Exception handling in Python (webapp2, Google App Engine)

Question

I tried to use the functions suggested on how to handle exceptions:

http://webapp-improved.appspot.com/guide/exceptions.html

in main.py:

def handle_404(request, response, exception):
        logging.exception(exception)
        response.write('404 Error')
        response.set_status(404)

def handle_500(request, response, exception):
    logging.exception(exception)
    response.write('A server error occurred!')
    response.set_status(500)

class AdminPage(webapp2.RequestHandler):
    def get(self):
    ...
    admin_id = admin.user_id()
    queues = httpRequests.get_queues(admin_id)

app = webapp2.WSGIApplication(...)

app.error_handlers[404] = handle_404
app.error_handlers[500] = handle_500

The function in httpRequests.py:

def get_queues(admin_id):

    url = "http://localhost:8080/api/" + admin_id + "/queues"
    result = urlfetch.fetch(url)

    if (result.status_code == 200):
        received_data = json.loads(result.content)
        return received_data

The function which is being called in the API:

class Queues(webapp2.RequestHandler): 
    def get(self, admin_id): 
        queues = queues(admin_id)
        if queues == []:
            self.abort(404)
        else:
            self.response.write(json.dumps(queues))

I'm stuck at get_queues in httpRequests.py. How to handle the HTTP exceptions with urlfetch?

Answer 1

Another approach for handling errors is to make a BaseHandler with handle_exception and have all your other handlers extend this one. A fully working example will look like this:

import webapp2
from google.appengine.api import urlfetch

class BaseHandler(webapp2.RequestHandler):
  def handle_exception(self, exception, debug_mode):
    if isinstance(exception, urlfetch.DownloadError):
      self.response.out.write('Oups...!')
    else:
      # Display a generic 500 error page.
      pass

class MainHandler(BaseHandler):
  def get(self):
    url = "http://www.google.commm/"
    result = urlfetch.fetch(url)
    self.response.write('Hello world!')


app = webapp2.WSGIApplication([
    ('/', MainHandler)
  ], debug=True)

An even better solution would be to throw the exception as they are when running in debug mode and handling them in a more friendly way when running on production. Taken from another example you could do something like this for your BaseHandler and expand it as you wish:

class BaseHandler(webapp2.RequestHandler):
  def handle_exception(self, exception, debug_mode):
    if not debug_mode:
      super(BaseHandler, self).handle_exception(exception, debug_mode)
    else:
      if isinstance(exception, urlfetch.DownloadError):
        # Display a download-specific error page
        pass
      else:
        # Display a generic 500 error page.
        pass