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<div class="section" id="method-could-be-a-function">
<h1>Method could be a function<a class="headerlink" href="#method-could-be-a-function" title="Permalink to this headline">¶</a></h1>
<p>When a method is not preceded by the <code class="docutils literal notranslate"><span class="pre">@staticmethod</span></code> or <code class="docutils literal notranslate"><span class="pre">@classmethod</span></code> decorators and does not contain any references to the class or instance (via keywords like <code class="docutils literal notranslate"><span class="pre">cls</span></code> or <code class="docutils literal notranslate"><span class="pre">self</span></code>), Python raises the <code class="docutils literal notranslate"><span class="pre">Method</span> <span class="pre">could</span> <span class="pre">be</span> <span class="pre">a</span> <span class="pre">function</span></code> error. This is not a critical error, but you should check the code in question in order to determine if this section of code really needs to be defined as a method of this class.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>In the <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> class below the <code class="docutils literal notranslate"><span class="pre">area</span></code> method calculates the area of any rectangle given a width and a height.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
<span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
<span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
<span class="c1"># should be preceded by @staticmethod here</span>
<span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
<span class="k">return</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">area</span></code> causes the <code class="docutils literal notranslate"><span class="pre">Method</span> <span class="pre">could</span> <span class="pre">be</span> <span class="pre">a</span> <span class="pre">function</span></code> error because it is ambiguous. It does not reference the instance or class using the <code class="docutils literal notranslate"><span class="pre">self</span></code> or <code class="docutils literal notranslate"><span class="pre">cls</span></code> keywords and it is not preceded by the <code class="docutils literal notranslate"><span class="pre">@staticmethod</span></code> decorator.</p>
<div class="section" id="class-method-is-not-preceded-by-classmethod-decorator">
<h3>Class method is not preceded by <code class="docutils literal notranslate"><span class="pre">@classmethod</span></code> decorator<a class="headerlink" href="#class-method-is-not-preceded-by-classmethod-decorator" title="Permalink to this headline">¶</a></h3>
<p>In the <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> class below the <code class="docutils literal notranslate"><span class="pre">print_class_name</span></code> method prints the name of the class. Again, Python raises the <code class="docutils literal notranslate"><span class="pre">Method</span> <span class="pre">could</span> <span class="pre">be</span> <span class="pre">a</span> <span class="pre">function</span></code> error because the method does not reference any class members or methods and is not preceded by the <code class="docutils literal notranslate"><span class="pre">@classmethod</span></code> decorator.</p>
<p>Furthermore, the first argument of a class method must be a reference to the class itself.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
<span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
<span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
<span class="c1"># should be preceded by @classmethod here</span>
<span class="c1"># missing required first argument "cls"</span>
<span class="k">def</span> <span class="nf">print_class_name</span><span class="p">():</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"class name: Rectangle"</span><span class="p">)</span>
</pre></div>
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</div>
<div class="section" id="best-practices">
<h2>Best practices<a class="headerlink" href="#best-practices" title="Permalink to this headline">¶</a></h2>
<div class="section" id="add-the-staticmethod-decorator-before-the-static-method">
<h3>Add the <code class="docutils literal notranslate"><span class="pre">@staticmethod</span></code> decorator before the static method<a class="headerlink" href="#add-the-staticmethod-decorator-before-the-static-method" title="Permalink to this headline">¶</a></h3>
<p>All static methods must be preceded by the <code class="docutils literal notranslate"><span class="pre">@staticmethod</span></code> decorator.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
<span class="c1"># clarifies that this is a static method and belongs here</span>
<span class="nd">@staticmethod</span>
<span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
<span class="k">return</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
</pre></div>
</div>
</div>
<div class="section" id="add-the-classmethod-decorator-before-the-class-method">
<h3>Add the <code class="docutils literal notranslate"><span class="pre">@classmethod</span></code> decorator before the class method<a class="headerlink" href="#add-the-classmethod-decorator-before-the-class-method" title="Permalink to this headline">¶</a></h3>
<p>All class methods must be preceded by the <code class="docutils literal notranslate"><span class="pre">@classmethod</span></code> decorator. Furthermore, the first argument of any class method must be <code class="docutils literal notranslate"><span class="pre">cls</span></code>, which is a reference to the class itself.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">print_class_name</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
<span class="c1"># "class name: Rectangle"</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"class name: </span><span class="si">{0}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">cls</span><span class="p">))</span>
</pre></div>
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<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://pylint.readthedocs.io/en/latest/user_guide/messages/refactor/old-no-self-use.html">PyLint - R0201, no-self-use</a></p></li>
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