| 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() |
|
|
| |
| @after_this_request |
| def remember_language(response): |
| response.set_cookie('user_lang', language) |
| return response |
|
|
| g.language = language |
|
|
