HTTP Exceptions — Werkzeug documentation

HTTP Exceptions

Error Classes

The following error classes exist in Werkzeug:

Baseclass

All the exceptions implement this common interface:

Special HTTP Exceptions

Starting with Werkzeug 0.3 some of the builtin classes raise exceptions that look like regular python exceptions (eg KeyError) but are BadRequest HTTP exceptions at the same time. This decision was made to simplify a common pattern where you want to abort if the client tampered with the submitted form data in a way that the application can’t recover properly and should abort with 400 BAD REQUEST.

Assuming the application catches all HTTP exceptions and reacts to them properly a view function could do the following safely and doesn’t have to check if the keys exist:

def new_post(request):
    post = Post(title=request.form['title'], body=request.form['body'])
    post.save()
    return redirect(post.url)

If title or body are missing in the form, a special key error will be raised which behaves like a KeyError but also a BadRequest exception.

Simple Aborting

Sometimes it’s convenient to just raise an exception by the error code, without importing the exception and looking up the name etc. For this purpose there is the abort() function.

If you want to use this functionality with custom exceptions you can create an instance of the aborter class:

Custom Errors

As you can see from the list above not all status codes are available as errors. Especially redirects and other non 200 status codes that do not represent errors are missing. For redirects you can use the redirect() function from the utilities.

If you want to add an error yourself you can subclass HTTPException:

from werkzeug.exceptions import HTTPException

class PaymentRequired(HTTPException):
    code = 402
    description = '<p>Payment required.</p>'

This is the minimal code you need for your own exception. If you want to add more logic to the errors you can override the get_description(), get_body(), get_headers() and get_response() methods. In any case you should have a look at the sourcecode of the exceptions module.

You can override the default description in the constructor with the description parameter:

raise BadRequest(description='Request failed because X was not present')