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<li class="nav-item nav-item-this"><a href="">Application Setup</a></li>
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<section id="application-setup">
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<h1>Application Setup<a class="headerlink" href="#application-setup" title="Link to this heading">¶</a></h1>
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<p>A Flask application is an instance of the <a class="reference internal" href="../api.html#flask.Flask" title="flask.Flask"><code class="xref py py-class docutils literal notranslate"><span class="pre">Flask</span></code></a> class.
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Everything about the application, such as configuration and URLs, will
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be registered with this class.</p>
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<p>The most straightforward way to create a Flask application is to create
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a global <a class="reference internal" href="../api.html#flask.Flask" title="flask.Flask"><code class="xref py py-class docutils literal notranslate"><span class="pre">Flask</span></code></a> instance directly at the top of your code, like
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how the “Hello, World!” example did on the previous page. While this is
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simple and useful in some cases, it can cause some tricky issues as the
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project grows.</p>
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<p>Instead of creating a <a class="reference internal" href="../api.html#flask.Flask" title="flask.Flask"><code class="xref py py-class docutils literal notranslate"><span class="pre">Flask</span></code></a> instance globally, you will create
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it inside a function. This function is known as the <em>application
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factory</em>. Any configuration, registration, and other setup the
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application needs will happen inside the function, then the application
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will be returned.</p>
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<section id="the-application-factory">
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<h2>The Application Factory<a class="headerlink" href="#the-application-factory" title="Link to this heading">¶</a></h2>
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<p>It’s time to start coding! Create the <code class="docutils literal notranslate"><span class="pre">flaskr</span></code> directory and add the
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<code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> file. The <code class="docutils literal notranslate"><span class="pre">__init__.py</span></code> serves double duty: it will
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contain the application factory, and it tells Python that the <code class="docutils literal notranslate"><span class="pre">flaskr</span></code>
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directory should be treated as a package.</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ mkdir flaskr
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</pre></div>
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</div>
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<div class="literal-block-wrapper docutils container" id="id1">
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<div class="code-block-caption"><span class="caption-text"><code class="docutils literal notranslate"><span class="pre">flaskr/__init__.py</span></code></span><a class="headerlink" href="#id1" title="Link to this code">¶</a></div>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">os</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">flask</span><span class="w"> </span><span class="kn">import</span> <span class="n">Flask</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">create_app</span><span class="p">(</span><span class="n">test_config</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
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<span class="c1"># create and configure the app</span>
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<span class="n">app</span> <span class="o">=</span> <span class="n">Flask</span><span class="p">(</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">instance_relative_config</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
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<span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">from_mapping</span><span class="p">(</span>
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<span class="n">SECRET_KEY</span><span class="o">=</span><span class="s1">'dev'</span><span class="p">,</span>
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<span class="n">DATABASE</span><span class="o">=</span><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">app</span><span class="o">.</span><span class="n">instance_path</span><span class="p">,</span> <span class="s1">'flaskr.sqlite'</span><span class="p">),</span>
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<span class="p">)</span>
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<span class="k">if</span> <span class="n">test_config</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
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<span class="c1"># load the instance config, if it exists, when not testing</span>
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<span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">from_pyfile</span><span class="p">(</span><span class="s1">'config.py'</span><span class="p">,</span> <span class="n">silent</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
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<span class="k">else</span><span class="p">:</span>
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<span class="c1"># load the test config if passed in</span>
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<span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">from_mapping</span><span class="p">(</span><span class="n">test_config</span><span class="p">)</span>
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<span class="c1"># ensure the instance folder exists</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">app</span><span class="o">.</span><span class="n">instance_path</span><span class="p">)</span>
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<span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
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<span class="k">pass</span>
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<span class="c1"># a simple page that says hello</span>
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<span class="nd">@app</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s1">'/hello'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">hello</span><span class="p">():</span>
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<span class="k">return</span> <span class="s1">'Hello, World!'</span>
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<span class="k">return</span> <span class="n">app</span>
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</pre></div>
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</div>
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</div>
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<p><code class="docutils literal notranslate"><span class="pre">create_app</span></code> is the application factory function. You’ll add to it
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later in the tutorial, but it already does a lot.</p>
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<ol class="arabic simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">app</span> <span class="pre">=</span> <span class="pre">Flask(__name__,</span> <span class="pre">instance_relative_config=True)</span></code> creates the
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<a class="reference internal" href="../api.html#flask.Flask" title="flask.Flask"><code class="xref py py-class docutils literal notranslate"><span class="pre">Flask</span></code></a> instance.</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">__name__</span></code> is the name of the current Python module. The app
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needs to know where it’s located to set up some paths, and
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<code class="docutils literal notranslate"><span class="pre">__name__</span></code> is a convenient way to tell it that.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">instance_relative_config=True</span></code> tells the app that
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configuration files are relative to the
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<a class="reference internal" href="../config.html#instance-folders"><span class="std std-ref">instance folder</span></a>. The instance folder
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is located outside the <code class="docutils literal notranslate"><span class="pre">flaskr</span></code> package and can hold local
