – hllau Sep 16 '12 at 06:29
3

Yes, `application/json`. – FogleBird Sep 16 '12 at 18:02
23

@FogleBird which security reason ? – Jocelyn delalande Sep 30 '14 at 13:24
8

@Jocelyndelalande: http://flask.pocoo.org/docs/0.10/security/#json-security – David Röthlisberger Dec 04 '14 at 10:55
Thanks for letting me know the security reason. :D – mxi1 Feb 05 '15 at 15:36
4

Security is no longer a valid concern, so Flask's `jsonify` now supports dumping lists: http://stackoverflow.com/a/35000418/770425 – Jeff Widman Apr 15 '16 at 20:16
1

The link @DavidRöthlisberger posted now 404s, but the information is in the 1.0 docs too: http://flask.pocoo.org/docs/1.0/security/#json-security also see https://stackoverflow.com/q/3503102/557406 – Charles L. Jun 12 '18 at 20:26

score 21 · Answer 3 · edited Dec 28 '18 at 10:30

21

This is working for me. Which version of Flask are you using?

from flask import jsonify

...

@app.route('/test/json')
def test_json():
    list = [
            {'a': 1, 'b': 2},
            {'a': 5, 'b': 10}
           ]
    return jsonify(results = list)

edited Dec 28 '18 at 10:30

Andrey

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answered Sep 15 '12 at 08:29

Geordee Naliyath

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Well I am using Flask 0.9 but it seems this would output a hash json rather than a list json. – hllau Sep 16 '12 at 06:30
6

recommending against using `list` as identifier of a variable – TMOTTM Jan 29 '21 at 16:39

Jeff Widman · Answer 4 · 2017-01-24T21:21:07.110

12

Flask's jsonify() method now serializes top-level arrays as of this commit, available in Flask 0.11 onwards.

For convenience, you can either pass in a Python list: jsonify([1,2,3]) Or pass in a series of args: jsonify(1,2,3)

Both will be serialized to a JSON top-level array: [1,2,3]

Details here: http://flask.pocoo.org/docs/dev/api/#flask.json.jsonify

edited Jan 24 '17 at 21:21

answered Jan 25 '16 at 19:09

Jeff Widman

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score 7 · Answer 5 · answered Oct 31 '14 at 17:17

7

Solved, no fuss. You can be lazy and use jsonify, all you need to do is pass in items=[your list].

Take a look here for the solution

https://github.com/mitsuhiko/flask/issues/510

answered Oct 31 '14 at 17:17

stonefury

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score 5 · Answer 6 · answered Mar 12 '16 at 14:15

5

A list in a flask can be easily jsonify using jsonify like:

from flask import Flask,jsonify
app = Flask(__name__)

tasks = [
    {
        'id':1,
        'task':'this is first task'
    },
    {
        'id':2,
        'task':'this is another task'
    }
]

@app.route('/app-name/api/v0.1/tasks',methods=['GET'])
def get_tasks():
    return jsonify({'tasks':tasks})  #will return the json

if(__name__ == '__main__'):
    app.run(debug = True)

answered Mar 12 '16 at 14:15

Hiro

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`jsonify({'tasks': tasks})` Worked perfectly this has definitely saved me a lot of time – A Star Jul 05 '17 at 23:10

score 1 · Answer 7 · edited Mar 08 '22 at 13:39

1

josonify works... But if you intend to just pass an array without the 'results' key, you can use JSON library from python. The following conversion works for me.

import json

@app.route('/test/json')
def test_json():
    mylist = [
        {'a': 1, 'b': 2},
        {'a': 5, 'b': 10}
        ]
    return json.dumps(mylist)

edited Mar 08 '22 at 13:39

G M

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answered Jan 07 '14 at 14:42

Madhan Ganesh

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I think this should be the accepted answer, is the only that returns the correct data structure. Thansk a lot – G M Mar 08 '22 at 13:40

score 1 · Answer 8 · answered Jul 06 '17 at 06:04

If you are searching literally the way to return a JSON list in flask and you are completly sure that your variable is a list then the easy way is (where bin is a list of 1's and 0's):

   return jsonify({'ans':bin}), 201

Finally, in your client you will obtain something like

{ "ans": [ 0.0, 0.0, 1.0, 1.0, 0.0 ] }

How do I `jsonify` a list in Flask?

8 Answers8

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