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<div class="section" id="view-decorators">
<h1>View Decorators<a class="headerlink" href="#view-decorators" title="Permalink to this headline">¶</a></h1>
<p>Python has a really interesting feature called function decorators. This
allow some really neat things for web applications. Because each view in
Flask is a function decorators can be used to inject additional
functionality to one or more functions. The <a class="reference internal" href="../api.html#flask.Flask.route" title="flask.Flask.route"><tt class="xref py py-meth docutils literal"><span class="pre">route()</span></tt></a>
decorator is the one you probably used already. But there are use cases
for implementing your own decorator. For instance, imagine you have a
view that should only be used by people that are logged in to. If a user
goes to the site and is not logged in, they should be redirected to the
login page. This is a good example of a use case where a decorator is an
excellent solution.</p>
<div class="section" id="login-required-decorator">
<h2>Login Required Decorator<a class="headerlink" href="#login-required-decorator" title="Permalink to this headline">¶</a></h2>
<p>So let’s implement such a decorator. A decorator is a function that
returns a function. Pretty simple actually. The only thing you have to
keep in mind when implementing something like this is to update the
<cite>__name__</cite>, <cite>__module__</cite> and some other attributes of a function. This is
often forgotten, but you don’t have to do that by hand, there is a
function for that that is used like a decorator (<a class="reference external" href="http://docs.python.org/dev/library/functools.html#functools.wraps" title="(in Python v3.3)"><tt class="xref py py-func docutils literal"><span class="pre">functools.wraps()</span></tt></a>).</p>
<p>This example assumes that the login page is called <tt class="docutils literal"><span class="pre">'login'</span></tt> and that
the current user is stored as <cite>g.user</cite> and <cite>None</cite> if there is no-one
logged in:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="n">wraps</span>
<span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">g</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">redirect</span><span class="p">,</span> <span class="n">url_for</span>
<span class="k">def</span> <span class="nf">login_required</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="nd">@wraps</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">decorated_function</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">if</span> <span class="n">g</span><span class="o">.</span><span class="n">user</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">return</span> <span class="n">redirect</span><span class="p">(</span><span class="n">url_for</span><span class="p">(</span><span class="s">'login'</span><span class="p">,</span> <span class="nb">next</span><span class="o">=</span><span class="n">request</span><span class="o">.</span><span class="n">url</span><span class="p">))</span>
<span class="k">return</span> <span class="n">f</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="n">decorated_function</span>
</pre></div>
</div>
<p>So how would you use that decorator now? Apply it as innermost decorator
to a view function. When applying further decorators, always remember
that the <a class="reference internal" href="../api.html#flask.Flask.route" title="flask.Flask.route"><tt class="xref py py-meth docutils literal"><span class="pre">route()</span></tt></a> decorator is the outermost:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@app.route</span><span class="p">(</span><span class="s">'/secret_page'</span><span class="p">)</span>
<span class="nd">@login_required</span>
<span class="k">def</span> <span class="nf">secret_page</span><span class="p">():</span>
<span class="k">pass</span>
</pre></div>
</div>
</div>
<div class="section" id="caching-decorator">
<h2>Caching Decorator<a class="headerlink" href="#caching-decorator" title="Permalink to this headline">¶</a></h2>
<p>Imagine you have a view function that does an expensive calculation and
because of that you would like to cache the generated results for a
certain amount of time. A decorator would be nice for that. We’re
assuming you have set up a cache like mentioned in <a class="reference internal" href="caching.html#caching-pattern"><em>Caching</em></a>.</p>
<p>Here an example cache function. It generates the cache key from a
specific prefix (actually a format string) and the current path of the
request. Notice that we are using a function that first creates the
decorator that then decorates the function. Sounds awful? Unfortunately
it is a little bit more complex, but the code should still be
straightforward to read.</p>
<p>The decorated function will then work as follows</p>
<ol class="arabic simple">
<li>get the unique cache key for the current request base on the current
path.</li>
<li>get the value for that key from the cache. If the cache returned
something we will return that value.</li>
<li>otherwise the original function is called and the return value is
stored in the cache for the timeout provided (by default 5 minutes).</li>
</ol>
<p>Here the code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="n">wraps</span>
<span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">request</span>
<span class="k">def</span> <span class="nf">cached</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">5</span> <span class="o">*</span> <span class="mi">60</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="s">'view/</span><span class="si">%s</span><span class="s">'</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">decorator</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="nd">@wraps</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">decorated_function</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="n">cache_key</span> <span class="o">=</span> <span class="n">key</span> <span class="o">%</span> <span class="n">request</span><span class="o">.</span><span class="n">path</span>
<span class="n">rv</span> <span class="o">=</span> <span class="n">cache</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">cache_key</span><span class="p">)</span>
<span class="k">if</span> <span class="n">rv</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">return</span> <span class="n">rv</span>
