Internal redirects in Flask

description.md

Internal redirects in Flask

Goal: Instead of redirecting a client to a new path, the request should be redirected internally. The response should be returned without changing the clients (browser) URL.

Simple internal redirect

The simplest way I found to achieve internal redirecting is by calling the app.full_dispatch_request with a modified context.

def internal_redirect(path):
    env = request.environ  # take the original environment object
    env['PATH_INFO'] = path  # replace PATH_INFO with the new path
    with landing.request_context(env):  # push new request_context to stack
        response = landing.full_dispatch_request()  # handle original request with new path
    return response

You could probably create more complex scenarios by manually pushing and popping the context.

env = request.environ  # take the original environment object
env['PATH_INFO'] = path  # replace PATH_INFO with the new path
ctx = landing.request_context(env)  # create new request_context
ctx.push()  # make new context active by pushing
response = some_view_function(foo='bar')  # render view within original request but with new path
ctx.pop()  # destroy new context
return response

Background: Flask request handling

This is my understanding of how Flask handles requests. Most of it comes from:

• The purpose of the flask context stack
• How context works

1. The Flask app creates an app.request_context(environ) based on an environment variable that is passed from the WSGI server.
2. The ctx = request_context() is pushed to the request_context_stack via ctx.push(). Now ctx is the currently active context.
3. The request is processed by the Flask app calling the app.full_dispatch_request() function (plus some error handling).
4. While app.dispatch_request would only call the view_function, full_dispatch_request also does the following:
   1. Before each request, before_request() functions are executed. If one of these functions return a response, the other functions are no longer called. In any case however the return value is treated as a replacement for the view’s return value.
   2. If the before_request() functions did not return a response, the regular request handling (app.dispatch_request) kicks in and the view function that was matched has the chance to return a response.
   3. The return value of the view is then converted into an actual response object and handed over to the after_request()functions which have the chance to replace it or modify it in place.
   4. At the end of the request the teardown_request()functions are executed. This always happens, even in case of an unhandled exception down the road or if a before-request handler was not executed yet or at all (for example in test environments sometimes you might want to not execute before-request callbacks).

flask_internal_redirect.py

def internal_redirect(path):
    env = request.environ  # take the original environment object
    env['PATH_INFO'] = path  # replace PATH_INFO with the new path
    with landing.request_context(env):  # push new request_context to stack
        response = landing.full_dispatch_request()  # handle original request with new path
    return response