HTTP Exceptions

HTTP Exceptions

werkzeug.exceptions

This module implements a number of Python exceptions you can raise fromwithin your views to trigger a standard non-200 response.

Usage Example

from werkzeug.wrappers import BaseRequest
from werkzeug.wsgi import responder
from werkzeug.exceptions import HTTPException, NotFound
 
def view(request):
    raise NotFound()
 
@responder
def application(environ, start_response):
    request = BaseRequest(environ)
    try:
        return view(request)
    except HTTPException as e:
        return e

As you can see from this example those exceptions are callable WSGIapplications. Because of Python 2.4 compatibility those do not extendfrom the response objects but only from the python exception class.

As a matter of fact they are not Werkzeug response objects. However youcan get a response object by calling get_response() on a HTTPexception.

Keep in mind that you have to pass an environment to get_response()because some errors fetch additional information from the WSGIenvironment.

If you want to hook in a different exception page to say, a 404 statuscode, you can add a second except for a specific subclass of an error:

@responderdef application(environ, start_response):    request = BaseRequest(environ)    try:        return view(request)    except NotFound, e:        return not_found(request)    except HTTPException, e:        return e

Error Classes

The following error classes exist in Werkzeug:

exception werkzeug.exceptions.BadRequest(description=None, response=None)
400__Bad Request

Raise if the browser sends something to the application the applicationor server cannot handle.

exception werkzeug.exceptions.Unauthorized(description=None, response=None, www_authenticate=None)
401 Unauthorized

Raise if the user is not authorized to access a resource.

The www_authenticate argument should be used to set theWWW-Authenticate header. This is used for HTTP basic auth andother schemes. Use WWWAuthenticateto create correctly formatted values. Strictly speaking a 401response is invalid if it doesn’t provide at least one value forthis header, although real clients typically don’t care.

Parameters:

description – Override the default message used for the bodyof the response.
www-authenticate – A single value, or list of values, for theWWW-Authenticate header.

Changed in version 0.15.3: If the www_authenticate argument is not set, theWWW-Authenticate header is not set.

Changed in version 0.15.3: The response argument was restored.

Changed in version 0.15.1: description was moved back as the first argument, restoring its previous position.

Changed in version 0.15.0: www_authenticate was added as the first argument, ahead ofdescription.

exception werkzeug.exceptions.Forbidden(description=None, response=None)
403__Forbidden

Raise if the user doesn’t have the permission for the requested resourcebut was authenticated.

exception werkzeug.exceptions.NotFound(description=None, response=None)
404__Not Found

Raise if a resource does not exist and never existed.

exception werkzeug.exceptions.MethodNotAllowed(valid_methods=None, description=None)
405__Method Not Allowed

Raise if the server used a method the resource does not handle. Forexample POST if the resource is view only. Especially useful for REST.

The first argument for this exception should be a list of allowed methods.Strictly speaking the response would be invalid if you don’t provide validmethods in the header which you can do with that list.

exception werkzeug.exceptions.NotAcceptable(description=None, response=None)
406__Not Acceptable

Raise if the server can’t return any content conforming to theAccept headers of the client.

exception werkzeug.exceptions.RequestTimeout(description=None, response=None)
408__Request Timeout

Raise to signalize a timeout.

exception werkzeug.exceptions.Conflict(description=None, response=None)
409__Conflict

Raise to signal that a request cannot be completed because it conflictswith the current state on the server.

New in version 0.7.

exception werkzeug.exceptions.Gone(description=None, response=None)
410__Gone

Raise if a resource existed previously and went away without new location.

exception werkzeug.exceptions.LengthRequired(description=None, response=None)
411__Length Required

Raise if the browser submitted data but no Content-Length header whichis required for the kind of processing the server does.

exception werkzeug.exceptions.PreconditionFailed(description=None, response=None)
412__Precondition Failed

Status code used in combination with If-Match, If-None-Match, orIf-Unmodified-Since.

exception werkzeug.exceptions.RequestEntityTooLarge(description=None, response=None)
413__Request Entity Too Large

The status code one should return if the data submitted exceeded a givenlimit.

exception werkzeug.exceptions.RequestURITooLarge(description=None, response=None)
414__Request URI Too Large

Like 413 but for too long URLs.

exception werkzeug.exceptions.UnsupportedMediaType(description=None, response=None)
415__Unsupported Media Type

The status code returned if the server is unable to handle the media typethe client transmitted.

exception werkzeug.exceptions.RequestedRangeNotSatisfiable(length=None, units='bytes', description=None)
416__Requested Range Not Satisfiable

The client asked for an invalid part of the file.

New in version 0.7.

exception werkzeug.exceptions.ExpectationFailed(description=None, response=None)
417__Expectation Failed

The server cannot meet the requirements of the Expect request-header.

New in version 0.7.

exception werkzeug.exceptions.ImATeapot(description=None, response=None)
418__I’m a teapot

The server should return this if it is a teapot and someone attemptedto brew coffee with it.

