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<header id="title-block-header">
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<h1 class="title">9.9 The <code>object</code> Superclass</h1>
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<section>
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<p>In our very first chapter, we described every piece of data as an <em>object</em>, and have continued to use this term throughout this course. It turns out that “object” is not merely a theoretical concept, but made explicit in the Python language. Python has a special class called <code>object</code>, which is an <em>ancestor class</em><label for="sn-0" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-0" class="margin-toggle"/><span class="sidenote">By “ancestor” we mean either a parent class, or a parent of a parent class, etc.</span> of every other class, both built-in classes like <code>int</code> or our custom data classes and the classes we’ve defined in this chapter. And this includes abstract classes like <code>Stack</code>!</p>
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<p>By default, whenever we define a new class (including data classes), if we do not specify a superclass in parentheses, <code>object</code> is the <em>implicit</em> superclass, which is why we can write <code>class Stack:</code> instead of <code>class Stack(object):</code>.</p>
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<p><img src="images/object_inheritance_diagram.png" /></p>
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<h2 id="the-object-special-methods">The <code>object</code> special methods</h2>
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<p>This <code>object</code> class defines several special methods as part of its shared public interface, including:<label for="sn-1" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-1" class="margin-toggle"/><span class="sidenote"> The Python convention is to name methods that have a special purpose with double underscores. These are sometimes called “dunder” methods (<strong>d</strong>ouble <strong>under</strong>score).</span></p>
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<ul>
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<li><code>__init__(self, ...)</code>, the initializer</li>
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<li><code>__str__(self)</code>, which returns a <code>str</code> representation of the object.</li>
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</ul>
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<h3 id="method-inheritance">Method inheritance</h3>
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<p>Unlike our <code>Stack</code> abstract class earlier this chapter, the <code>object</code> class is actually <em>not abstract</em>, and implements each of these methods. We can use this to illustrate a different form of inheritance, where the superclass is a concrete class. In this case, inheritance is used not just to define a shared public interface, but also to provide <em>default implementations</em> for each method in the interface.</p>
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<p>For example, suppose we create a dummy class with a completely empty body:</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">class</span> Donut:</span>
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<span id="cb1-2"><a href="#cb1-2"></a> <span class="co">"""A donut, because why not?"</span></span></code></pre></div>
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<p>This class inherits the <code>object.__init__</code> method, which allows us to create new <code>Donut</code> instances.</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb2-1"><a href="#cb2-1"></a><span class="op">>>></span> donut <span class="op">=</span> Donut()</span>
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<span id="cb2-2"><a href="#cb2-2"></a><span class="op">>>></span> <span class="bu">type</span>(donut)</span>
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<span id="cb2-3"><a href="#cb2-3"></a><span class="op"><</span><span class="kw">class</span> <span class="st">'__main__.Donut'</span><span class="op">></span></span></code></pre></div>
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<p>Similarly, this class inherits the <code>object.__str__</code> method, which returns a string that states the class name and memory location of the object:</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb3-1"><a href="#cb3-1"></a><span class="op">>>></span> d <span class="op">=</span> Donut()</span>
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<span id="cb3-2"><a href="#cb3-2"></a><span class="op">>>></span> d.<span class="fu">__str__</span>()</span>
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<span id="cb3-3"><a href="#cb3-3"></a><span class="co">'<__main__.Donut object at 0x7fc299d7b588>'</span></span></code></pre></div>
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<p>We can use the built-in <code>dir</code> function to see all of the special methods that <code>Donut</code> has inherited from <code>object</code>:<label for="sn-2" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-2" class="margin-toggle"/><span class="sidenote">Though this list includes few special attributes set directly by the Python interpreter, which are beyond the scope of this course.</span></p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb4-1"><a href="#cb4-1"></a><span class="op">>>></span> <span class="bu">dir</span>(Donut)</span>
