Global error handler for any exception

You can use @app.errorhandler(Exception):

Demo (the HTTPException check ensures that the status code is preserved):

from flask import Flask, abort, jsonify
from werkzeug.exceptions import HTTPException

app = Flask('test')

@app.errorhandler(Exception)
def handle_error(e):
    code = 500
    if isinstance(e, HTTPException):
        code = e.code
    return jsonify(error=str(e)), code

@app.route("https://stackoverflow.com/")
def index():
    abort(409)

app.run(port=1234)

Output:

$ http get http://127.0.0.1:1234/
HTTP/1.0 409 CONFLICT
Content-Length: 31
Content-Type: application/json
Date: Sun, 29 Mar 2015 17:06:54 GMT
Server: Werkzeug/0.10.1 Python/3.4.3

{
    "error": "409: Conflict"
}

$ http get http://127.0.0.1:1234/notfound
HTTP/1.0 404 NOT FOUND
Content-Length: 32
Content-Type: application/json
Date: Sun, 29 Mar 2015 17:06:58 GMT
Server: Werkzeug/0.10.1 Python/3.4.3

{
    "error": "404: Not Found"
}

If you also want to override the default HTML exceptions from Flask (so that they also return JSON), add the following before app.run:

from werkzeug.exceptions import default_exceptions
for ex in default_exceptions:
    app.register_error_handler(ex, handle_error)

For older Flask versions (<=0.10.1, i.e. any non-git/master version at the moment), add the following code to your application to register the HTTP errors explicitly:

from werkzeug import HTTP_STATUS_CODES
for code in HTTP_STATUS_CODES:
    app.register_error_handler(code, handle_error)