Flask: Converting Python dict to json object for client api

Question

I need to create a json object from a flask a query result. I then need to pass the json object to the route to create an API.

In searching for some slick ways to create dicts from my instances I stumbled upon a method in this post to use the internal dict of the instance and add a jsond method to the model class. Here is the Model with the custom method, 'jsond':

from app import db  
class Rest(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    name = db.Column(db.String(100), unique = True)
    street = db.Column(db.Text)
    zipcd = db.Column(db.Integer)
    comments = db.relationship('Comment', backref='rest', lazy='dynamic')
    lat = db.Column(db.Float(6))
    lng = db.Column(db.Float(6))

    def __init__(self,name,street,zipcd):
        self.name = name
        self.street = street
        self.zipcd = zipcd

    def __repr__(self):
        return '{}'.format(self.name)

    def name_slug(self):
        return self.name

    def jsond(self):
        instDict = self.__dict__.copy()
        if instDict.has_key('_sa_instance_state'):
            del instDict['_sa_instance_state']
        return instDict

And here's my view function:

from app import app, db
from flask import render_template, flash, redirect, session, url_for, request, g,      jsonify, make_response
from flask.json import dumps
from flask.ext import restful 
from flask.ext.httpauth import HTTPBasicAuth
from models import Comment, Rest, Badge
from helper import make_badges, make_inspections, loc_query
import operator
auth = HTTPBasicAuth()

@app.route('/api',methods=['GET'])
def makeApi():

    ###Query Parameters###
    lim = request.args.get('limit', 10)
    off = request.args.get('offset', 0)
    loc = request.args.get('location', "39.94106,-75.173192")
    lat, lng = loc.split(",")
    radius = request.args.get('radius',2)

    query = loc_query(lat,lng,radius,off,lim)

    results = Rest.query.from_statement(query).all()


    rest_json = []
    for rest in results:
        rest_json.append(rest.jsond())

    return make_response(jsonify({'count':len(rest_json),'rests':rest_json}))

So when in the python API command line I can run queries successfully and create a dict from a particular instances in a query with all fields present (using the custom jsond method). However when I go to the makeApi route using my view I get a json object with ONLY THE 'id' field present:

rests: [
{
id: 28450
},
{
id: 28795
},
{
id: 30439
},
{
id: 29325
},
{
id: 29765
},
{
id: 29928
},
{
id: 30383
},
{
id: 29064
},
{
id: 29862
},
{
id: 28610
}
]
}

I've been going in circles for hours and have no idea why the view behavior would differ from the python API. Perhaps its something Im doing wrong with jsonify but i dont think so.

Solution

Generally is a bad idea your approach on jsonifying a model:

• self.__dict__ may contain a lot of undocumented keys
• column types: you can't jsonify relationship and column types directly. And notably a datetime column.
• security: sometimes you might want to hide some fields (for untrusted users consuming your APIs for example)

A good approach is to create a method to return a json serializable dictionary:

class Foo(db.Model):
    field1 = db.Column(...)
    field2 = db.Column(...)

    def as_dict(self):
        obj_d = {
            'field1': self.field1,
            'field2': self.field2,
            ...
        }
        return obj_d

Then in your view:

foos = Foo.query.all()
results = [ foo.as_dict() for foo in foos ]

return jsonify({count: len(results), results: results)

Based on your application you can make as_dict smarter by converting fields (notably datetime fileds) in javascript friendly formats or adding convenient fields like following a relationship.

Comments

[user2957824]

@paolocasiello is there way to pass a dict object for use in the template directly? if so is this ever beneficial?

[Paolo Casciello]

You mean Jinja templates? You can pass the whole model to the templates render_template(..., mod=model) and use functions/attributes directly from the template

[qazimusab]

Thanks so much! This helped me a ton!