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