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<div class="section" id="step-4-request-database-connections">
<span id="tutorial-dbcon"></span><h1>Step 4: Request Database Connections<a class="headerlink" href="#step-4-request-database-connections" title="Permalink to this headline">ΒΆ</a></h1>
<p>Now we know how we can open database connections and use them for scripts,
but how can we elegantly do that for requests? We will need the database
connection in all our functions so it makes sense to initialize them
before each request and shut them down afterwards.</p>
<p>Flask allows us to do that with the <a class="reference internal" href="../api.html#flask.Flask.before_request" title="flask.Flask.before_request"><tt class="xref py py-meth docutils literal"><span class="pre">before_request()</span></tt></a>,
<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> and <a class="reference internal" href="../api.html#flask.Flask.teardown_request" title="flask.Flask.teardown_request"><tt class="xref py py-meth docutils literal"><span class="pre">teardown_request()</span></tt></a>
decorators:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@app.before_request</span>
<span class="k">def</span> <span class="nf">before_request</span><span class="p">():</span>
<span class="n">g</span><span class="o">.</span><span class="n">db</span> <span class="o">=</span> <span class="n">connect_db</span><span class="p">()</span>
<span class="nd">@app.teardown_request</span>
<span class="k">def</span> <span class="nf">teardown_request</span><span class="p">(</span><span class="n">exception</span><span class="p">):</span>
<span class="n">g</span><span class="o">.</span><span class="n">db</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
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<p>Functions marked with <a class="reference internal" href="../api.html#flask.Flask.before_request" title="flask.Flask.before_request"><tt class="xref py py-meth docutils literal"><span class="pre">before_request()</span></tt></a> are called before
a request and passed no arguments. Functions marked with
<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> are called after a request and
passed the response that will be sent to the client. They have to return
that response object or a different one. They are however not guaranteed
to be executed if an exception is raised, this is where functions marked with
<a class="reference internal" href="../api.html#flask.Flask.teardown_request" title="flask.Flask.teardown_request"><tt class="xref py py-meth docutils literal"><span class="pre">teardown_request()</span></tt></a> come in. They get called after the
response has been constructed. They are not allowed to modify the request, and
their return values are ignored. If an exception occurred while the request was
being processed, it is passed to each function; otherwise, <cite>None</cite> is passed in.</p>
<p>We store our current database connection on the special <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 that Flask provides for us. This object stores information for one
request only and is available from within each function. Never store such
things on other objects because this would not work with threaded
environments. That special <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 does some magic behind
the scenes to ensure it does the right thing.</p>
<p>Continue to <a class="reference internal" href="views.html#tutorial-views"><em>Step 5: The View Functions</em></a>.</p>
<div class="admonition hint">
<p class="first admonition-title">Hint</p>
<p>Where do I put this code?</p>
<p>If you’ve been following along in this tutorial, you might be wondering
where to put the code from this step and the next. A logical place is to
group these module-level functions together, and put your new
<tt class="docutils literal"><span class="pre">before_request</span></tt> and <tt class="docutils literal"><span class="pre">teardown_request</span></tt> functions below your existing
<tt class="docutils literal"><span class="pre">init_db</span></tt> function (following the tutorial line-by-line).</p>
<p class="last">If you need a moment to find your bearings, take a look at how the <a class="reference external" href="http://github.com/mitsuhiko/flask/tree/master/examples/flaskr/">example
source</a> is organized. In Flask, you can put all of your application code
into a single Python module. You don’t have to, and if your app <a class="reference internal" href="../patterns/packages.html#larger-applications"><em>grows
larger</em></a>, it’s a good idea not to.</p>
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