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<span id="cb4-2"><a href="#cb4-2"></a>[<span class="st">'__class__'</span>, <span class="st">'__delattr__'</span>, <span class="st">'__dict__'</span>, <span class="st">'__dir__'</span>, <span class="st">'__doc__'</span>, <span class="st">'__eq__'</span>, <span class="st">'__format__'</span>, <span class="st">'__ge__'</span>, <span class="st">'__getattribute__'</span>, <span class="st">'__gt__'</span>, <span class="st">'__hash__'</span>, <span class="st">'__init__'</span>, <span class="st">'__init_subclass__'</span>, <span class="st">'__le__'</span>, <span class="st">'__lt__'</span>, <span class="st">'__module__'</span>, <span class="st">'__ne__'</span>, <span class="st">'__new__'</span>, <span class="st">'__reduce__'</span>, <span class="st">'__reduce_ex__'</span>, <span class="st">'__repr__'</span>, <span class="st">'__setattr__'</span>, <span class="st">'__sizeof__'</span>, <span class="st">'__str__'</span>, <span class="st">'__subclasshook__'</span>, <span class="st">'__weakref__'</span>]</span></code></pre></div>
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<p>There is another reason these methods are special beyond simply being inherited from the <code>object</code> superclass: they are often called by other functions or parts of Python syntax. For example, we have already seen how the <code>__init__</code> method is called when a new object is initialized.</p>
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<p>The <code>__str__</code> method is called when we attempt to convert an object to a string by calling <code>str</code> on it:</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb5-1"><a href="#cb5-1"></a><span class="op">>>></span> d <span class="op">=</span> Donut()</span>
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<span id="cb5-2"><a href="#cb5-2"></a><span class="op">>>></span> d.<span class="fu">__str__</span>()</span>
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<span id="cb5-3"><a href="#cb5-3"></a><span class="co">'<__main__.Donut object at 0x7fc299d7b588>'</span></span>
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<span id="cb5-4"><a href="#cb5-4"></a><span class="op">>>></span> <span class="bu">str</span>(d)</span>
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<span id="cb5-5"><a href="#cb5-5"></a><span class="co">'<__main__.Donut object at 0x7fc299d7b588>'</span></span></code></pre></div>
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<p>Similarly, the built-in <code>print</code> function actually first converts its arguments into strings using their <code>__str__</code> methods, and then prints out the resulting text.</p>
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<h3 id="method-overriding">Method overriding</h3>
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<p>Now, even though the <code>object</code> superclass contains default implementations of <code>__init__</code> and <code>__str__</code>, in practice we often want to define our own custom implementations of these methods.</p>
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<p>Every time we’ve defined our own <code>__init__</code> in a class, we have <strong>overridden</strong> the <code>object.__init__</code> method. Formally, we say that a class <code>C</code> <strong>overrides</strong> a method <code>m</code> when the method <code>m</code> is defined in the superclass of <code>C</code>, and is also given a concrete implementation in the body of <code>C</code>.<label for="sn-3" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-3" class="margin-toggle"/><span class="sidenote"> This definition applies whether the superclass of <code>C</code> has <code>m</code> as an abstract or concrete method. For example, we could say that <code>Stack1</code> overrides the <code>push</code> and <code>pop</code> method from its abstract superclass <code>Stack</code>.</span></p>
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<p>Similarly, when we defined a custom exception class in <a href="05-section.html">Section 9.5</a>,</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">class</span> EmptyStackError(<span class="pp">Exception</span>):</span>
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<span id="cb6-2"><a href="#cb6-2"></a> <span class="co">"""Exception raised when calling pop on an empty stack."""</span></span>
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<span id="cb6-3"><a href="#cb6-3"></a></span>
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<span id="cb6-4"><a href="#cb6-4"></a> <span class="kw">def</span> <span class="fu">__str__</span>(<span class="va">self</span>) <span class="op">-></span> <span class="bu">str</span>:</span>
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<span id="cb6-5"><a href="#cb6-5"></a> <span class="co">"""Return a string representation of this error."""</span></span>
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<span id="cb6-6"><a href="#cb6-6"></a> <span class="cf">return</span> <span class="st">'pop may not be called on an empty stack'</span></span></code></pre></div>
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<p>this class <em>overrode</em> the <code>__str__</code> method to use its own string representation, which is displayed when this exception is raised.</p>
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