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<div class="section" id="not-using-defaultdict">
<h1>Not using <code class="docutils literal"><span class="pre">defaultdict()</span></code><a class="headerlink" href="#not-using-defaultdict" title="Permalink to this headline">¶</a></h1>
<p>When a dict is created using <code class="docutils literal"><span class="pre">defaultdict()</span></code>, the value for each key in the dict will default to the value provided as the first argument of <code class="docutils literal"><span class="pre">defaultdict()</span></code>. This is more concise and less error-prone than manually setting the value of each key.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>The code below defines an empty dict and then manually initializes the keys of the dict. Although there is nothing wrong with this code, there is a more concise and less error-prone way to achieve the same idea, as explained in the solution below.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">if</span> <span class="s2">"k"</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">d</span><span class="p">:</span>
<span class="n">d</span><span class="p">[</span><span class="s2">"k"</span><span class="p">]</span> <span class="o">=</span> <span class="mi">6</span>
<span class="n">d</span><span class="p">[</span><span class="s2">"k"</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="nb">print</span><span class="p">(</span><span class="n">d</span><span class="p">[</span><span class="s2">"k"</span><span class="p">])</span> <span class="c1"># 7</span>
</pre></div>
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<div class="section" id="best-practice">
<h2>Best practice<a class="headerlink" href="#best-practice" title="Permalink to this headline">¶</a></h2>
<div class="section" id="use-defaultdict-to-initialize-dict-keys">
<h3>Use <code class="docutils literal"><span class="pre">defaultdict()</span></code> to initialize dict keys<a class="headerlink" href="#use-defaultdict-to-initialize-dict-keys" title="Permalink to this headline">¶</a></h3>
<p>The modified code below uses <code class="docutils literal"><span class="pre">defaultdict</span></code> to initialize the dict. Whenever a new key is created, the default value for that key is 6. This code is functionally equivalent to the previous code, but this one is more concise and less error-prone, because every key automatically initializes to 6 with no work on the part of the programmer.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">collections</span> <span class="k">import</span> <span class="n">defaultdict</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">defaultdict</span><span class="p">(</span><span class="k">lambda</span> <span class="p">:</span> <span class="mi">6</span><span class="p">)</span>
<span class="n">d</span><span class="p">[</span><span class="s2">"k"</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="nb">print</span><span class="p">(</span><span class="n">d</span><span class="p">[</span><span class="s2">"k"</span><span class="p">])</span> <span class="c1"># 7</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><a class="reference external" href="https://docs.python.org/2/library/collections.html#collections.defaultdict">Python Standard Library - collections.defaultdict</a></li>
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