Q:

Global error handler for any exception

Is there a way to add a global catch-all error handler in which I can change the response to a generic JSON response?
I can't use the got_request_exception signal, as it is not allowed to modify the response (http://flask.pocoo.org/docs/0.10/signals/).

In contrast all signal handlers are executed in undefined order and do not modify any data.

I would prefer to not wrap the app.handle_exception function as that feels like internal API. I guess I'm after something like:
 @app.errorhandler()
 def handle_global_error(e):
     return "Global error"

Note the errorhandler does not take any parameters, meaning it would catch all exceptions/status codes which does not have a specific error handler attached to them. I know I can use errorhandler(500) or errorhandler(Exception) to catch exceptions, but if I do abort(409) for example, it will still return a HTML response.

A:

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('/')
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)

A:

This is Flask 0.12 compatible, and a very good solution to the problem (it allows one to render errors in JSON or any other format)
from functools import wraps
from flask import Flask, redirect, jsonify
app = Flask(__name__)

def get_http_exception_handler(app):
    """Overrides the default http exception handler to return JSON."""
    handle_http_exception = app.handle_http_exception
    @wraps(handle_http_exception)
    def ret_val(exception):
        exc = handle_http_exception(exception)    
        return jsonify({'code':exc.code, 'message':exc.description}), exc.code
    return ret_val

# Override the HTTP exception handler.
app.handle_http_exception = get_http_exception_handler(app)

https://github.com/pallets/flask/issues/671#issuecomment-12746738

A:

Far from elegant, but the following works for tying all subclasses of HTTPException to a single error handler:
from flask import jsonify
from werkzeug.exceptions import HTTPException

def handle_error(error):
    code = 500
    if isinstance(error, HTTPException):
        code = error.code
    return jsonify(error='error', code=code)

for cls in HTTPException.__subclasses__():
    app.register_error_handler(cls, handle_error)

