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<meta name="author" content="Thomas Loiret">
<meta name="description" content="A couple of tricks to understand the path taken to a certain point in the code.">
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<h1 itemprop="name">How did I get there?</h1>
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Thomas Loiret
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<span class="meta__date" itemprop="datePublished" content="2022-02-15">
<span class="glyphicon glyphicon-calendar"></span> Tue 15 February 2022
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<p>You have a complex piece of code and you don't understand how you end up in a specific method? <strong>The solution is to get the stack trace.</strong></p>
<h1>Print the stack trace</h1>
<p>The solution is to use the <code>traceback</code> module:</p>
<div class="highlight"><pre><span></span><code><span class="kn">from</span> <span class="nn">traceback</span> <span class="kn">import</span> <span class="n">print_stack</span>
<span class="n">print_stack</span><span class="p">()</span>
</code></pre></div>
<p><strong>Note:</strong> this will print it to <code>stderr</code>. If you want to print to <code>stdout</code> use <code>print_stack(file=sys.stdout)</code>.</p>
<h1>While using <code>pdb</code></h1>
<p>You can easily get the stack trace while using the <a href="https://docs.python.org/3/library/pdb.html">Python Debugger</a> (<code>pdb</code>). To do so, simply type <code>w</code> (or <code>where</code>). It will print the stack trace.</p>
<h1>Example</h1>
<p>Let's take the following file <code>script.py</code> as example:</p>
<div class="highlight"><pre><span></span><code><span class="kn">from</span> <span class="nn">traceback</span> <span class="kn">import</span> <span class="n">print_stack</span>
<span class="k">def</span> <span class="nf">bar</span><span class="p">():</span>
<span class="k">pass</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">():</span>
<span class="n">print_stack</span><span class="p">()</span>
<span class="n">bar</span><span class="p">()</span>
<span class="n">foo</span><span class="p">()</span>
</code></pre></div>
<p>The output will be:</p>
<div class="highlight"><pre><span></span><code> <span class="n">File</span> <span class="s2">"script.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">10</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="n">foo</span><span class="p">()</span>
<span class="n">File</span> <span class="s2">"script.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">7</span><span class="p">,</span> <span class="ow">in</span> <span class="n">foo</span>
<span class="n">print_stack</span><span class="p">()</span>
</code></pre></div>
<p>Each entry is composed of 2 lines:</p>
<ul>
<li>the first one contains the location (i.e. file, line and current method),</li>
<li>the second line indicates the method that led to the next step.</li>
</ul>
<p><strong>Note:</strong> The stack trace contains only the methods that are still ongoing when we hit the <code>print_stack()</code>. In the previous example, there is no trace of the <code>bar</code> method in the output as its execution was already done when we reached the <code>print_stack()</code>.</p>
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