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<div class="section" id="using-sqlite-3-with-flask">
<span id="sqlite3"></span><h1>Using SQLite 3 with Flask<a class="headerlink" href="#using-sqlite-3-with-flask" title="Permalink to this headline">¶</a></h1>
<p>In Flask you can implement the opening of database connections at the
beginning of the request and closing at the end 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> 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 in combination with the special <a class="reference internal" href="../api.html#flask.g" title="flask.g"><tt class="xref py py-class docutils literal"><span class="pre">g</span></tt></a> object.</p>
<p>So here is a simple example of how you can use SQLite 3 with Flask:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sqlite3</span>
<span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">g</span>
<span class="n">DATABASE</span> <span class="o">=</span> <span class="s">'/path/to/database.db'</span>
<span class="k">def</span> <span class="nf">connect_db</span><span class="p">():</span>
<span class="k">return</span> <span class="n">sqlite3</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">DATABASE</span><span class="p">)</span>
<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="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">g</span><span class="p">,</span> <span class="s">'db'</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>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Please keep in mind that the teardown request functions are always
executed, even if a before-request handler failed or was never
executed. Because of this we have to make sure here that the database
is there before we close it.</p>
</div>
<div class="section" id="connect-on-demand">
<h2>Connect on Demand<a class="headerlink" href="#connect-on-demand" title="Permalink to this headline">¶</a></h2>
<p>The downside of this approach is that this will only work if Flask
executed the before-request handlers for you. If you are attempting to
use the database from a script or the interactive Python shell you would
have to do something like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">with</span> <span class="n">app</span><span class="o">.</span><span class="n">test_request_context</span><span class="p">():</span>
<span class="n">app</span><span class="o">.</span><span class="n">preprocess_request</span><span class="p">()</span>
<span class="c"># now you can use the g.db object</span>
</pre></div>
</div>
<p>In order to trigger the execution of the connection code. You won’t be
able to drop the dependency on the request context this way, but you could
make it so that the application connects when necessary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">get_connection</span><span class="p">():</span>
<span class="n">db</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">g</span><span class="p">,</span> <span class="s">'_db'</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="k">if</span> <span class="n">db</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">db</span> <span class="o">=</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="k">return</span> <span class="n">db</span>
</pre></div>
</div>
<p>Downside here is that you have to use <tt class="docutils literal"><span class="pre">db</span> <span class="pre">=</span> <span class="pre">get_connection()</span></tt> instead of
just being able to use <tt class="docutils literal"><span class="pre">g.db</span></tt> directly.</p>
</div>
<div class="section" id="easy-querying">
<span id="id1"></span><h2>Easy Querying<a class="headerlink" href="#easy-querying" title="Permalink to this headline">¶</a></h2>
<p>Now in each request handling function you can access <cite>g.db</cite> to get the
current open database connection. To simplify working with SQLite, a
helper function can be useful:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">query_db</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">one</span><span class="o">=</span><span class="bp">False</span><span class="p">):</span>
<span class="n">cur</span> <span class="o">=</span> <span class="n">g</span><span class="o">.</span><span class="n">db</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">args</span><span class="p">)</span>
<span class="n">rv</span> <span class="o">=</span> <span class="p">[</span><span class="nb">dict</span><span class="p">((</span><span class="n">cur</span><span class="o">.</span><span class="n">description</span><span class="p">[</span><span class="n">idx</span><span class="p">][</span><span class="mi">0</span><span class="p">],</span> <span class="n">value</span><span class="p">)</span>
<span class="k">for</span> <span class="n">idx</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">row</span><span class="p">))</span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">cur</span><span class="o">.</span><span class="n">fetchall</span><span class="p">()]</span>
<span class="k">return</span> <span class="p">(</span><span class="n">rv</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="k">if</span> <span class="n">rv</span> <span class="k">else</span> <span class="bp">None</span><span class="p">)</span> <span class="k">if</span> <span class="n">one</span> <span class="k">else</span> <span class="n">rv</span>
</pre></div>
</div>
<p>This handy little function makes working with the database much more
pleasant than it is by just using the raw cursor and connection objects.</p>
<p>Here is how you can use it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">query_db</span><span class="p">(</span><span class="s">'select * from users'</span><span class="p">):</span>
<span class="k">print</span> <span class="n">user</span><span class="p">[</span><span class="s">'username'</span><span class="p">],</span> <span class="s">'has the id'</span><span class="p">,</span> <span class="n">user</span><span class="p">[</span><span class="s">'user_id'</span><span class="p">]</span>
</pre></div>
</div>
<p>Or if you just want a single result:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">user</span> <span class="o">=</span> <span class="n">query_db</span><span class="p">(</span><span class="s">'select * from users where username = ?'</span><span class="p">,</span>
<span class="p">[</span><span class="n">the_username</span><span class="p">],</span> <span class="n">one</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">if</span> <span class="n">user</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'No such user'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="n">the_username</span><span class="p">,</span> <span class="s">'has the id'</span><span class="p">,</span> <span class="n">user</span><span class="p">[</span><span class="s">'user_id'</span><span class="p">]</span>
</pre></div>
</div>
<p>To pass variable parts to the SQL statement, use a question mark in the
statement and pass in the arguments as a list. Never directly add them to
the SQL statement with string formatting because this makes it possible
to attack the application using <a class="reference external" href="http://en.wikipedia.org/wiki/SQL_injection">SQL Injections</a>.</p>
</div>
<div class="section" id="initial-schemas">
<h2>Initial Schemas<a class="headerlink" href="#initial-schemas" title="Permalink to this headline">¶</a></h2>
<p>Relational databases need schemas, so applications often ship a
<cite>schema.sql</cite> file that creates the database. It’s a good idea to provide
a function that creates the database based on that schema. This function
can do that for you:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">closing</span>
<span class="k">def</span> <span class="nf">init_db</span><span class="p">():</span>
<span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">connect_db</span><span class="p">())</span> <span class="k">as</span> <span class="n">db</span><span class="p">:</span>
<span class="k">with</span> <span class="n">app</span><span class="o">.</span><span class="n">open_resource</span><span class="p">(</span><span class="s">'schema.sql'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="n">db</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span><span class="o">.</span><span class="n">executescript</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">())</span>
<span class="n">db</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
</pre></div>
</div>
<p>You can then create such a database from the python shell:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">yourapplication</span> <span class="kn">import</span> <span class="n">init_db</span>
<span class="gp">>>> </span><span class="n">init_db</span><span class="p">()</span>
</pre></div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Using SQLite 3 with Flask</a><ul>
<li><a class="reference internal" href="#connect-on-demand">Connect on Demand</a></li>
<li><a class="reference internal" href="#easy-querying">Easy Querying</a></li>
<li><a class="reference internal" href="#initial-schemas">Initial Schemas</a></li>
</ul>
</li>
</ul>
<h3>Related Topics</h3>
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<li><a href="../index.html">Documentation overview</a><ul>
<li><a href="index.html">Patterns for Flask</a><ul>
<li>Previous: <a href="fabric.html" title="previous chapter">Deploying with Fabric</a></li>
<li>Next: <a href="sqlalchemy.html" title="next chapter">SQLAlchemy in Flask</a></li>
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