Request / Response Objects

The request and response objects wrap the WSGI environment or the return value from a WSGI application so that it is another WSGI application (wraps a whole application).

How they Work

Your WSGI application is always passed two arguments. The WSGI “environment” and the WSGI start_response function that is used to start the response phase. The Request class wraps the environ for easier access to request variables (form data, request headers etc.).

The Response on the other hand is a standard WSGI application that you can create. The simple hello world in Werkzeug looks like this:

from werkzeug.wrappers import Response
application = Response('Hello World!')

To make it more useful you can replace it with a function and do some processing:

from werkzeug.wrappers import Request, Response

def application(environ, start_response):
    request = Request(environ)
    response = Response(f"Hello {request.args.get('name', 'World!')}!")
    return response(environ, start_response)

Because this is a very common task the Request object provides a helper for that. The above code can be rewritten like this:

from werkzeug.wrappers import Request, Response

@Request.application
def application(request):
    return Response(f"Hello {request.args.get('name', 'World!')}!")

The application is still a valid WSGI application that accepts the environment and start_response callable.


Mutability and Reusability of Wrappers

The implementation of the Werkzeug request and response objects are trying to guard you from common pitfalls by disallowing certain things as much as possible. This serves two purposes: high performance and avoiding of pitfalls.

For the request object the following rules apply:

  1. The request object is immutable. Modifications are not supported by default, you may however replace the immutable attributes with mutable attributes if you need to modify it.
  2. The request object may be shared in the same thread, but is not thread safe itself. If you need to access it from multiple threads, use locks around calls.
  3. It’s not possible to pickle the request object.

For the response object the following rules apply:

  1. The response object is mutable
  2. The response object can be pickled or copied after freeze() was called.
  3. Since Werkzeug 0.6 it’s safe to use the same response object for multiple WSGI responses.
  4. It’s possible to create copies using copy.deepcopy.


Wrapper Classes