flask/docs/patterns/deferredcallbacks.rst

Deferred Request Callbacks
==========================

One of the design principles of Flask is that response objects are created and
passed down a chain of potential callbacks that can modify them or replace
them. When the request handling starts, there is no response object yet. It is
created as necessary either by a view function or by some other component in
the system.

What happens if you want to modify the response at a point where the response
does not exist yet?  A common example for that would be a
:meth:`~flask.Flask.before_request` callback that wants to set a cookie on the
response object.

One way is to avoid the situation. Very often that is possible. For instance
you can try to move that logic into a :meth:`~flask.Flask.after_request`
callback instead. However, sometimes moving code there makes it
more complicated or awkward to reason about.

As an alternative, you can use :func:`~flask.after_this_request` to register
callbacks that will execute after only the current request. This way you can
defer code execution from anywhere in the application, based on the current
request.

At any time during a request, we can register a function to be called at the
end of the request. For example you can remember the current language of the
user in a cookie in a :meth:`~flask.Flask.before_request` callback::

    from flask import request, after_this_request

    @app.before_request
    def detect_user_language():
        language = request.cookies.get('user_lang')

        if language is None:
            language = guess_language_from_request()

            # when the response exists, set a cookie with the language
            @after_this_request
            def remember_language(response):
                response.set_cookie('user_lang', language)
                return response

        g.language = language