HTTP Exceptions

Implements a number of Python exceptions which can be raised from within a view to trigger a standard HTTP non-200 response.

Usage Example

from werkzeug.wrappers.request import Request
from werkzeug.exceptions import HTTPException, NotFound

def view(request):
    raise NotFound()

@Request.application
def application(request):
    try:
        return view(request)
    except HTTPException as e:
        return e

As you can see from this example those exceptions are callable WSGI applications. However, they are not Werkzeug response objects. You can get a response object by calling get_response() on a HTTP exception.

Keep in mind that you may have to pass an environ (WSGI) or scope (ASGI) to get_response() because some errors fetch additional information relating to the request.

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

@Request.application
def application(request):
    try:
        return view(request)
    except NotFound as e:
        return not_found(request)
    except HTTPException as 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 application or server cannot handle.

Parameters:
Return type:

None

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 the WWW-Authenticate header. This is used for HTTP basic auth and other schemes. Use WWWAuthenticate to create correctly formatted values. Strictly speaking a 401 response is invalid if it doesn’t provide at least one value for this header, although real clients typically don’t care.

Parameters:

  • description (str | None) – Override the default message used for the body of the response.

  • www-authenticate – A single value, or list of values, for the WWW-Authenticate header(s).

  • response (Response | None)

  • www_authenticate (None | (WWWAuthenticate | t.Iterable[WWWAuthenticate]))

Return type:

None

Changelog

Changed in version 2.0: Serialize multiple www_authenticate items into multiple WWW-Authenticate headers, rather than joining them into a single value, for better interoperability.

Changed in version 0.15.3: If the www_authenticate argument is not set, the WWW-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 of description.

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

403 Forbidden

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

Parameters:
Return type:

None

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

404 Not Found

Raise if a resource does not exist and never existed.

Parameters:
Return type:

None

exception werkzeug.exceptions.MethodNotAllowed(valid_methods=None, description=None, response=None)

405 Method Not Allowed

Raise if the server used a method the resource does not handle. For example 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 valid methods in the header which you can do with that list.

Takes an optional list of valid http methods starting with werkzeug 0.3 the list will be mandatory.

Parameters:

  • valid_methods (t.Iterable[str] | None)

  • description (str | None)

  • response (Response | None)

Return type:

None

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

406 Not Acceptable

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

Parameters:
Return type:

None

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

408 Request Timeout

Raise to signalize a timeout.

Parameters:
Return type:

None

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

409 Conflict

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

Changelog

Added in version 0.7.

Parameters:
Return type:

None

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

410 Gone

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

Parameters:
Return type:

None

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

411 Length Required

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

Parameters:
Return type:

None

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

412 Precondition Failed

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

Parameters:
Return type:

None

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 given limit.

Parameters:
Return type:

None

exception werkzeug.exceptions.RequestURITooLarge(description=None, response=None)

414 Request URI Too Large

Like 413 but for too long URLs.

Parameters:
Return type:

None

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 type the client transmitted.

Parameters:
Return type:

None

exception werkzeug.exceptions.RequestedRangeNotSatisfiable(length=None, units='bytes', description=None, response=None)

416 Requested Range Not Satisfiable

The client asked for an invalid part of the file.

Changelog

Added in version 0.7.

Takes an optional Content-Range header value based on length parameter.

Parameters:

  • length (int | None)

  • units (str)

  • description (str | None)

  • response (Response | None)

Return type:

None

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

417 Expectation Failed

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

Changelog

Added in version 0.7.

Parameters:
Return type:

None

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 attempted to brew coffee with it.

Changelog

Added in version 0.7.

Parameters:
Return type:

None

exception werkzeug.exceptions.MisdirectedRequest(description=None, response=None)

421 Misdirected Request

Indicates that the request was directed to a server that is not able to produce a response.

Added in version 3.1.

Parameters:
Return type:

None

exception werkzeug.exceptions.UnprocessableEntity(description=None, response=None)

422 Unprocessable Entity

Used if the request is well formed, but the instructions are otherwise incorrect.

Parameters:
Return type:

None

exception werkzeug.exceptions.Locked(description=None, response=None)

423 Locked

Used if the resource that is being accessed is locked.

Parameters:
Return type:

None

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

424 Failed Dependency

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

Parameters:
Return type:

None

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

428 Precondition Required

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

Parameters:
Return type:

None

exception werkzeug.exceptions.TooManyRequests(description=None, response=None, retry_after=None)

429 Too Many Requests

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

Parameters:

  • retry_after (datetime | int | None) – If given, set the Retry-After header to this value. May be an int number of seconds or a datetime.

