Catch-All global exception handler in App Engine for Python

Question

Is it possible to create a catch-all global exception handler in Google App Engine using Python?

Basically, I want to catch all un-caught exceptions and gracefully handle it, while sending an email with the traceback to me.

Currently, for all uncaught errors, the users see a stacktrace with a snippet of code in it. This is undesirable.

Answer 1

Yes it is possible.
You can do it using the ereporter package that allows to receive exception reports from your application by email.

Ereporter will report two kind of exceptions:

• exceptions logged with logging.exception('Your handled exception')
• any uncaught exceptions

To catch all the exceptions, I would create a custom BaseHandler class overriding the handle_exception() method; all your request handlers should inherit from this Base class.
Have a look to Custom Error Responses too.

Here is a simple example of BaseHandler class:

class BaseHandler(webapp.RequestHandler):

    def handle_exception(self, exception, debug_mode):
        if debug_mode:
            webapp.RequestHandler.handle_exception(self, exception, debug_mode)
        else:
            logging.exception(exception)
            self.error(500)
            self.response.out.write(template.render('templdir/error.html', {}))

Answer 2

You might want to call the original handle_exception by calling the following in your BaseHandler:

webapp.RequestHandler.handle_exception(self, exception, debug_mode)

Here it is in context.

from google.appengine.ext import webapp
import sys
import traceback

class BaseHandler(webapp.RequestHandler):
    def handle_exception(self, exception, debug_mode):
        from main import emaildevs
        emaildevs('An error occurred on example.com', ''.join(traceback.format_exception(*sys.exc_info())))
        webapp.RequestHandler.handle_exception(self, exception, debug_mode)