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data that shouldn’t be committed to version control, such as
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configuration secrets and the database file.</p></li>
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</ul>
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</li>
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<li><p><a class="reference internal" href="../api.html#flask.Config.from_mapping" title="flask.Config.from_mapping"><code class="xref py py-meth docutils literal notranslate"><span class="pre">app.config.from_mapping()</span></code></a> sets
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some default configuration that the app will use:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="../config.html#SECRET_KEY" title="SECRET_KEY"><code class="xref py py-data docutils literal notranslate"><span class="pre">SECRET_KEY</span></code></a> is used by Flask and extensions to keep data
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safe. It’s set to <code class="docutils literal notranslate"><span class="pre">'dev'</span></code> to provide a convenient value
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during development, but it should be overridden with a random
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value when deploying.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">DATABASE</span></code> is the path where the SQLite database file will be
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saved. It’s under
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<a class="reference internal" href="../api.html#flask.Flask.instance_path" title="flask.Flask.instance_path"><code class="xref py py-attr docutils literal notranslate"><span class="pre">app.instance_path</span></code></a>, which is the
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path that Flask has chosen for the instance folder. You’ll learn
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more about the database in the next section.</p></li>
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</ul>
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</li>
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<li><p><a class="reference internal" href="../api.html#flask.Config.from_pyfile" title="flask.Config.from_pyfile"><code class="xref py py-meth docutils literal notranslate"><span class="pre">app.config.from_pyfile()</span></code></a> overrides
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the default configuration with values taken from the <code class="docutils literal notranslate"><span class="pre">config.py</span></code>
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file in the instance folder if it exists. For example, when
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deploying, this can be used to set a real <code class="docutils literal notranslate"><span class="pre">SECRET_KEY</span></code>.</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">test_config</span></code> can also be passed to the factory, and will be
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used instead of the instance configuration. This is so the tests
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you’ll write later in the tutorial can be configured
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independently of any development values you have configured.</p></li>
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</ul>
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</li>
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<li><p><a class="reference external" href="https://docs.python.org/3/library/os.html#os.makedirs" title="(in Python v3.13)"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.makedirs()</span></code></a> ensures that
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<a class="reference internal" href="../api.html#flask.Flask.instance_path" title="flask.Flask.instance_path"><code class="xref py py-attr docutils literal notranslate"><span class="pre">app.instance_path</span></code></a> exists. Flask
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doesn’t create the instance folder automatically, but it needs to be
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created because your project will create the SQLite database file
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there.</p></li>
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<li><p><a class="reference internal" href="../api.html#flask.Flask.route" title="flask.Flask.route"><code class="xref py py-meth docutils literal notranslate"><span class="pre">@app.route()</span></code></a> creates a simple route so you can
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see the application working before getting into the rest of the
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tutorial. It creates a connection between the URL <code class="docutils literal notranslate"><span class="pre">/hello</span></code> and a
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function that returns a response, the string <code class="docutils literal notranslate"><span class="pre">'Hello,</span> <span class="pre">World!'</span></code> in
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this case.</p></li>
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</ol>
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</section>
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<section id="run-the-application">
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<h2>Run The Application<a class="headerlink" href="#run-the-application" title="Link to this heading">¶</a></h2>
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<p>Now you can run your application using the <code class="docutils literal notranslate"><span class="pre">flask</span></code> command. From the
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terminal, tell Flask where to find your application, then run it in
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debug mode. Remember, you should still be in the top-level
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<code class="docutils literal notranslate"><span class="pre">flask-tutorial</span></code> directory, not the <code class="docutils literal notranslate"><span class="pre">flaskr</span></code> package.</p>
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<p>Debug mode shows an interactive debugger whenever a page raises an
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exception, and restarts the server whenever you make changes to the
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code. You can leave it running and just reload the browser page as you
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follow the tutorial.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ flask --app flaskr run --debug
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</pre></div>
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</div>
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<p>You’ll see output similar to this:</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>* Serving Flask app "flaskr"
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* Debug mode: on
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* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
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* Restarting with stat
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* Debugger is active!
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* Debugger PIN: nnn-nnn-nnn
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</pre></div>
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</div>
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<p>Visit <a class="reference external" href="http://127.0.0.1:5000/hello">http://127.0.0.1:5000/hello</a> in a browser and you should see the
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“Hello, World!” message. Congratulations, you’re now running your Flask
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web application!</p>
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<p>If another program is already using port 5000, you’ll see
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<code class="docutils literal notranslate"><span class="pre">OSError:</span> <span class="pre">[Errno</span> <span class="pre">98]</span></code> or <code class="docutils literal notranslate"><span class="pre">OSError:</span> <span class="pre">[WinError</span> <span class="pre">10013]</span></code> when the
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server tries to start. See <a class="reference internal" href="../server.html#address-already-in-use"><span class="std std-ref">Address already in use</span></a> for how to
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handle that.</p>
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<p>Continue to <a class="reference internal" href="database.html"><span class="doc">Define and Access the Database</span></a>.</p>
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</section>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Application Setup</a><ul>
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<li><a class="reference internal" href="#the-application-factory">The Application Factory</a></li>
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<li><a class="reference internal" href="#run-the-application">Run The Application</a></li>
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