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<li class="toctree-l1"><a class="reference internal" href="../intro.html">Python Like You Mean It</a></li>
<li class="toctree-l1"><a class="reference internal" href="../module_1.html">Module 1: Getting Started with Python</a></li>
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<li class="toctree-l2"><a class="reference internal" href="ClassDefinition.html">Defining a New Class of Object</a></li>
<li class="toctree-l2"><a class="reference internal" href="ClassInstances.html">Instances of a Class</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Applications_of_OOP.html">Applications of Object Oriented Programming</a></li>
<li class="toctree-l2"><a class="reference internal" href="Special_Methods.html">Special Methods</a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">Inheritance</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#Summary-of-Inheritance">Summary of Inheritance</a></li>
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<div class="section" id="Inheritance">
<h1>Inheritance<a class="headerlink" href="#Inheritance" title="Permalink to this headline">¶</a></h1>
<p>A final topic for us to discuss in this introduction to object oriented programming is the concept of inheritance. Working with inheritance provides powerful abstractions and elegant code re-use - it permits a class to inherit and build off of the attributes of another class.</p>
<p>Let’s immediately consider an example of inheritance in action. Let’s revisit the <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> class that we wrote in the introduction to this module.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
<span class="sd">""" A class of Python object that describes the properties of a rectangle"""</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">,</span> <span class="n">center</span><span class="o">=</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
<span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
<span class="bp">self</span><span class="o">.</span><span class="n">center</span> <span class="o">=</span> <span class="n">center</span>
<span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s2">"Rectangle(width=</span><span class="si">{w}</span><span class="s2">, height=</span><span class="si">{h}</span><span class="s2">, center=</span><span class="si">{c}</span><span class="s2">)"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">h</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">height</span><span class="p">,</span>
<span class="n">w</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">width</span><span class="p">,</span>
<span class="n">c</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">center</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">compute_area</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">*</span> <span class="bp">self</span><span class="o">.</span><span class="n">height</span>
</pre></div>
</div>
<p>Now suppose that we also want to write a <code class="docutils literal notranslate"><span class="pre">Square</span></code> class, such that only a single side length need be specified to determine its size. Recognize that a square is a special type of rectangle - one whose width and height are equal. In light of this, we ought to leverage the code that we already wrote for <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>. We can do this by defining a <code class="docutils literal notranslate"><span class="pre">Square</span></code> class that is a <em>subclass</em> of <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>. This means that <code class="docutils literal notranslate"><span class="pre">Square</span></code> will <em>inherit</em> all of the attributes of <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>, including its
methods. Let’s proceed with writing this subclass:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Creating Square, a subclass of Rectangle</span>
<span class="k">class</span> <span class="nc">Square</span><span class="p">(</span><span class="n">Rectangle</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">side</span><span class="p">,</span> <span class="n">center</span><span class="o">=</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)):</span>
<span class="c1"># equivalent to `Rectangle.__init__(self, side, side, center)`</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">side</span><span class="p">,</span> <span class="n">side</span><span class="p">,</span> <span class="n">center</span><span class="p">)</span>
</pre></div>
</div>
<p>Specifying <code class="docutils literal notranslate"><span class="pre">class</span> <span class="pre">Square(Rectangle)</span></code> signals that <code class="docutils literal notranslate"><span class="pre">Square</span></code> is a subclass of <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> and thus it will have inherited the attributes of <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>. Next, see that we overwrote the <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method that <code class="docutils literal notranslate"><span class="pre">Square</span></code> inherited; instead of accepting a height and a width, <code class="docutils literal notranslate"><span class="pre">Square</span></code> should by specified by a single side length. Within this new <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method, we pass in that single side length as both the width and height to <code class="docutils literal notranslate"><span class="pre">Rectangle.__init__</span></code>. <code class="docutils literal notranslate"><span class="pre">super</span></code> always refers to the
“super class” or “parent class” of a given class, thus <code class="docutils literal notranslate"><span class="pre">super</span></code> is <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> here.</p>
