Custom Error Documents

In this article we will configure a Pecan application to display a custom error page whenever the server returns a 404 Page Not Found status.

This article assumes that you have already created a test application as described in Creating Your First Pecan Application.

Note

While this example focuses on the HTTP 404 message, the same technique may be applied to define custom actions for any of the HTTP status response codes in the 400 and 500 range. You are well advised to use this power judiciously.

Overview

Pecan makes it simple to customize error documents in two simple steps:

• Configure Routing of the HTTP status messages you want to handle in your application’s config.py
• Write Custom Controllers to handle the status messages you have configured

Configure Routing

Let’s configure our application test_project to route HTTP 404 Page Not Found messages to a custom controller.

First, let’s update test_project/config.py to specify a new error-handler.

# Pecan Application Configurations
app = {
    'root'            : 'test_project.controllers.root.RootController',
    'modules'         : ['test_project'],
    'static_root'     : '%(confdir)s/public',
    'template_path'   : '%(confdir)s/test_project/templates',
    'reload'          : True,
    'debug'           : True,

    # modify the 'errors' key to direct HTTP status codes to a custom
    # controller
    'errors'          : {
        #404           : '/error/404',
        404           : '/notfound',
        '__force_dict__' : True
    }
}

Instead of the default error page, Pecan will now route 404 messages to the controller method notfound.

Write Custom Controllers

The easiest way to implement the error handler is to add it to test_project.root.RootController class (typically in test_project/controllers/root.py).

from pecan import expose
from webob.exc import status_map


class RootController(object):

    @expose(generic=True, template='index.html')
    def index(self):
        return dict()

    @index.when(method='POST')
    def index_post(self, q):
        redirect('https://pecan.readthedocs.io/en/latest/search.html?q=%s' % q)


    ## custom handling of '404 Page Not Found' messages
    @expose('error.html')
    def notfound(self):
        return dict(status=404, message="test_project does not have this page")


    @expose('error.html')
    def error(self, status):
        try:
            status = int(status)
        except ValueError:
            status = 0
        message = getattr(status_map.get(status), 'explanation', '')
        return dict(status=status, message=message)

And that’s it!

Notice that the only bit of code we added to our RootController was:

## custom handling of '404 Page Not Found' messages
@expose('error.html')
def notfound(self):
    return dict(status=404, message="test_project does not have this page")

We simply expose() the notfound controller with the error.html template (which was conveniently generated for us and placed under test_project/templates/ when we created test_project). As with any Pecan controller, we return a dictionary of variables for interpolation by the template renderer.

Now we can modify the error template, or write a brand new one to make the 404 error status page of test_project as pretty or fancy as we want.