httpnotfound aiohttp

When building web applications, you'll inevitably need to handle 404 errors - those pesky "page not found" messages when a user tries to access a URL that doesn't exist.

The Python aiohttp framework makes it easy to customize your 404 pages and handle these errors gracefully. Here's how:

Raising 404 Errors

First, you need to raise a 404 error in your aiohttp application when a route isn't found. You can do this with the aiohttp.web.HTTPNotFound exception:

from aiohttp import web

routes = web.RouteTableDef()

@routes.get('/page')
async def handle(request):
  raise web.HTTPNotFound()

This will raise a 404 error that aiohttp will catch.

Custom Error Handler

Next, setup a custom error handler that will render your 404 page:

async def error_handler(request, error):
  if isinstance(error, web.HTTPNotFound):
    return web.Response(text="Sorry, page not found", status=404)
  else:
    raise error
  
app = web.Application()
app.router.add_routes(routes)
app.add_routes([web.get('/{tail:.*}', error_handler)])

The key thing here is to check if the error is a 404, and handle that custom case accordingly.

Templating

For full customization, you'll want to render a template instead of just returning text. Make sure your template engine is configured with aiohttp, and then render the template:

return web.Response(
  text=self.template.render('404.html'), 
  status=404
)

Final Touches

Add some nice styling to the page, perhaps some helpful links, and you've got yourself a polished 404 page that doesn't look like an ugly default error.

Handling 404s gracefully is essential for a good user experience. With aiohttp, it's straightforward to raise the errors and handle them properly. Add custom templates and styling, and those pesky 404s won't be so bad after all!