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<div class="section" id="deferred-request-callbacks">
<span id="deferred-callbacks"></span><h1>Deferred Request Callbacks<a class="headerlink" href="#deferred-request-callbacks" title="Permalink to this headline">¶</a></h1>
<p>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.</p>
<p>But 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
before-request function that wants to set a cookie on the response object.</p>
<p>One way is to avoid the situation. Very often that is possible. For
instance you can try to move that logic into an after-request callback
instead. Sometimes however moving that code there is just not a very
pleasant experience or makes code look very awkward.</p>
<p>As an alternative possibility you can attach a bunch of callback functions
to the <a class="reference internal" href="../api.html#flask.g" title="flask.g"><tt class="xref py py-data docutils literal"><span class="pre">g</span></tt></a> object and call then at the end of the request.
This way you can defer code execution from anywhere in the application.</p>
<div class="section" id="the-decorator">
<h2>The Decorator<a class="headerlink" href="#the-decorator" title="Permalink to this headline">¶</a></h2>
<p>The following decorator is the key. It registers a function on a list on
the <a class="reference internal" href="../api.html#flask.g" title="flask.g"><tt class="xref py py-data docutils literal"><span class="pre">g</span></tt></a> object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">g</span>
<span class="k">def</span> <span class="nf">after_this_request</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="k">if</span> <span class="ow">not</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">g</span><span class="p">,</span> <span class="s">'after_request_callbacks'</span><span class="p">):</span>
<span class="n">g</span><span class="o">.</span><span class="n">after_request_callbacks</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">g</span><span class="o">.</span><span class="n">after_request_callbacks</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="k">return</span> <span class="n">f</span>
</pre></div>
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<div class="section" id="calling-the-deferred">
<h2>Calling the Deferred<a class="headerlink" href="#calling-the-deferred" title="Permalink to this headline">¶</a></h2>
<p>Now you can use the <cite>after_this_request</cite> decorator to mark a function to
be called at the end of the request. But we still need to call them. For
this the following function needs to be registered as
<a class="reference internal" href="../api.html#flask.Flask.after_request" title="flask.Flask.after_request"><tt class="xref py py-meth docutils literal"><span class="pre">after_request()</span></tt></a> callback:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@app.after_request</span>
<span class="k">def</span> <span class="nf">call_after_request_callbacks</span><span class="p">(</span><span class="n">response</span><span class="p">):</span>
<span class="k">for</span> <span class="n">callback</span> <span class="ow">in</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">g</span><span class="p">,</span> <span class="s">'after_request_callbacks'</span><span class="p">,</span> <span class="p">()):</span>
<span class="n">response</span> <span class="o">=</span> <span class="n">callback</span><span class="p">(</span><span class="n">response</span><span class="p">)</span>
<span class="k">return</span> <span class="n">response</span>
</pre></div>
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<div class="section" id="a-practical-example">
<h2>A Practical Example<a class="headerlink" href="#a-practical-example" title="Permalink to this headline">¶</a></h2>
<p>Now we can easily at any point in time register a function to be called at
the end of this particular request. For example you can remember the
current language of the user in a cookie in the before-request function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">request</span>
<span class="nd">@app.before_request</span>
<span class="k">def</span> <span class="nf">detect_user_language</span><span class="p">():</span>
<span class="n">language</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">cookies</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'user_lang'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">language</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">language</span> <span class="o">=</span> <span class="n">guess_language_from_request</span><span class="p">()</span>
<span class="nd">@after_this_request</span>
<span class="k">def</span> <span class="nf">remember_language</span><span class="p">(</span><span class="n">response</span><span class="p">):</span>
<span class="n">response</span><span class="o">.</span><span class="n">set_cookie</span><span class="p">(</span><span class="s">'user_lang'</span><span class="p">,</span> <span class="n">language</span><span class="p">)</span>
<span class="n">g</span><span class="o">.</span><span class="n">language</span> <span class="o">=</span> <span class="n">language</span>
</pre></div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Deferred Request Callbacks</a><ul>
<li><a class="reference internal" href="#the-decorator">The Decorator</a></li>
<li><a class="reference internal" href="#calling-the-deferred">Calling the Deferred</a></li>
<li><a class="reference internal" href="#a-practical-example">A Practical Example</a></li>
</ul>
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