Last Updated: May 25, 2017
· calsaverini
2010 12 08 130808

JSON exception handler for Flask

If you want Flask to spit json objects when exceptions are raised inside the application code, this exception handler can be useful:

from flask import jsonify
from werkzeug.exceptions import default_exceptions
from werkzeug.exceptions import HTTPException

class JSONExceptionHandler(object):

    def __init__(self, app=None):
        if app:

    def std_handler(self, error):
        response = jsonify(message=error.message)
        response.status_code = error.code if isinstance(error, HTTPException) else 500
        return response

    def init_app(self, app):
        self.app = app
        for code, v in default_exceptions.iteritems():

    def register(self, exception_or_code, handler=None):
        self.app.errorhandler(exception_or_code)(handler or self.std_handler)

To test it, try running the following app:

from exception_handler import *
from werkzeug.exceptions import default_exceptions, NotFound
from flask import Flask

app = Flask(__name__)
handler = JSONExceptionHandler(app)

class UserNotFound(NotFound):
    def __init__(self, uid):
        super(UserNotFound, self).__init__()
        self.message = "Couldn't found a user with uid={}.".format(uid)

def home(uid):
    if uid != 0:
        raise UserNotFound(uid)
        return jsonify({'user': "Foo", "uid": 0})

if __name__ == "__main__":

If you run this app, a GET request at http://localhost:5000/0 will return a JSON:

{"user": "Foo", "uid": 0}

and a GET request at http://localhost:5000/1 will return a JSON:

{"message": "Couldn't found a user with uid=1."}

You could also create custom exception handlers:

def zero_division_handler(error):
    return jsonify({"message": "Don't divide by zero!!!!!1111"})

handler.register(ZeroDivisionError, handler=zero_division_handler)
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Hi I tried your code above, but it doesnt work for 405 Method Not Allowed
it seems fallback to Flask exception, any idea ?
Also where is this reference to? from exception_handler import *


7 months ago ·