Flask API failing to decode JSON data. Error: "message": "Failed to decode JSON object: Expecting value: line 1 column 1 (char 0)"

Question

I'm setting up a simple rest api using flask and flask-restful. Right now all I'm trying to do is create a post request with some Json data, and then return it just to see if it works. I always get the same error "message": "Failed to decode JSON object: Expecting value: line 1 column 1 (char 0)"

Below is my code

from flask import Flask, jsonify, request
from flask_restful import Resource, Api

app = Flask(__name__)
api = Api(app)


class Tester(Resource):
   def get(self):
       return {'about': 'Hello World'}

   def post(self):
       data_json = request.get_json(force=True)
       return {'you sent': data_json}


api.add_resource(Tester, '/')

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

The curl request I am making to test this is below, I've also tried making request using postman

curl -H "Content-Type: application/json" -X POST -d '{'username':"abc",'password':"abc"}' http://localhost:5000

In postman, do you have raw-> JSON (application/json) option selected? — bc291
– bc291, Commented Feb 27, 2019 at 14:59
@needtobe No I did not. Just tried it and I got a 200 response. Thank you. That said why doesn't it seem to work in the curl request? — FProlog
– FProlog, Commented Feb 27, 2019 at 15:04
Try curl -i -H "Content-Type: application/json" -X POST -d "{\"username\":\"abc\", \"password\":\"abc\"}" 127.0.0.1:5000 instead. — bc291
– bc291, Commented Feb 27, 2019 at 15:24

alessiosavi · Accepted Answer · 2020-04-15 16:59:15Z

13

You need to select raw -> JSON (application/json) in Postman like this:

When it comes to your cURL request, answer explains that windows's command line lacks support of strings with single quotes, so use:

curl -i -H "Content-Type: application/json" -X POST -d "{\"username\":\"abc\", \"password\":\"abc\"}" 127.0.0.1:5000

instead:

curl -H "Content-Type: application/json" -X POST -d '{'username':"abc",'password':"abc"}' http://localhost:5000

\ escapes " character.

edited Apr 15, 2020 at 16:59

alessiosavi

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answered Feb 27, 2019 at 15:08

bc291

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Comments

Anon · Accepted Answer · 2021-07-23 04:32:31Z

4

You also need to have the Content-Length header enabled.

answered Jul 23, 2021 at 4:32

Anon

1098 bronze badges

1 Comment

Ryan Detzel Over a year ago

This was it for me, so strange!

ChrisQIU · Accepted Answer · 2022-05-20 14:33:21Z

2

Some additional information in case you are using python script to test your flask api like me - you have to dumps before adding the dictionary to the data field.

import requests
import json

response = requests.post(
    'http://localhost:5000', 
    data = json.dumps({'username':"abc",'password':"abc"}),
    headers = {"Content-Type": "application/json"}
    )

answered May 20, 2022 at 14:33

ChrisQIU

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1 Comment

Banty Over a year ago

This answer solved my problem. Got rid of the JSON decode error and also converts datetime.datetime(2019, 4, 3, 9, 47, 9, 427000) to 2019-04-03 09:47:09.427000 on the fly. >>> json.dumps({'datetime':'datetime.datetime(2019, 4, 3, 9, 47, 9, 427000)'}) output: {'datetime':'2019-04-03 09:47:09.427000'}

DhoTjai · Accepted Answer · 2019-02-27 16:38:14Z

0

The curl request (payload) is incorrect. Use double quotes in the payload.

curl -H "Content-Type: application/json" -X POST -d '{"username":"abc","password":"abc"}' http://localhost:5000

edited Feb 27, 2019 at 16:38

answered Feb 27, 2019 at 15:06

DhoTjai

1511 silver badge6 bronze badges

1 Comment

DhoTjai Over a year ago

Single quote in your payload --> '{'.

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