Global error handler for any exception

Question

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.

Answer 1

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) 

Answer 2

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