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<div class="section" id="dynamically-creating-variable-method-function-names">
<h1>Dynamically creating variable/method/function names<a class="headerlink" href="#dynamically-creating-variable-method-function-names" title="Permalink to this headline">¶</a></h1>
<p>Sometimes a programmer gets an idea to make his/her work easier by creating magically working code that uses <code class="docutils literal"><span class="pre">setattr()</span></code> and <code class="docutils literal"><span class="pre">getattr()</span></code> functions to set some variable. While this may look like a good idea, because there is no need to write all the methods by hand, you are asking for trouble down the road.</p>
<div class="section" id="example">
<h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<p>Consider the following code. You have some data and want to update the <em>class</em> with all of the data. Of course you don’t want to do this by hand, especially if there are tons of items in <code class="docutils literal"><span class="pre">data_dict</span></code>. However, when refactoring this kind of code after several years, and you’d like to know where some variable is added to this class, you’d usually use <code class="docutils literal"><span class="pre">grep</span></code> or <code class="docutils literal"><span class="pre">ack_grep</span></code> to find it. But when setting variables/methods/functions like this, you’re screwed.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">data_dict</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'var1'</span><span class="p">:</span> <span class="s1">'Data1'</span><span class="p">,</span> <span class="s1">'var2'</span><span class="p">:</span> <span class="s1">'Data2'</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">MyAwesomeClass</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">data_dict</span><span class="p">):</span>
<span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">data_dict</span><span class="o">.</span><span class="n">iteritems</span><span class="p">():</span>
<span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
</pre></div>
</div>
<p>While previous example may look easy to find and debug, consider this:</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">data_list</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'dat1'</span><span class="p">,</span> <span class="s1">'dat2'</span><span class="p">,</span> <span class="s1">'dat3'</span><span class="p">]</span>
<span class="n">data_dict</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'dat1'</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span>
<span class="s1">'dat2'</span><span class="p">:</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">],</span>
<span class="s1">'dat3'</span><span class="p">:</span> <span class="p">[</span><span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">9</span><span class="p">],</span>
<span class="s1">'dat4'</span><span class="p">:</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">6</span><span class="p">]}</span>
<span class="k">class</span> <span class="nc">MyAwesomeClass</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">data_list</span><span class="p">,</span> <span class="n">data_dict</span><span class="p">):</span>
<span class="n">counter</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">data_dict</span><span class="o">.</span><span class="n">iteritems</span><span class="p">():</span>
<span class="k">if</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">data_list</span><span class="p">:</span>
<span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="s1">'unknown'</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">counter</span><span class="p">),</span> <span class="n">value</span><span class="p">)</span>
<span class="n">counter</span> <span class="o">+=</span> <span class="mi">1</span>
</pre></div>
</div>
<p>Now the class contains also <code class="docutils literal"><span class="pre">unknownX</span></code> variables indexed by their count. Well, what a nice mess we created here. Try to find a year later where these variables come from.</p>
</div>
<div class="section" id="solutions">
<h2>Solutions<a class="headerlink" href="#solutions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="find-another-way">
<h3>Find another way<a class="headerlink" href="#find-another-way" title="Permalink to this headline">¶</a></h3>
<p>While the approach in the examples above may be the easiest to write, it is the worst to maintain later. You should always try to find another way to solve your problem.</p>
<p>Typical examples:</p>
<ul class="simple">
<li>Use function to parse incoming data</li>
<li>Use the data dict/list itself without class</li>
</ul>
<p>This however depends on the task at hand.</p>
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