Google App Engine and 404 error

A significantly simpler way to do this without requiring any CPU cycles is to place this handler at the bottom of your app.yaml

- url: /.*
    static_files: views/404.html
    upload: views/404.html

This then allows you to place a static 404.html file in your views directory. No need for a python handler. Anything that isn’t handled in your app.yaml already will hit that.

google app engine now has Custom Error Responses

so you can now add an error_handlers section to your app.yaml, as in this example:

error_handlers:

- file: default_error.html

- error_code: over_quota
    file: over_quota.html

The dev_appserver is already returning 404 responses for anything that doesn’t match the mapping, or does match the mapping but doesn’t exist. The 404 response itself has no body, but it’s still a 404:

$ wget -O - http://127.0.0.1:8080/foo
--2010-10-28 10:54:51--  http://127.0.0.1:8080/foo
Connecting to 127.0.0.1:8080... connected.
HTTP request sent, awaiting response... 404 
2010-10-28 10:54:51 ERROR 404: (no description).

$ wget -O - http://127.0.0.1:8080/foo/
--2010-10-28 10:54:54--  http://127.0.0.1:8080/foo/
Connecting to 127.0.0.1:8080... connected.
HTTP request sent, awaiting response... 404 
2010-10-28 10:54:54 ERROR 404: (no description).

If you want to return a more user-friendly error page, follow jonmiddleton’s advice and specify a custom 404 page.

I can’t comment on jonmiddleton’s answer, but the custom error responses is for App engine specific errors by the look of it.
I don’t see a way to specify a custom 404 page.

Django let’s you specify one though.

You can create a function to handle your errors for any of the status codes. You’re case being 404, define a function like this:

def Handle404(request, response, exception):
     response.out.write("Your error message") 
     response.set_status(404)`

You can pass anything – HTML / plain-text / templates in the response.out.write function. Now, add the following declaration after your app declaration.

app.error_handlers[404] = Handle404

This worked for me.

My approach is to handle both 404 and permanent redirects in a catch all handler that I put as the last one. This is usefull when I redesign and app and rename/substitute urls:

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


class ErrorsHandler(webapp2.RequestHandler):
    def get(self):
        p = self.request.path_qs
        if p in ['/index.html', 'resources-that-I-removed']:  
            return self.redirect('/and-substituted-with-this', permanent=True)
        else: 
            self.error(404)
            template = jinja_environment.get_template('404.html')
            context =  {
                'page_title': '404',
            }
            self.response.out.write(template.render(context))

webapp2 provides the error_handlers dictionary that you can use to serve custom error pages.
Example below:

def handle_404(request, response, exception):
    logging.warn(str(exception))
    response.set_status(404)
    h = YourAppBaseHandler(request, response)
    h.render_template('notfound')

def handle_500(request, response, exception):
    logging.error(str(exception))
    response.set_status(500)
    h = YourAppBaseHandler(request, response)
    h.render_template('servererror')

app = webapp2.WSGIApplication([
    webapp2.Route('/', MainHandler, name='home')
    ], debug=True)
app.error_handlers[404] = handle_404
app.error_handlers[500] = handle_500

More details are available on webapp2‘s documentation pages: http://webapp-improved.appspot.com/guide/app.html#error-handlers

I have reviewed all the above given answers and used the following at the end as the most universal 404 solution:

Add this link at the end of app.yaml

- url: /(.*) 
  script: 404.app

and create 404.py with the following content

import webapp2
from google.appengine.ext.webapp import template

class NotFound(webapp2.RequestHandler):
  def get(self):
    self.error(404)
    self.response.out.write(template.render('404.html', {}))

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

This will display contents of 404.html file with 404 error code.

The advantage of this solution is simplicity, correctness of bahaviour and flexibility, as it allows to use a static 404.html file as error page content.

I also want to warn against some of the solutions suggested above.

• Custom Error Responses don not work with 404 error
• Also don’t use static files, as they would return 200 instead of 404 error. This can cause a lot of headache ahead.