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<li class="nav-item nav-item-this"><a href="">Lazily Loading Views</a></li>
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<section id="lazily-loading-views">
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<h1>Lazily Loading Views<a class="headerlink" href="#lazily-loading-views" title="Link to this heading">¶</a></h1>
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<p>Flask is usually used with the decorators. Decorators are simple and you
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have the URL right next to the function that is called for that specific
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URL. However there is a downside to this approach: it means all your code
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that uses decorators has to be imported upfront or Flask will never
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actually find your function.</p>
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<p>This can be a problem if your application has to import quick. It might
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have to do that on systems like Google’s App Engine or other systems. So
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if you suddenly notice that your application outgrows this approach you
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can fall back to a centralized URL mapping.</p>
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<p>The system that enables having a central URL map is the
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<a class="reference internal" href="../api.html#flask.Flask.add_url_rule" title="flask.Flask.add_url_rule"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_url_rule()</span></code></a> function. Instead of using decorators,
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you have a file that sets up the application with all URLs.</p>
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<section id="converting-to-centralized-url-map">
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<h2>Converting to Centralized URL Map<a class="headerlink" href="#converting-to-centralized-url-map" title="Link to this heading">¶</a></h2>
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<p>Imagine the current application looks somewhat like this:</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">Flask</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="nd">@app</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">pass</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">'/user/<username>'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">user</span><span class="p">(</span><span class="n">username</span><span class="p">):</span>
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<span class="k">pass</span>
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</pre></div>
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</div>
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<p>Then, with the centralized approach you would have one file with the views
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(<code class="file docutils literal notranslate"><span class="pre">views.py</span></code>) but without any decorator:</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">index</span><span class="p">():</span>
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<span class="k">pass</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">user</span><span class="p">(</span><span class="n">username</span><span class="p">):</span>
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<span class="k">pass</span>
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</pre></div>
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</div>
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<p>And then a file that sets up an application which maps the functions to
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URLs:</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">Flask</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication</span><span class="w"> </span><span class="kn">import</span> <span class="n">views</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">add_url_rule</span><span class="p">(</span><span class="s1">'/'</span><span class="p">,</span> <span class="n">view_func</span><span class="o">=</span><span class="n">views</span><span class="o">.</span><span class="n">index</span><span class="p">)</span>
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<span class="n">app</span><span class="o">.</span><span class="n">add_url_rule</span><span class="p">(</span><span class="s1">'/user/<username>'</span><span class="p">,</span> <span class="n">view_func</span><span class="o">=</span><span class="n">views</span><span class="o">.</span><span class="n">user</span><span class="p">)</span>
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</pre></div>
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</div>
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</section>
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<section id="loading-late">
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<h2>Loading Late<a class="headerlink" href="#loading-late" title="Link to this heading">¶</a></h2>
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<p>So far we only split up the views and the routing, but the module is still
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loaded upfront. The trick is to actually load the view function as needed.
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This can be accomplished with a helper class that behaves just like a
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function but internally imports the real function on first use:</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">werkzeug.utils</span><span class="w"> </span><span class="kn">import</span> <span class="n">import_string</span><span class="p">,</span> <span class="n">cached_property</span>
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<span class="k">class</span><span class="w"> </span><span class="nc">LazyView</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
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<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">import_name</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="vm">__module__</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__name__</span> <span class="o">=</span> <span class="n">import_name</span><span class="o">.</span><span class="n">rsplit</span><span class="p">(</span><span class="s1">'.'</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">import_name</span> <span class="o">=</span> <span class="n">import_name</span>
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<span class="nd">@cached_property</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">view</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="k">return</span> <span class="n">import_string</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">import_name</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="fm">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
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<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">view</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>What’s important here is is that <code class="code docutils literal notranslate"><span class="pre">__module__</span></code> and <code class="code docutils literal notranslate"><span class="pre">__name__</span></code> are properly
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set. This is used by Flask internally to figure out how to name the
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URL rules in case you don’t provide a name for the rule yourself.</p>
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<p>Then you can define your central place to combine the views like this:</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">Flask</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">yourapplication.helpers</span><span class="w"> </span><span class="kn">import</span> <span class="n">LazyView</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">add_url_rule</span><span class="p">(</span><span class="s1">'/'</span><span class="p">,</span>
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<span class="n">view_func</span><span class="o">=</span><span class="n">LazyView</span><span class="p">(</span><span class="s1">'yourapplication.views.index'</span><span class="p">))</span>
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<span class="n">app</span><span class="o">.</span><span class="n">add_url_rule</span><span class="p">(</span><span class="s1">'/user/<username>'</span><span class="p">,</span>
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<span class="n">view_func</span><span class="o">=</span><span class="n">LazyView</span><span class="p">(</span><span class="s1">'yourapplication.views.user'</span><span class="p">))</span>
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</pre></div>
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</div>
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<p>You can further optimize this in terms of amount of keystrokes needed to
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write this by having a function that calls into
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<a class="reference internal" href="../api.html#flask.Flask.add_url_rule" title="flask.Flask.add_url_rule"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_url_rule()</span></code></a> by prefixing a string with the project
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name and a dot, and by wrapping <code class="code docutils literal notranslate"><span class="pre">view_func</span></code> in a <code class="code docutils literal notranslate"><span class="pre">LazyView</span></code> as needed.</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">url</span><span class="p">(</span><span class="n">import_name</span><span class="p">,</span> <span class="n">url_rules</span><span class="o">=</span><span class="p">[],</span> <span class="o">**</span><span class="n">options</span><span class="p">):</span>
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<span class="n">view</span> <span class="o">=</span> <span class="n">LazyView</span><span class="p">(</span><span class="sa">f</span><span class="s2">"yourapplication.</span><span class="si">{</span><span class="n">import_name</span><span class="si">}</span><span class="s2">"</span><span class="p">)</span>
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<span class="k">for</span> <span class="n">url_rule</span> <span class="ow">in</span> <span class="n">url_rules</span><span class="p">:</span>
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<span class="n">app</span><span class="o">.</span><span class="n">add_url_rule</span><span class="p">(</span><span class="n">url_rule</span><span class="p">,</span> <span class="n">view_func</span><span class="o">=</span><span class="n">view</span><span class="p">,</span> <span class="o">**</span><span class="n">options</span><span class="p">)</span>
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<span class="c1"># add a single route to the index view</span>
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<span class="n">url</span><span class="p">(</span><span class="s1">'views.index'</span><span class="p">,</span> <span class="p">[</span><span class="s1">'/'</span><span class="p">])</span>
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<span class="c1"># add two routes to a single function endpoint</span>
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<span class="n">url_rules</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'/user/'</span><span class="p">,</span><span class="s1">'/user/<username>'</span><span class="p">]</span>
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<span class="n">url</span><span class="p">(</span><span class="s1">'views.user'</span><span class="p">,</span> <span class="n">url_rules</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>One thing to keep in mind is that before and after request handlers have
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to be in a file that is imported upfront to work properly on the first
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request. The same goes for any kind of remaining decorator.</p>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Lazily Loading Views</a><ul>
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<li><a class="reference internal" href="#converting-to-centralized-url-map">Converting to Centralized URL Map</a></li>
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<li><a class="reference internal" href="#loading-late">Loading Late</a></li>
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