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<section id="application-factories">
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<h1>Application Factories<a class="headerlink" href="#application-factories" title="Link to this heading">¶</a></h1>
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<p>If you are already using packages and blueprints for your application
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(<a class="reference internal" href="../blueprints.html"><span class="doc">Modular Applications with Blueprints</span></a>) there are a couple of really nice ways to further improve
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the experience. A common pattern is creating the application object when
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the blueprint is imported. But if you move the creation of this object
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into a function, you can then create multiple instances of this app later.</p>
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<p>So why would you want to do this?</p>
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<ol class="arabic simple">
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<li><p>Testing. You can have instances of the application with different
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settings to test every case.</p></li>
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<li><p>Multiple instances. Imagine you want to run different versions of the
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same application. Of course you could have multiple instances with
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different configs set up in your webserver, but if you use factories,
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you can have multiple instances of the same application running in the
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same application process which can be handy.</p></li>
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</ol>
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<p>So how would you then actually implement that?</p>
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<section id="basic-factories">
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<h2>Basic Factories<a class="headerlink" href="#basic-factories" title="Link to this heading">¶</a></h2>
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<p>The idea is to set up the application in a function. Like this:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">create_app</span><span class="p">(</span><span class="n">config_filename</span><span class="p">):</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>
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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="n">config_filename</span><span class="p">)</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication.model</span><span class="w"> </span><span class="kn">import</span> <span class="n">db</span>
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<span class="n">db</span><span class="o">.</span><span class="n">init_app</span><span class="p">(</span><span class="n">app</span><span class="p">)</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication.views.admin</span><span class="w"> </span><span class="kn">import</span> <span class="n">admin</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication.views.frontend</span><span class="w"> </span><span class="kn">import</span> <span class="n">frontend</span>
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<span class="n">app</span><span class="o">.</span><span class="n">register_blueprint</span><span class="p">(</span><span class="n">admin</span><span class="p">)</span>
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<span class="n">app</span><span class="o">.</span><span class="n">register_blueprint</span><span class="p">(</span><span class="n">frontend</span><span class="p">)</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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<p>The downside is that you cannot use the application object in the blueprints
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at import time. You can however use it from within a request. How do you
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get access to the application with the config? Use
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<a class="reference internal" href="../api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><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">current_app</span><span class="p">,</span> <span class="n">Blueprint</span><span class="p">,</span> <span class="n">render_template</span>
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<span class="n">admin</span> <span class="o">=</span> <span class="n">Blueprint</span><span class="p">(</span><span class="s1">'admin'</span><span class="p">,</span> <span class="vm">__name__</span><span class="p">,</span> <span class="n">url_prefix</span><span class="o">=</span><span class="s1">'/admin'</span><span class="p">)</span>
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<span class="nd">@admin</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s1">'/'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">index</span><span class="p">():</span>
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<span class="k">return</span> <span class="n">render_template</span><span class="p">(</span><span class="n">current_app</span><span class="o">.</span><span class="n">config</span><span class="p">[</span><span class="s1">'INDEX_TEMPLATE'</span><span class="p">])</span>
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</pre></div>
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</div>
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<p>Here we look up the name of a template in the config.</p>
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</section>
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<section id="factories-extensions">
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<h2>Factories & Extensions<a class="headerlink" href="#factories-extensions" title="Link to this heading">¶</a></h2>
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<p>It’s preferable to create your extensions and app factories so that the
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extension object does not initially get bound to the application.</p>
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<p>Using <a class="reference external" href="https://flask-sqlalchemy.palletsprojects.com/">Flask-SQLAlchemy</a>,
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as an example, you should not do something along those lines:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">create_app</span><span class="p">(</span><span class="n">config_filename</span><span class="p">):</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>
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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="n">config_filename</span><span class="p">)</span>
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<span class="n">db</span> <span class="o">=</span> <span class="n">SQLAlchemy</span><span class="p">(</span><span class="n">app</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>But, rather, in model.py (or equivalent):</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">db</span> <span class="o">=</span> <span class="n">SQLAlchemy</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>and in your application.py (or equivalent):</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">create_app</span><span class="p">(</span><span class="n">config_filename</span><span class="p">):</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>
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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="n">config_filename</span><span class="p">)</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication.model</span><span class="w"> </span><span class="kn">import</span> <span class="n">db</span>
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<span class="n">db</span><span class="o">.</span><span class="n">init_app</span><span class="p">(</span><span class="n">app</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>Using this design pattern, no application-specific state is stored on the
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extension object, so one extension object can be used for multiple apps.
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For more information about the design of extensions refer to <a class="reference internal" href="../extensiondev.html"><span class="doc">Flask Extension Development</span></a>.</p>
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</section>
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<section id="using-applications">
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<h2>Using Applications<a class="headerlink" href="#using-applications" title="Link to this heading">¶</a></h2>
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<p>To run such an application, you can use the <strong class="command">flask</strong> command:</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ flask --app hello run
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</pre></div>
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</div>
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<p>Flask will automatically detect the factory if it is named
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<code class="docutils literal notranslate"><span class="pre">create_app</span></code> or <code class="docutils literal notranslate"><span class="pre">make_app</span></code> in <code class="docutils literal notranslate"><span class="pre">hello</span></code>. You can also pass arguments
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to the factory like this:</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ flask --app 'hello:create_app(local_auth=True)' run
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</pre></div>
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</div>
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<p>Then the <code class="docutils literal notranslate"><span class="pre">create_app</span></code> factory in <code class="docutils literal notranslate"><span class="pre">hello</span></code> is called with the keyword
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argument <code class="docutils literal notranslate"><span class="pre">local_auth=True</span></code>. See <a class="reference internal" href="../cli.html"><span class="doc">Command Line Interface</span></a> for more detail.</p>
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</section>
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<section id="factory-improvements">
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<h2>Factory Improvements<a class="headerlink" href="#factory-improvements" title="Link to this heading">¶</a></h2>
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<p>The factory function above is not very clever, but you can improve it.
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The following changes are straightforward to implement:</p>
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<ol class="arabic simple">
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<li><p>Make it possible to pass in configuration values for unit tests so that
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you don’t have to create config files on the filesystem.</p></li>
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<li><p>Call a function from a blueprint when the application is setting up so
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that you have a place to modify attributes of the application (like
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hooking in before/after request handlers etc.)</p></li>
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<li><p>Add in WSGI middlewares when the application is being created if necessary.</p></li>
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</ol>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Application Factories</a><ul>
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<li><a class="reference internal" href="#basic-factories">Basic Factories</a></li>
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<li><a class="reference internal" href="#factories-extensions">Factories & Extensions</a></li>
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<li><a class="reference internal" href="#using-applications">Using Applications</a></li>
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<li><a class="reference internal" href="#factory-improvements">Factory Improvements</a></li>
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