How to JSON format an HTTP error response in webapp2 - json

Question

I am using webapp2 for development in App Engine. What I would like to do is to send a custom JSON formatted response in case of an error. For example when the request length is larger that a threshold, to respond with HTTP 400 and response body
{'error':'InvalidMessageLength'}
In webapp2, there is the option to assign error handlers for certain exceptions. For example:
app.error_handlers[400] = handle_error_400
Where handle_error_400 is the following:
def handle_error_400(request, response, exception):
response.write(exception)
response.set_status(400)
When webapp2.RequestHandler.abort(400) is executed, the above code is executed.
How is it possible to have different response formats (HTML and JSON) dynamically based on the above setup? That is, how it is possible to call different versions of handle_error_400 function?

Answer 1

Here is a fully working example that demonstrates how to have the same error handler for all kind of errors and if your URL starts with /json then the response will be an application/json (use your imagination on how you could make a good use of the request object to decide what kind of response you should provide):
import webapp2
import json
def handle_error(request, response, exception):
if request.path.startswith('/json'):
response.headers.add_header('Content-Type', 'application/json')
result = {
'status': 'error',
'status_code': exception.code,
'error_message': exception.explanation,
}
response.write(json.dumps(result))
else:
response.write(exception)
response.set_status(exception.code)
app = webapp2.WSGIApplication()
app.error_handlers[404] = handle_error
app.error_handlers[400] = handle_error
In the above example you can easily test the different behaviours by visting the following URLs that will return a 404 which is the easiest error to test:
http://localhost:8080/404
http://localhost:8080/json/404