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  <div class="section" id="not-using-with-to-open-files">
<h1>Not using <code class="docutils literal"><span class="pre">with</span></code> to open files<a class="headerlink" href="#not-using-with-to-open-files" title="Permalink to this headline">¶</a></h1>
<p>In Python 2.5, the <code class="docutils literal"><span class="pre">file</span></code> class was equipped with special methods that are automatically called whenever a file is opened via a <code class="docutils literal"><span class="pre">with</span></code> statement (e.g. <code class="docutils literal"><span class="pre">with</span> <span class="pre">open(&quot;file.txt&quot;,</span> <span class="pre">&quot;r&quot;)</span> <span class="pre">as</span> <span class="pre">file</span></code>). These special methods ensure that the file is properly and safely opened and closed.</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 does not use <code class="docutils literal"><span class="pre">with</span></code> to open a file. This code depends on the programmer remembering to manually close the file via <code class="docutils literal"><span class="pre">close()</span></code> when finished. Even if the programmer remembers to call <code class="docutils literal"><span class="pre">close()</span></code> the code is still dangerous, because if an exception occurs before the call to <code class="docutils literal"><span class="pre">close()</span></code> then <code class="docutils literal"><span class="pre">close()</span></code> will not be called and the memory issues can occur, or the file can be corrupted.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;file.txt&quot;</span><span class="p">,</span> <span class="s2">&quot;r&quot;</span><span class="p">)</span>
<span class="n">content</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span>  <span class="c1"># ZeroDivisionError</span>
<span class="c1"># never executes, possible memory issues or file corruption</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</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-with-to-open-a-file">
<h3>Use <code class="docutils literal"><span class="pre">with</span></code> to open a file<a class="headerlink" href="#use-with-to-open-a-file" title="Permalink to this headline">¶</a></h3>
<p>The modified code below is the safest way to open a file. The <code class="docutils literal"><span class="pre">file</span></code> class has some special built-in methods called <code class="docutils literal"><span class="pre">__enter__()</span></code> and <code class="docutils literal"><span class="pre">__exit__()</span></code> which are automatically called when the file is opened and closed, respectively. Python guarantees that these special methods are always called, even if an exception occurs.</p>
<div class="code python highlight-default"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;file.txt&quot;</span><span class="p">,</span> <span class="s2">&quot;r&quot;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">content</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
    <span class="c1"># Python still executes f.close() even though an exception occurs</span>
    <span class="mi">1</span> <span class="o">/</span> <span class="mi">0</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>
<p><a class="reference external" href="http://effbot.org/zone/python-with-statement.htm">effbot - Understanding Python’s with statement</a></p>
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