Pass user built json encoder into Flask's jsonify

Question

I want to pass a numpy JSON serializer I wrote into Flask’s jsonify function, but I cannot find a way to do this. I cannot use json.dumps, because I have to set the status_code of the Flask response when handling an error message. Is there a way to pass the JSON serializer in as a parameter, similar to using the cls parameter in json.dumps as shown here: Convert numpy type to python?
My code is below; thanks.

import json
import numpy as np
from flask import Flask, jsonify

class JSON_Improved(json.JSONEncoder):
    '''
    Used to help jsonify numpy arrays or lists that contain numpy data types.
    '''
    def default(self, obj):
        if isinstance(obj, np.integer):
            return int(obj)
        elif isinstance(obj, np.floating):
            return float(obj)
        elif isinstance(obj, np.ndarray):
            return obj.tolist()
        else:
            return super(MyEncoder, self).default(obj)

app = Flask(__name__)
@app.errorhandler(ErrorMessage)
def handle_invalid_usage(error):
    response = jsonify(error.to_dict())
    response.status_code = error.status_code
    return response

Asked By: Alex

||

Source

Answer 1

You can custom the json encoder of Flask app with app.json_encoder = JSON_Improved. JSON_Improved inherit from flask.json.JSONEncoder

class JSON_Improved(JSONEncoder):

    pass

There is a Flask Snippets about it in https://web.archive.org/web/20190128005233/http://flask.pocoo.org/snippets/119

Answered By: stamaimer

Answer 2

Since json_encoder got deprecated in Flask 2.2 and would be removed in Flask 2.3, the following update might be useful.

The documentation states that:

To use a different provider, either subclass Flask and set
json_provider_class to a provider class, or set app.json to an
instance of the class.

In my case, assigning the only json_provider_class wasn’t sufficient. But either of the following worked:

app = Flask(__name__)
app.json_provider_class = CustomJSONProvider
app.json = CustomJSONProvider(app)

or just:

app = Flask(__name__)
app.json = CustomJSONProvider(app)

Given the already defined JSON_Improved encoder, sample class CustomJSONProvider would be:

from flask.json.provider import JSONProvider


class CustomJsonProvider(JSONProvider):

    def dumps(self, obj, **kwargs):
        return json.dumps(obj, **kwargs, cls=JSON_Improved)

    def loads(self, s: str | bytes, **kwargs):
        return json.loads(s, **kwargs)

Answered By: Dmitry

Pass user built json encoder into Flask's jsonify

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