<span class="n">rv</span> <span class="o">=</span> <span class="n">f</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="n">cache</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="n">cache_key</span><span class="p">,</span> <span class="n">rv</span><span class="p">,</span> <span class="n">timeout</span><span class="o">=</span><span class="n">timeout</span><span class="p">)</span>
<span class="k">return</span> <span class="n">rv</span>
<span class="k">return</span> <span class="n">decorated_function</span>
<span class="k">return</span> <span class="n">decorator</span>
</pre></div>
</div>
<p>Notice that this assumes an instantiated <cite>cache</cite> object is available, see
<a class="reference internal" href="caching.html#caching-pattern"><em>Caching</em></a> for more information.</p>
</div>
<div class="section" id="templating-decorator">
<h2>Templating Decorator<a class="headerlink" href="#templating-decorator" title="Permalink to this headline">¶</a></h2>
<p>A common pattern invented by the TurboGears guys a while back is a
templating decorator. The idea of that decorator is that you return a
dictionary with the values passed to the template from the view function
and the template is automatically rendered. With that, the following
three examples do exactly the same:</p>
<div class="highlight-python"><div class="highlight"><pre><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">return</span> <span class="n">render_template</span><span class="p">(</span><span class="s">'index.html'</span><span class="p">,</span> <span class="n">value</span><span class="o">=</span><span class="mi">42</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="nd">@templated</span><span class="p">(</span><span class="s">'index.html'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">():</span>
<span class="k">return</span> <span class="nb">dict</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="mi">42</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="nd">@templated</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">():</span>
<span class="k">return</span> <span class="nb">dict</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>
</pre></div>
</div>
<p>As you can see, if no template name is provided it will use the endpoint
of the URL map with dots converted to slashes + <tt class="docutils literal"><span class="pre">'.html'</span></tt>. Otherwise
the provided template name is used. When the decorated function returns,
the dictionary returned is passed to the template rendering function. If
<cite>None</cite> is returned, an empty dictionary is assumed, if something else than
a dictionary is returned we return it from the function unchanged. That
way you can still use the redirect function or return simple strings.</p>
<p>Here the code for that decorator:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="n">wraps</span>
<span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">request</span>
<span class="k">def</span> <span class="nf">templated</span><span class="p">(</span><span class="n">template</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">decorator</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="nd">@wraps</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">decorated_function</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="n">template_name</span> <span class="o">=</span> <span class="n">template</span>
<span class="k">if</span> <span class="n">template_name</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">template_name</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">endpoint</span> \
<span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="s">'.'</span><span class="p">,</span> <span class="s">'/'</span><span class="p">)</span> <span class="o">+</span> <span class="s">'.html'</span>
<span class="n">ctx</span> <span class="o">=</span> <span class="n">f</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">if</span> <span class="n">ctx</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">ctx</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">elif</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="nb">dict</span><span class="p">):</span>
<span class="k">return</span> <span class="n">ctx</span>
<span class="k">return</span> <span class="n">render_template</span><span class="p">(</span><span class="n">template_name</span><span class="p">,</span> <span class="o">**</span><span class="n">ctx</span><span class="p">)</span>
<span class="k">return</span> <span class="n">decorated_function</span>
<span class="k">return</span> <span class="n">decorator</span>
</pre></div>
</div>
</div>
<div class="section" id="endpoint-decorator">
<h2>Endpoint Decorator<a class="headerlink" href="#endpoint-decorator" title="Permalink to this headline">¶</a></h2>
<p>When you want to use the werkzeug routing system for more flexibility you
need to map the endpoint as defined in the <a class="reference external" href="http://werkzeug.pocoo.org/docs/routing/#werkzeug.routing.Rule" title="(in Werkzeug v0.7)"><tt class="xref py py-class docutils literal"><span class="pre">Rule</span></tt></a>
to a view function. This is possible with this decorator. For example:</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">werkzeug.routing</span> <span class="kn">import</span> <span class="n">Rule</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">url_map</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">Rule</span><span class="p">(</span><span class="s">'/'</span><span class="p">,</span> <span class="n">endpoint</span><span class="o">=</span><span class="s">'index'</span><span class="p">))</span>
<span class="nd">@app.endpoint</span><span class="p">(</span><span class="s">'index'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_index</span><span class="p">():</span>
<span class="k">return</span> <span class="s">"Hello world"</span>
</pre></div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">View Decorators</a><ul>
<li><a class="reference internal" href="#login-required-decorator">Login Required Decorator</a></li>
<li><a class="reference internal" href="#caching-decorator">Caching Decorator</a></li>
<li><a class="reference internal" href="#templating-decorator">Templating Decorator</a></li>
<li><a class="reference internal" href="#endpoint-decorator">Endpoint Decorator</a></li>
</ul>
</li>
</ul>
<h3>Related Topics</h3>
<ul>
<li><a href="../index.html">Documentation overview</a><ul>
<li><a href="index.html">Patterns for Flask</a><ul>
<li>Previous: <a href="caching.html" title="previous chapter">Caching</a></li>
<li>Next: <a href="wtforms.html" title="next chapter">Form Validation with WTForms</a></li>
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