mirror of https://github.com/pallets/flask.git
46 lines
1.8 KiB
ReStructuredText
46 lines
1.8 KiB
ReStructuredText
.. _deferred-callbacks:
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Deferred Request Callbacks
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==========================
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One of the design principles of Flask is that response objects are created and
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passed down a chain of potential callbacks that can modify them or replace
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them. When the request handling starts, there is no response object yet. It is
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created as necessary either by a view function or by some other component in
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the system.
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What happens if you want to modify the response at a point where the response
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does not exist yet? A common example for that would be a
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:meth:`~flask.Flask.before_request` callback that wants to set a cookie on the
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response object.
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One way is to avoid the situation. Very often that is possible. For instance
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you can try to move that logic into a :meth:`~flask.Flask.after_request`
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callback instead. However, sometimes moving code there makes it more
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more complicated or awkward to reason about.
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As an alternative, you can use :func:`~flask.after_this_request` to register
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callbacks that will execute after only the current request. This way you can
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defer code execution from anywhere in the application, based on the current
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request.
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At any time during a request, we can register a function to be called at the
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end of the request. For example you can remember the current language of the
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user in a cookie in a :meth:`~flask.Flask.before_request` callback::
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from flask import request, after_this_request
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@app.before_request
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def detect_user_language():
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language = request.cookies.get('user_lang')
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if language is None:
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language = guess_language_from_request()
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# when the response exists, set a cookie with the language
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@after_this_request
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def remember_language(response):
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response.set_cookie('user_lang', language)
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g.language = language
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