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Signed-off-by: Edgar Alvarado Taleno <edgar.alvaradotaleno@ucr.ac.cr>

flask-docs/_sources/patterns/deferredcallbacks.rst.txt

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+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