Bossy Lobster

A blog by Danny Hermes; musing on tech, mathematics, etc.

Handling errors in Google App Engine...and failing

After spending a nontrivial amount of my nights and weekends working on an App Engine app, I wanted a good way to monitor the logs without checking in on them every day. After a particularly frustrating weekend of updates that exposed unnoticed bugs that had yet to be triggered by the app, I set out to find such a way. I set out to find a Pythonic way.

Since I knew the App Engine Mail API was super easy to configure, I figured I would just email myself every time there was an exception, before serving my default 500 error page. To do so, I just needed to subclass the default RequestHandler with my own handle_exception method. (OK, prepare yourselves, a bunch of code is about to happen. See the necessary imports at the bottom of the post.)

class ExtendedHandler(RequestHandler):

    def handle_exception(self, exception, debug_mode):
        traceback_info = ''.join(format_exception(*sys.exc_info()))
        email_admins(traceback_info, defer_now=True)


Awesome! By making all my handlers inherit from ExtendedHandler, I can use the native Python modules traceback and sys to get the traceback and my handy dandy

def email_admins(error_msg, defer_now=False):
    if defer_now:
        defer(email_admins, error_msg, defer_now=False)

    sender = 'YOUR APP Errors <>'
    to = 'Robert Admin <[email protected]>, James Nekbehrd <[email protected]>'
    subject = 'YOUR APP Error: Admin Notify'
    body = '\n'.join(['Dearest Admin,',
                      'An error has occurred in YOUR APP:',

    mail.send_mail(sender=sender, to=to,
                   subject=subject, body=body)

to send out the email in the deferred queue as not to hold up the handler serving the page. Mission accomplished, right? WRONG!

Unfortunately, handle_exception only handles the "right" kind of exceptions. That is, exceptions which inherit directly from Python's Exception. From the horse's mouth:

Exceptions should typically be derived from the Exception class, either directly or indirectly.

But. But! If the app fails because a request times out, a DeadlineExceededError is thrown and handle_exception falls on its face. Why? Because DeadlineExceededError inherits directly from Exception's parent class:BaseException.

It's OK little ones, in my next post I explain how I did it while keeping my code Pythonic by using metaclasses.


from google.appengine.api import mail
from google.appengine.ext.deferred import defer
from google.appengine.ext.webapp import RequestHandler
import sysfrom traceback import format_exception
from SOME_APP_SPECIFIC_LIBRARY import serve_500


An idea or piece of code which closely follows the most common idioms of the Python language, rather than implementing code using concepts common to other languages.

Deferred Queue:

Make sure to enable the deferred library in your app.yaml by using deferred: on in your builtins.

App Engine Deferred Library Exception Google App Engine Mail API Metaclass Python Pythonic Request Handler Task Queue API