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<div class="section" id="extending-cementapp">
<h1>Extending CementApp<a class="headerlink" href="#extending-cementapp" title="Permalink to this headline">¶</a></h1>
<p>CementApp provides a convenient <code class="docutils literal"><span class="pre">extend</span></code> mechanism that allows plugins,
extensions, or the app itself to add objects/functions to the global
application object. For example, a plugin might extend the CementApp with an
<code class="docutils literal"><span class="pre">api</span></code> member allowing developers to call <code class="docutils literal"><span class="pre">app.api.get(...)</span></code>. The
application itself does not provide <code class="docutils literal"><span class="pre">app.api</span></code> however the plugin does. As
plugins are often third party, it is not possible for the plugin developer to
simply sub-class the CementApp and add the functionality because the CementApp
is already instantiated by the time plugins are loaded.</p>
<p>Take the following for example:</p>
<p><strong>myapp.py</strong></p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">cement.core.foundation</span> <span class="kn">import</span> <span class="n">CementApp</span>
<span class="k">with</span> <span class="n">CementApp</span><span class="p">(</span><span class="s1">'myapp'</span><span class="p">)</span> <span class="k">as</span> <span class="n">app</span><span class="p">:</span>
<span class="n">app</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>
</pre></div>
</div>
<p>The above is a very simple Cement application, which obviously doesn’t do
much. That said, we can add a plugin that extends the application to add an
API client object, for example, pretty easily. Note the following is an
arbitrary and non-functional example using dRest:</p>
<p><strong>/etc/myapp/plugins.d/api.conf</strong></p>
<div class="highlight-console"><div class="highlight"><pre><span></span><span class="go">[api]</span>
<span class="go">enable_plugin = true</span>
<span class="go">endpoint = https://example.com/api/v1</span>
<span class="go">user = john.doe</span>
<span class="go">password = XXXXXXXXXXXX</span>
</pre></div>
</div>
<p><strong>/var/lib/myapp/plugins/api.py</strong></p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">drest</span>
<span class="kn">from</span> <span class="nn">cement.core</span> <span class="kn">import</span> <span class="n">hook</span>
<span class="k">def</span> <span class="nf">extend_api_object</span><span class="p">(</span><span class="n">app</span><span class="p">):</span>
<span class="c1"># get api info from this plugins configuration</span>
<span class="n">endpoint</span> <span class="o">=</span> <span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'api'</span><span class="p">,</span> <span class="s1">'endpoint'</span><span class="p">)</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'api'</span><span class="p">,</span> <span class="s1">'user'</span><span class="p">)</span>
<span class="n">password</span> <span class="o">=</span> <span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'api'</span><span class="p">,</span> <span class="s1">'password'</span><span class="p">)</span>
<span class="c1"># create an api object and authenticate</span>
<span class="n">my_api_client</span> <span class="o">=</span> <span class="n">drest</span><span class="o">.</span><span class="n">API</span><span class="p">(</span><span class="n">endpoint</span><span class="p">)</span>
<span class="n">my_api_client</span><span class="o">.</span><span class="n">auth</span><span class="p">(</span><span class="n">username</span><span class="p">,</span> <span class="n">password</span><span class="p">)</span>
<span class="c1"># extend the global app object with an ``api`` member</span>
<span class="n">app</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="s1">'api'</span><span class="p">,</span> <span class="n">my_api_client</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">load</span><span class="p">(</span><span class="n">app</span><span class="p">):</span>
<span class="n">hook</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="s1">'pre_run'</span><span class="p">,</span> <span class="n">extend_api_object</span><span class="p">)</span>
</pre></div>
</div>
<p>In the above plugin, we simply created a dRest API client within a
<code class="docutils literal"><span class="pre">pre_run</span></code> hook and then extended the global <code class="docutils literal"><span class="pre">app</span></code> with it. The developer
can now reference <code class="docutils literal"><span class="pre">app.api</span></code> anywhere that the global <code class="docutils literal"><span class="pre">app</span></code> object is
accessible.</p>
<p>Our application code could now look like:</p>
<p><strong>myapp.py</strong></p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">cement.core.foundation</span> <span class="kn">import</span> <span class="n">CementApp</span>
<span class="k">with</span> <span class="n">CementApp</span><span class="p">(</span><span class="s1">'myapp'</span><span class="p">)</span> <span class="k">as</span> <span class="n">app</span><span class="p">:</span>
<span class="n">app</span><span class="o">.</span><span class="n">run</span><span class="p">()</span>
<span class="c1"># use the api object that the plugin provides</span>
<span class="n">app</span><span class="o">.</span><span class="n">api</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
</pre></div>
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