Flask TypeError 'is not JSON serializable' - nested dictionary

Question

i am using Flask as framework for my server, and while returning a response i get the following error:

> Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\flask\app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "C:\Python27\lib\site-packages\flask\app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "C:\Python27\lib\site-packages\flask_restful\__init__.py", line 480, in wrapper
    resp = resource(*args, **kwargs)
  File "C:\Python27\lib\site-packages\flask\views.py", line 84, in view
    return self.dispatch_request(*args, **kwargs)
  File "C:\Python27\lib\site-packages\flask_restful\__init__.py", line 595, in dispatch_request
    resp = meth(*args, **kwargs)
  File "rest.py", line 27, in get
    return jsonify(**solution)
  File "C:\Python27\lib\site-packages\flask\json.py", line 263, in jsonify
    (dumps(data, indent=indent, separators=separators), '\n'),
  File "C:\Python27\lib\site-packages\flask\json.py", line 123, in dumps
    rv = _json.dumps(obj, **kwargs)
  File "C:\Python27\lib\json\__init__.py", line 251, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "C:\Python27\lib\json\encoder.py", line 209, in encode
    chunks = list(chunks)
  File "C:\Python27\lib\json\encoder.py", line 434, in _iterencode
    for chunk in _iterencode_dict(o, _current_indent_level):
  File "C:\Python27\lib\json\encoder.py", line 408, in _iterencode_dict
    for chunk in chunks:
  File "C:\Python27\lib\json\encoder.py", line 332, in _iterencode_list
    for chunk in chunks:
  File "C:\Python27\lib\json\encoder.py", line 332, in _iterencode_list
    for chunk in chunks:
  File "C:\Python27\lib\json\encoder.py", line 442, in _iterencode
    o = _default(o)
  File "C:\Python27\lib\site-packages\flask\json.py", line 80, in default
    return _json.JSONEncoder.default(self, o)
  File "C:\Python27\lib\json\encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: {'origin': u'porto', 'dest': u'lisboa', 'price': '31', 'date': '2017-12-23', 'url': u'https://www.google.pt/flights/#search;f=opo;t=lis;d=2017-12-23;r=2017-12-24'} is not JSON serializable

i have the following function:

from flask import Flask, request, jsonify
from flask_restful import Resource, Api
from flask_cors import CORS, cross_origin
from json import dumps
import flights
import solveProblem

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

class Flights(Resource):
    def get(self, data):
        print 'received data from client: ' + data
        solution = solveProblem.solve(data)
        print 'got the solution from the script! \nSOLUTION: \n'
        print solution
        return jsonify(solution)

api.add_resource(Flights, '/flights/<string:data>')

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

while debugging the problem, i found the following solutions which did not work:

1) return solution instead of {'solution': solution}

2) do jsonify(solution)

3) do jsonify(**solution)

none of the above worked for me; i wonder why this happens, when i am trying to return a valid dictionary:

{'flights': [[{'origin': u'porto', 'dest': u'lisboa', 'price': '31', 'date': '2017-12-23', 'url': u'https://www.google.pt/flights/#search;f=opo;t=lis;d=2017-12-23;r=2017-12-24'}]], 'cost': '31'}

any help is appreciated. Thanks

Answer 1

My guess is when you were creating 'solution', the data that got assigned to it was an incorrectly formatted dictionary

    {'item', 'value'}

Instead of:

    {'item': 'value'}

Thus creating a set instead of a dict