<p>Having defined our subclass, we can leverage the other methods of <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> as-is. Let’s see <code class="docutils literal notranslate"><span class="pre">Square</span></code> in action:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># create a square of side-length 2</span>
<span class="o">>>></span> <span class="n">my_square</span> <span class="o">=</span> <span class="n">Square</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="c1"># using the inherited `get_area` method</span>
<span class="o">>>></span> <span class="n">my_square</span><span class="o">.</span><span class="n">get_area</span><span class="p">()</span>
<span class="mi">4</span>
<span class="c1"># a square is a rectangle with equal height/width</span>
<span class="o">>>></span> <span class="n">my_square</span>
<span class="n">Rectangle</span><span class="p">(</span><span class="n">width</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">height</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">center</span><span class="o">=</span><span class="p">(</span><span class="mf">0.0</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">))</span>
<span class="o">>>></span> <span class="n">my_square</span><span class="o">.</span><span class="n">width</span> <span class="o">==</span> <span class="n">my_square</span><span class="o">.</span><span class="n">height</span>
<span class="kc">True</span>
</pre></div>
</div>
<p>The built-in <code class="docutils literal notranslate"><span class="pre">issubclass</span></code> function allows us to verify the relationship between <code class="docutils literal notranslate"><span class="pre">Square</span></code> and <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># `Square` and `Rectangle` are distinct classes</span>
<span class="o">>>></span> <span class="n">Square</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">Rectangle</span>
<span class="kc">True</span>
<span class="c1"># `Square` is a subclass of `Rectangle`</span>
<span class="o">>>></span> <span class="nb">issubclass</span><span class="p">(</span><span class="n">Square</span><span class="p">,</span> <span class="n">Rectangle</span><span class="p">)</span>
<span class="kc">True</span>
<span class="c1"># `my_square is an both an instance of `Square` and `Rectangle`</span>
<span class="o">>>></span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">my_square</span><span class="p">,</span> <span class="n">Square</span><span class="p">)</span>
<span class="kc">True</span>
<span class="o">>>></span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">my_square</span><span class="p">,</span> <span class="n">Rectangle</span><span class="p">)</span>
<span class="kc">True</span>
</pre></div>
</div>
<div class="section" id="Summary-of-Inheritance">
<h2>Summary of Inheritance<a class="headerlink" href="#Summary-of-Inheritance" title="Permalink to this headline">¶</a></h2>
<p>In general, if you have a class <code class="docutils literal notranslate"><span class="pre">A</span></code>, then you can define a subclass of <code class="docutils literal notranslate"><span class="pre">A</span></code> via:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">A</span><span class="p">:</span>
<span class="n">attr</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="mi">0</span>
<span class="c1"># `B` is a subclass of `A`</span>
<span class="k">class</span> <span class="nc">B</span><span class="p">(</span><span class="n">A</span><span class="p">):</span>
<span class="c1"># inherits `attr` and `method`</span>
<span class="n">b_attr</span> <span class="o">=</span> <span class="o">-</span><span class="mi">2</span> <span class="c1"># class attribute distinct to `B`</span>
<span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># overwrites inherited `method`</span>
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">B</span></code> will have inherited all of the attributes and methods of <code class="docutils literal notranslate"><span class="pre">A</span></code>. Defining attributes and methods within the definition of <code class="docutils literal notranslate"><span class="pre">B</span></code> will overwrite those that already exist in <code class="docutils literal notranslate"><span class="pre">A</span></code>. <code class="docutils literal notranslate"><span class="pre">B</span></code> is also free to have its own distinct attributes and methods be defined, irrespective of <code class="docutils literal notranslate"><span class="pre">A</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">issubclass</span><span class="p">(</span><span class="n">B</span><span class="p">,</span> <span class="n">A</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">A</span><span class="o">.</span><span class="n">attr</span>
<span class="go">0</span>
<span class="gp">>>> </span><span class="n">A</span><span class="p">()</span><span class="o">.</span><span class="n">method</span><span class="p">()</span>
<span class="go">0</span>
<span class="gp">>>> </span><span class="n">B</span><span class="o">.</span><span class="n">attr</span>
<span class="go">0</span>
<span class="gp">>>> </span><span class="n">B</span><span class="p">()</span><span class="o">.</span><span class="n">method</span><span class="p">()</span>
<span class="go">-1</span>
<span class="gp">>>> </span><span class="n">B</span><span class="o">.</span><span class="n">b_attr</span>
<span class="go">-2</span>
</pre></div>
</div>
<p>We have only scratched the surface of the topic of class inheritance. That being said, this section does convey the essential functionality and utility of class inheritance.</p>
</div>
<div class="section" id="Links-to-Official-Documentation">
<h2>Links to Official Documentation<a class="headerlink" href="#Links-to-Official-Documentation" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/classes.html#inheritance">Official Tutorial: Inheritance</a></p></li>
</ul>
</div>
</div>
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