  • description (str | None)

  • response (Response | None)

Return type:

None

Changelog

Changed in version 1.0: Added retry_after parameter.

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 too large. One or more individual fields may be too large, or the set of all headers is too large.

Parameters:
Return type:

None

exception werkzeug.exceptions.UnavailableForLegalReasons(description=None, response=None)

451 Unavailable For Legal Reasons

This status code indicates that the server is denying access to the resource as a consequence of a legal demand.

Parameters:
Return type:

None

exception werkzeug.exceptions.InternalServerError(description=None, response=None, original_exception=None)

500 Internal Server Error

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

Changelog

Changed in version 1.0.0: Added the original_exception attribute.

Parameters:
Return type:

None

original_exception

The original exception that caused this 500 error. Can be used by frameworks to provide context when handling unexpected errors.

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

501 Not Implemented

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

Parameters:
Return type:

None

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

502 Bad Gateway

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

Parameters:
Return type:

None

exception werkzeug.exceptions.ServiceUnavailable(description=None, response=None, retry_after=None)

503 Service Unavailable

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

Parameters:

  • retry_after (datetime | int | None) – If given, set the Retry-After header to this value. May be an int number of seconds or a datetime.

  • description (str | None)

  • response (Response | None)

Return type:

None

Changelog

Changed in version 1.0: Added retry_after parameter.

exception werkzeug.exceptions.GatewayTimeout(description=None, response=None)

504 Gateway Timeout

Status code you should return if a connection to an upstream server times out.

Parameters:
Return type:

None

exception werkzeug.exceptions.HTTPVersionNotSupported(description=None, response=None)

505 HTTP Version Not Supported

The server does not support the HTTP protocol version used in the request.

Parameters:
Return type:

None

exception werkzeug.exceptions.ClientDisconnected(description=None, response=None)

Internal exception that is raised if Werkzeug detects a disconnected client. Since the client is already gone at that point attempting to send the error message to the client might not work and might ultimately result in another exception in the server. Mainly this is here so that it is silenced by default as far as Werkzeug is concerned.

Since disconnections cannot be reliably detected and are unspecified by WSGI to a large extent this might or might not be raised if a client is gone.

Changelog

Added in version 0.8.

Parameters:
Return type:

None

exception werkzeug.exceptions.SecurityError(description=None, response=None)

Raised if something triggers a security error. This is otherwise exactly like a bad request error.

Changelog

Added in version 0.9.

Parameters:
Return type:

None

Baseclass

All the exceptions implement this common interface:

exception werkzeug.exceptions.HTTPException(description=None, response=None)

The base class for all HTTP exceptions. This exception can be called as a WSGI application to render a default error page or you can catch the subclasses of it independently and render nicer error messages.

Changelog

Changed in version 2.1: Removed the wrap class method.

Parameters:
Return type:

None

get_response(environ=None, scope=None)

Get a response object. If one was passed to the exception it’s returned directly.

Parameters:

  • environ (WSGIEnvironment | WSGIRequest | None) – the optional environ for the request. This can be used to modify the response depending on how the request looked like.

  • scope (dict[str, t.Any] | None)

Returns:

a Response object or a subclass thereof.

Return type:

Response

__call__(environ, start_response)

Call the exception as WSGI application.

Parameters:

  • environ (WSGIEnvironment) – the WSGI environment.

  • start_response (StartResponse) – the response callable provided by the WSGI server.

Return type:

t.Iterable[bytes]

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.

exception werkzeug.exceptions.BadRequestKeyError(arg=None, *args, **kwargs)

An exception that is used to signal both a KeyError and a BadRequest. Used by many of the datastructures.

Parameters:

  • arg (object | None)

  • args (t.Any)

  • kwargs (t.Any)

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.

werkzeug.exceptions.abort(status, *args, **kwargs)

Raises an HTTPException for the given status code or WSGI application.

If a status code is given, it will be looked up in the list of exceptions and will raise that exception. If passed a WSGI application, it will wrap it in a proxy WSGI exception and raise that:

abort(404)  # 404 Not Found
abort(Response('Hello World'))
Parameters:
Return type:

t.NoReturn

If you want to use this functionality with custom exceptions you can create 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 as callable that raises exceptions. If the first argument to the callable is an integer it will be looked up in the mapping, if it’s a WSGI application it will be raised in a proxy exception.

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

Parameters:

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')