New in version 0.7.

exception werkzeug.exceptions.FailedDependency(description=None, response=None)
424__Failed Dependency

Used if the method could not be performed on the resourcebecause the requested action depended on another action and that action failed.

exception werkzeug.exceptions.PreconditionRequired(description=None, response=None)
428__Precondition Required

The server requires this request to be conditional, typically to preventthe lost update problem, which is a race condition between two or moreclients attempting to update a resource through PUT or DELETE. By requiringeach client to include a conditional header (“If-Match” or “If-Unmodified-Since”) with the proper value retained from a recent GET request, theserver ensures that each client has at least seen the previous revision ofthe resource.

exception werkzeug.exceptions.TooManyRequests(description=None, response=None)
429__Too Many Requests

The server is limiting the rate at which this user receives responses, andthis request exceeds that rate. (The server may use any convenient methodto identify users and their request rates). The server may include a“Retry-After” header to indicate how long the user should wait beforeretrying.

exception werkzeug.exceptions.RequestHeaderFieldsTooLarge(description=None, response=None)
431__Request Header Fields Too Large

The server refuses to process the request because the header fields are toolarge. One or more individual fields may be too large, or the set of allheaders is too large.

exception werkzeug.exceptions.InternalServerError(description=None, response=None)
500__Internal Server Error

Raise if an internal server error occurred. This is a good fallback if anunknown error occurred in the dispatcher.

exception werkzeug.exceptions.NotImplemented(description=None, response=None)
501__Not Implemented

Raise if the application does not support the action requested by thebrowser.

exception werkzeug.exceptions.BadGateway(description=None, response=None)
502__Bad Gateway

If you do proxying in your application you should return this status codeif you received an invalid response from the upstream server it accessedin attempting to fulfill the request.

exception werkzeug.exceptions.ServiceUnavailable(description=None, response=None)
503__Service Unavailable

Status code you should return if a service is temporarily unavailable.

exception werkzeug.exceptions.HTTPUnicodeError
This exception is used to signal unicode decode errors of requestdata. For more information see the Unicode chapter.

exception werkzeug.exceptions.ClientDisconnected(description=None, response=None)
Internal exception that is raised if Werkzeug detects a disconnectedclient. Since the client is already gone at that point attempting tosend the error message to the client might not work and might ultimatelyresult in another exception in the server. Mainly this is here so thatit is silenced by default as far as Werkzeug is concerned.

Since disconnections cannot be reliably detected and are unspecifiedby WSGI to a large extent this might or might not be raised if a clientis gone.

New in version 0.8.

exception werkzeug.exceptions.SecurityError(description=None, response=None)
Raised if something triggers a security error. This is otherwiseexactly like a bad request error.

New in version 0.9.

Baseclass

All the exceptions implement this common interface:

exception werkzeug.exceptions.HTTPException(description=None, response=None)
Baseclass for all HTTP exceptions. This exception can be called as WSGIapplication to render a default error page or you can catch the subclassesof it independently and render nicer error messages.
- call(environ, start_response)
- Call the exception as WSGI application.

Parameters:

- **environ** – the WSGI environment.
- **start_response** – the response callable provided by the WSGIserver.

getresponse(_environ=None)
Get a response object. If one was passed to the exceptionit’s returned directly.

Parameters:environ – the optional environ for the request. Thiscan be used to modify the response dependingon how the request looked like.Returns:a Response object or a subclass thereof.

Special HTTP Exceptions

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

Assuming the application catches all HTTP exceptions and reacts to themproperly a view function could do the following safely and doesn’t have tocheck 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 beraised which behaves like a KeyError but also a BadRequestexception.

exception werkzeug.exceptions.BadRequestKeyError(arg=None, *args, **kwargs)
An exception that is used to signal both a KeyError and aBadRequest. Used by many of the datastructures.

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 thispurpose there is the abort() function.

werkzeug.exceptions.abort(status, *args, **kwargs)
Raises an HTTPException for the given status code or WSGIapplication:

abort(404)  # 404 Not Found
abort(Response('Hello World'))

Can be passed a WSGI application or a status code. If a status code isgiven it’s looked up in the list of exceptions and will raise thatexception, if passed a WSGI application it will wrap it in a proxy WSGIexception and raise that:

abort(404)
abort(Response('Hello World'))

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

class werkzeug.exceptions.Aborter(mapping=None, extra=None)
When passed a dict of code -> exception items it can be used ascallable that raises exceptions. If the first argument to thecallable is an integer it will be looked up in the mapping, if it’sa WSGI application it will be raised in a proxy exception.

The rest of the arguments are forwarded to the exception constructor.

Custom Errors

As you can see from the list above not all status codes are available aserrors. Especially redirects and other non 200 status codes that do notrepresent 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 toadd more logic to the errors you can override theget_description(), get_body(),get_headers() and get_response()methods. In any case you should have a look at the sourcecode of theexceptions module.

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

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