Return JSON response from Flask view

Question

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

User · Answer

Flask 1 1 x supports returning a JSON dict without calling jsonify  If you want to return something besides a dict  you still need to call jsonify   app route  quot   quot   def index        return            quot api stuff quot    quot values quot          is equivalent to  app route  quot   quot   def index        return jsonify            quot api stuff quot    quot values quot           See the pull request that added this  https   github com pallets flask pull 3111

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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

User · Answer

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  lt arg gt     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

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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

User · Answer

Create a python dictionary to return as the response  response     key    value    Then you can use json dumps   or flask jsonify   to turn a python dictionary into JSON  from json import dumps  return dumps response   from flask import jsonify  return jsonify response    Note that flask jsonify   is made for flask and also it has some more advatages  You can use it like in the following code  from flask import jsonify  params          key    value     return jsonify   params   This is like using the following line  return jsonify key  value    Remeber in this code you don t need the response dictionary

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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

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I like this way       app route  quot  summary quot       def summary            responseBody      quot message quot    quot bla bla bla quot    quot summary quot   make summary             return make response jsonify responseBody   200

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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

User · Answer

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

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EDIT  As of Flask 1 1 0  as mentioned by Adrien Ball in comment below   you can now directly return a python dict and Flask will jsonify it behind the scenes for you  app route   summary   def summary        d   make summary       return d  if you want to return a different json-serializable object  you can import and use jsonify from flask import jsonify     return jsonify x

User · Answer

if its a dict  flask can return it directly  Version 1 0 2   def summary        d   make summary       return d  200

User · Answer

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

User · Answer

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

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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

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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

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