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<title>DRF Viewset: permission policy per method - Thomas Loiret - Random thoughts</title>
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<h1 itemprop="name">DRF Viewset: permission policy per method</h1>
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Thomas Loiret
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<span class="meta__date" itemprop="datePublished" content="2021-11-27">
<span class="glyphicon glyphicon-calendar"></span> Sat 27 November 2021
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<h2>Setting the permission policy in DRF</h2>
<p>As the Django Rest Framework <a href="https://www.django-rest-framework.org/api-guide/permissions/#setting-the-permission-policy">documentation</a> explains you can set the permission policy in 3 places:</p>
<ol>
<li>In the settings (<code>DEFAULT_PERMISSION_CLASSES</code>)</li>
<li>At the ViewSet level (using the <code>permission_classes</code> attribute of ViewSet)</li>
<li>At a ViewSet action level (using the <code>permission_classes</code> of the <code>@action</code> decorator)
oo</li>
</ol>
<p><strong>Note:</strong> It is important to know that each level of permission overrides the previous one. They are not cumulative.</p>
<h2>The issue</h2>
<p>It is already possible to set permission policy for a ViewSet action so we got some flexibility here. But what about the standard methods <code>create()</code>, <code>retrieve()</code>, <code>update()</code>, <code>partial_update()</code>, <code>destroy()</code> and <code>list()</code>? What if you want to set different permissions for <code>create</code> and <code>list</code> methods for example?</p>
<p>That is precisely the issue we will try to fix in this blog post.</p>
<h2>The solution - show me the code</h2>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">MyViewSet</span><span class="p">(</span><span class="n">PermissionPolicyMixin</span><span class="p">,</span> <span class="n">ModelViewSet</span><span class="p">):</span>
<span class="c1"># By default, we check that the user has the "write" permission</span>
<span class="n">permission_classes</span> <span class="o">=</span> <span class="p">[</span><span class="n">IsWriteUser</span><span class="p">]</span>
<span class="n">permission_classes_per_method</span> <span class="o">=</span> <span class="p">{</span>
<span class="c1"># except for list and retrieve where both users with "write" or "read-only"</span>
<span class="c1"># permissions can access the endpoints.</span>
<span class="s2">"list"</span><span class="p">:</span> <span class="p">[</span><span class="n">IsWriteUser</span> <span class="o">|</span> <span class="n">IsReadOnlyUser</span><span class="p">],</span>
<span class="s2">"retrieve"</span><span class="p">:</span> <span class="p">[</span><span class="n">IsWriteUser</span> <span class="o">|</span> <span class="n">IsReadOnlyUser</span><span class="p">]</span>
<span class="p">}</span>
</code></pre></div>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">PermissionPolicyMixin</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">check_permissions</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">request</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="c1"># This line is heavily inspired from `APIView.dispatch`.</span>
<span class="c1"># It returns the method associated with an endpoint.</span>
<span class="n">handler</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">request</span><span class="o">.</span><span class="n">method</span><span class="o">.</span><span class="n">lower</span><span class="p">())</span>
<span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span>
<span class="n">handler</span> <span class="o">=</span> <span class="kc">None</span>
<span class="k">if</span> <span class="p">(</span>
<span class="n">handler</span>
<span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">permission_classes_per_method</span>
<span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">permission_classes_per_method</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">handler</span><span class="o">.</span><span class="vm">__name__</span><span class="p">)</span>
<span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">permission_classes</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">permission_classes_per_method</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">handler</span><span class="o">.</span><span class="vm">__name__</span><span class="p">)</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">check_permissions</span><span class="p">(</span><span class="n">request</span><span class="p">)</span>
</code></pre></div>
<h2>Explanation</h2>
<p>With the <code>PermissionPolicyMixin</code> mixin we alter the permission process of Django Rest Framework. Before checking the permission for an endpoint we check if the method associated exists in the <code>permission_classes_per_method</code> attribute. If so, we set <code>self.permission_classes</code>.</p>
<p>This mixin also allows us to override the <code>permission_classes</code> set in the <code>@action()</code> decorator. So it can also help to define all the permission policies in one place.</p>
<p><strong>Note:</strong> When a ViewSet inherits of <code>PermissionPolicyMixin</code>, it is important that the mixin is set first, or at least before any classes like <code>*ViewSet</code> or <code>*ModelMixin</code>.</p>
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