I have a function that analyzes a CSV file with Pandas and produces a dict with summary information. I want to return the results as a response from a Flask view. How do I return a JSON response?

@app.route("/summary")
def summary():
    d = make_summary()
    # send it back as json

Pass the summary data to the jsonify function, which returns a JSON response.

from flask import jsonify

@app.route('/summary')
def summary():
    d = make_summary()
    return jsonify(d)

As of Flask 0.11, you can pass any JSON-serializable type, not just dict, as the top level object.

jsonify serializes the data you pass it to JSON. If you want to serialize the data yourself, do what jsonify does by building a response with status=200 and mimetype='application/json'.

from flask import json

@app.route('/summary')
def summary():
    data = make_summary()
    response = app.response_class(
        response=json.dumps(data),
        status=200,
        mimetype='application/json'
    )
    return response

Pass keyword arguments to flask.jsonify and they will be output as a JSON object.

@app.route('/_get_current_user')
def get_current_user():
    return jsonify(
        username=g.user.username,
        email=g.user.email,
        id=g.user.id
    )

{
    "username": "admin",
    "email": "admin@localhost",
    "id": 42
}

If you already have a dict, you can pass it directly as jsonify(d).

If you don’t want to use jsonify for some reason, you can do what it does manually. Call flask.json.dumps to create JSON data, then return a response with the application/json content type.

from flask import json

@app.route('/summary')
def summary():
    data = make_summary()
    response = app.response_class(
        response=json.dumps(data),
        mimetype='application/json'
    )
    return response

flask.json is distinct from the built-in json module. It will use the faster simplejson module if available, and enables various integrations with your Flask app.

If you want to analyze a file uploaded by the user, the Flask quickstart shows how to get files from users and access them. Get the file from request.files and pass it to the summary function.

from flask import request, jsonify
from werkzeug import secure_filename

@app.route('/summary', methods=['GET', 'POST'])
def summary():
    if request.method == 'POST':
        csv = request.files['data']
        return jsonify(
            summary=make_summary(csv),
            csv_name=secure_filename(csv.filename)
        )

    return render_template('submit_data.html')

Replace the 'data' key for request.files with the name of the file input in your HTML form.

To return a JSON response and set a status code you can use make_response:

from flask import jsonify, make_response

@app.route('/summary')
def summary():
    d = make_summary()
    return make_response(jsonify(d), 200)

Inspiration taken from this comment in the Flask issue tracker.

As of version 1.1.0 Flask, if a view returns a dict it will be turned into a JSON response.

@app.route("/users", methods=['GET'])
def get_user():
    return {
        "user": "John Doe",
    }

I use a decorator to return the result of jsonfiy. I think it is more readable when a view has multiple returns. This does not support returning a tuple like content, status, but I handle returning error statuses with app.errorhandler instead.

import functools
from flask import jsonify

def return_json(f):
    @functools.wraps(f)
    def inner(**kwargs):
        return jsonify(f(**kwargs))

    return inner

@app.route('/test/<arg>')
@return_json
def test(arg):
    if arg == 'list':
        return [1, 2, 3]
    elif arg == 'dict':
        return {'a': 1, 'b': 2}
    elif arg == 'bool':
        return True
    return 'none of them'

Prior to Flask 0.11, jsonfiy would not allow returning an array directly. Instead, pass the list as a keyword argument.

@app.route('/get_records')
def get_records():
    results = [
        {
          "rec_create_date": "12 Jun 2016",
          "rec_dietary_info": "nothing",
          "rec_dob": "01 Apr 1988",
          "rec_first_name": "New",
          "rec_last_name": "Guy",
        },
        {
          "rec_create_date": "1 Apr 2016",
          "rec_dietary_info": "Nut allergy",
          "rec_dob": "01 Feb 1988",
          "rec_first_name": "Old",
          "rec_last_name": "Guy",
        },
    ]
    return jsonify(results=list)

In Flask 1.1, if you return a dictionary and it will automatically be converted into JSON. So if make_summary() returns a dictionary, you can

from flask import Flask

app = Flask(__name__)

@app.route('/summary')
def summary():
    d = make_summary()
    return d

The SO that asks about including the status code was closed as a duplicate to this one. So to also answer that question, you can include the status code by returning a tuple of the form (dict, int). The dict is converted to JSON and the int will be the HTTP Status Code. Without any input, the Status is the default 200. So in the above example the code would be 200. In the example below it is changed to 201.

from flask import Flask

app = Flask(__name__)

@app.route('/summary')
def summary():
    d = make_summary()
    return d, 201  # 200 is the default

You can check the status code using

curl --request GET "http://127.0.0.1:5000/summary" -w "\ncode: %{http_code}\n\n"

if its a dict, flask can return it directly (Version 1.0.2)

def summary():
    d = make_summary()
    return d, 200

“”” Using Flask Class-base View “””

from flask import Flask, request, jsonify

from flask.views import MethodView

app = Flask(**__name__**)

app.add_url_rule('/summary/', view_func=Summary.as_view('summary'))

class Summary(MethodView):

    def __init__(self):
        self.response = dict()

    def get(self):
        self.response['summary'] = make_summary()  # make_summary is a method to calculate the summary.
        return jsonify(self.response)

Flask 1.1.x

now Flask support request return with json directly, jsonify not required anymore

@app.route("/")
def index():
    return {
        "api_stuff": "values",
    }

is equivalent to

@app.route("/")
def index():
    return jsonify({
        "api_stuff": "values",
    })

for more information read here https://medium.com/octopus-wealth/returning-json-from-flask-cf4ce6fe9aeb and https://github.com/pallets/flask/pull/3111