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<section id="using-a-dictionary-to-switch">
<span id="dict-as-switch"></span><h1>Using a Dictionary to <code class="docutils literal notranslate"><span class="pre">switch</span></code><a class="headerlink" href="#using-a-dictionary-to-switch" title="Link to this heading"></a></h1>
<p>Python does not have a <code class="docutils literal notranslate"><span class="pre">switch/case</span> <span class="pre">statement</span></code>. Why not?</p>
<p><a class="reference external" href="https://www.python.org/dev/peps/pep-3103/">https://www.python.org/dev/peps/pep-3103/</a></p>
<p>So what to use instead of “switch-case”?</p>
<section id="switch-case">
<h2><code class="docutils literal notranslate"><span class="pre">switch</span></code> / <code class="docutils literal notranslate"><span class="pre">case</span></code><a class="headerlink" href="#switch-case" title="Link to this heading"></a></h2>
<section id="what-is-switch">
<h3>What is <code class="docutils literal notranslate"><span class="pre">switch</span></code>?<a class="headerlink" href="#what-is-switch" title="Link to this heading"></a></h3>
<p>Many languages have a “switch-case” construct:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">switch</span><span class="p">(</span><span class="n">argument</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"zero"</span><span class="p">;</span>
<span class="k">case</span> <span class="mi">1</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"one"</span><span class="p">;</span>
<span class="k">case</span> <span class="mi">2</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"two"</span><span class="p">;</span>
<span class="n">default</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"nothing"</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>How do you say this in Python?</p>
</section>
<section id="if-elif-chains">
<h3><code class="docutils literal notranslate"><span class="pre">if-elif</span></code> chains<a class="headerlink" href="#if-elif-chains" title="Link to this heading"></a></h3>
<p>The obvious way to say it is a chain of <code class="docutils literal notranslate"><span class="pre">elif</span></code> statements:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">argument</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"zero"</span>
<span class="k">elif</span> <span class="n">argument</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"one"</span>
<span class="k">elif</span> <span class="n">argument</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"two"</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="s2">"nothing"</span>
</pre></div>
</div>
<p>And there is nothing wrong with that, but….</p>
</section>
<section id="id1">
<h3>dict as switch<a class="headerlink" href="#id1" title="Link to this heading"></a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">elif</span></code> chain is neither elegant nor efficient. There are a number of ways to say it in python – but one elegant one is to use a dict:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">arg_dict</span> <span class="o">=</span> <span class="p">{</span><span class="mi">0</span><span class="p">:</span><span class="s2">"zero"</span><span class="p">,</span> <span class="mi">1</span><span class="p">:</span><span class="s2">"one"</span><span class="p">,</span> <span class="mi">2</span><span class="p">:</span> <span class="s2">"two"</span><span class="p">}</span>
<span class="n">arg_dict</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">argument</span><span class="p">,</span> <span class="s2">"nothing"</span><span class="p">)</span>
</pre></div>
</div>
<p>Simple, elegant and fast.</p>
<p>You can do a dispatch table by putting functions as the value.</p>
<p>Example: The mailroom2 solution.</p>
</section>
<section id="switch-with-functions">
<h3>Switch with functions<a class="headerlink" href="#switch-with-functions" title="Link to this heading"></a></h3>
<p>What would this be like if you used functions instead? Think of the possibilities.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="k">def</span><span class="w"> </span><span class="nf">my_zero_func</span><span class="p">():</span>
<span class="go"> return "I'm zero"</span>
<span class="gp">In [12]: </span><span class="k">def</span><span class="w"> </span><span class="nf">my_one_func</span><span class="p">():</span>
<span class="go"> return "I'm one"</span>
<span class="gp">In [13]: </span><span class="n">switch_func_dict</span> <span class="o">=</span> <span class="p">{</span>
<span class="go"> 0: my_zero_func,</span>
<span class="go"> 1: my_one_func,</span>
<span class="go">}</span>
<span class="gp">In [14]: </span><span class="n">switch_func_dict</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="mi">0</span><span class="p">)()</span>
<span class="gh">Out[14]: </span><span class="go">"I'm zero"</span>
</pre></div>
</div>
<p>Again, fast and efficient.</p>
<p>This is possible because functions are “first class objects” in Python.</p>
</section>
<section id="oo-switch-case">
<h3>OO switch/case<a class="headerlink" href="#oo-switch-case" title="Link to this heading"></a></h3>
<p>Another way to do the equivalent of switch / case is subclassing.</p>
<p>If you haven’t learned about classes in Python this will be pretty confusing. But here’s a high level overview:</p>
<p>In C, before C++, a common idiom was something like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">switch</span><span class="p">(</span><span class="n">object_type</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="n">circle</span><span class="p">:</span>
<span class="n">draw_a_circle</span><span class="p">();</span>
<span class="k">case</span> <span class="n">square</span><span class="p">:</span>
<span class="n">draw_a_square</span><span class="p">();</span>
<span class="k">case</span> <span class="n">polygon</span><span class="p">:</span>
<span class="n">draw_a_polygon</span><span class="p">();</span>
<span class="n">default</span><span class="p">:</span>
<span class="n">draw_nothing</span><span class="p">();</span>
<span class="p">};</span>
</pre></div>
</div>
<p>That is, a different function is called depending on what type of “thing” you are dealing with.</p>
<p>This is actually a really common idiom in C. And even in modern OO code written by old C developers – I had a developer on my team do exactly this in a program we were working on. It was a map drawing program (written in Python), and there was code all over it like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">layer</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="s2">"tiles"</span><span class="p">:</span>
<span class="n">do_something_with_tiles</span>
<span class="k">elif</span> <span class="n">layer</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="s2">"grid"</span><span class="p">:</span>
<span class="n">do_somethign_with_grid</span>
</pre></div>
</div>
<p>This was a maintainability nightmare – if you added a new layer type, you had to find every one of these constructs and add another <code class="docutils literal notranslate"><span class="pre">elif</span></code> block to it.</p>
</section>
<section id="the-oo-way">
<h3>The OO way<a class="headerlink" href="#the-oo-way" title="Link to this heading"></a></h3>
<p>With object oriented programming, you can “subclass” objects, and use “polymorphism” to achieve this kind of selection. Say you have a bunch of objects you want to be able to draw. Give each of them a <code class="docutils literal notranslate"><span class="pre">draw()</span></code> method, and then the above switch statement becomes:</p>
<p>the_object.draw()</p>
<p>That’s IT!</p>
<p>You don’t have to test to see which type of object it is, you only have to know that it knows how to draw itself.</p>
<p>Now when you add a new object type – all you need to do is make sure it has a draw() method (and other needed methods) and then all the other code will know how to use it without your changing anything.</p>
</section>
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