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<!---
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This lesson was developed by Steven Peters for Python five-day course.
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Questions? Comments?
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1. Log an issue to this repo to alert me of a problem.
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2. Suggest an edit yourself by forking this repo, making edits, and submitting a pull request with your changes back to our master branch.
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3. Hit me up on Slack @steve.peters.
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--->
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<section id="section" class="level2 separator">
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<h2><img src="https://s3.amazonaws.com/python-ga/images/GA_Cog_Medium_White_RGB.png" /></h2>
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<h1>
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Python Programming: Class Inheritance
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</h1>
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<!--
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## Overview
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This lesson assumes knowledge of classes and dives straight into the concept of inheritance. After a We Do to build a child class, there's an I Do that walks through overwriting the parent variables and methods. If there's time, assign an exercise! It's in the `xx-additional-exercises` folder (it's too long for a slide). Otherwise, assign it for homework.
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## Important Notes or Prerequisites
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- Students have just learned about classes! Go slowly here. Make sure everyone's confident with classes.
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## Learning Objectives
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In this lesson, students will:
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- Implement inheritance.
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- Describe what has been inherited from one class to another.
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- Overwrite variables and methods.
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## Duration
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20 minutes
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## Suggested Agenda
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| Time | Activity |
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| --- | --- |
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| 0:00 - 0:03 | Welcome |
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| 0:03 - 0:10 | Creating a Child Class |
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| 0:10 - 0:18 | Overwriting Variables and Methods |
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| 0:18 - 0:20 | Summary |
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## Differentiation and Extensions
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- In the interest of time, this does not have a You Do for overwriting attributes or methods. It has a Knowledge Check question, but feel free to add exercises!
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- If students get this easily, introduce multiple inheritance — perhaps an `iPhone 8` class, inheriting from `IPhone`.
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- There is, in the `xx-additional-exercises` folder in the parent folder, an inheritance challenge that you should give in class if there's time. If not, give as homework. It's quite long.
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## In Class: Materials
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- Projector
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- Internet connection
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- Python 3
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-->
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<hr />
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</section>
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<section id="learning-objectives" class="level2">
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<h2>Learning Objectives</h2>
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<p><em>After this lesson, you will be able to…</em></p>
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<ul>
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<li>Implement inheritance.</li>
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<li>Describe what has been inherited from one class to another.</li>
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<li>Overwrite variables and methods.</li>
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</ul>
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<hr />
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</section>
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<section id="discussion-similar-classes" class="level2">
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<h2>Discussion: Similar Classes</h2>
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<p><code>Phone</code> is a class — there are hundreds of types of phones.</p>
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<ul>
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<li>What attributes and functions would a <code>Phone</code> have?</li>
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</ul>
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<p>What about an <code>iPhone</code>? Or <code>android_phone</code>?</p>
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<ul>
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<li><code>iPhone</code> and <code>android_phone</code> would be objects of the <code>Phone</code> class.</li>
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<li>But, there are different types of iPhones and Android phones.</li>
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<li>Should <code>IPhone</code> and <code>AndroidPhone</code> be classes themselves?</li>
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</ul>
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<p>What would you do?</p>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Have students brainstorm what a <code>Phone</code> class would contain.</li>
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</ul>
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<p><strong>Talking Points:</strong></p>
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You could go further: - You could say <code>IPhone</code> is a class — there are many types of iPhones. - What specific attributes would an <code>IPhone</code> have apart from other phones? - You could say <code>AndroidPhone</code> is a class — there are many types of Android phones. - What specific attributes would an <code>AndroidPhone</code> have apart from other phones?" - <code>AndroidPhone</code> and <code>IPhone</code> are separate classes. However, they are both types of <code>Phone</code>.
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</aside>
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<hr />
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</section>
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<section id="introduction-inheritance" class="level2">
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<h2>Introduction: Inheritance</h2>
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<p><code>AndroidPhone</code> and <code>IPhone</code> are separate classes <em>and</em> in the <code>Phone</code> class.</p>
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<p>This is called <strong>inheritance</strong>: making classes that are subsets of other classes.</p>
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<p><code>Phone</code> is the <strong>parent</strong> class. It’s a regular class! All phones:</p>
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<ul>
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<li>Have a phone number.</li>
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<li>Can place phone calls.</li>
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<li>Can send text messages.</li>
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</ul>
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<p><code>IPhone</code> is a <strong>child</strong> class. The child class <strong>inherits</strong> methods and properties from the parent class but can also define its own functionality. iPhones uniquely:</p>
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<ul>
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<li>Have an <code>unlock</code> method that accepts a fingerprint.</li>
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<li>Have a <code>set_fingerprint</code> method that accepts a fingerprint.</li>
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</ul>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Brainstorm the differences between the phone classes.</li>
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<li>This can be hard to wrap your head around in programming, so give real-world examples. Mystery novels are a type of book, but there are tons of those. Sofas are a type of couch, which are a type of chair.</li>
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</ul>
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<p><strong>Talking Points:</strong></p>
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<ul>
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<li>Inheritance allows us to extend functionality defined in a <code>parent</code> class (<code>Phone</code>) and create <code>child</code> classes (e.g., <code>IPhone</code>, <code>AndroidPhone</code>) that extend and compartmentalize different pieces of functionality.</li>
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<li>When we define a class to represent a <code>Phone</code>, we can add the basic properties and functionality that all phones have.</li>
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</ul>
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</aside>
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<hr />
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</section>
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<section id="we-do-inheritance" class="level2">
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<h2>We Do: Inheritance</h2>
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<p>All phones have a phone number, can place phone calls, and can send text messages.</p>
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<p>Start a new file, <code>Phone.py</code>. In it, let’s start and test the class:</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb1-1" data-line-number="1"><span class="kw">class</span> Phone:</a>
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<a class="sourceLine" id="cb1-2" data-line-number="2"> <span class="kw">def</span> <span class="fu">__init__</span>(<span class="va">self</span>, phone_number):</a>
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<a class="sourceLine" id="cb1-3" data-line-number="3"> <span class="va">self</span>.number <span class="op">=</span> phone_number</a>
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<a class="sourceLine" id="cb1-4" data-line-number="4"></a>
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<a class="sourceLine" id="cb1-5" data-line-number="5"> <span class="kw">def</span> call(<span class="va">self</span>, other_number):</a>
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<a class="sourceLine" id="cb1-6" data-line-number="6"> <span class="bu">print</span>(<span class="st">"Calling from"</span>, <span class="va">self</span>.number, <span class="st">"to"</span>, other_number)</a>
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<a class="sourceLine" id="cb1-7" data-line-number="7"></a>
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<a class="sourceLine" id="cb1-8" data-line-number="8"> <span class="kw">def</span> text(<span class="va">self</span>, other_number, msg):</a>
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<a class="sourceLine" id="cb1-9" data-line-number="9"> <span class="bu">print</span>(<span class="st">"Sending text from"</span>, <span class="va">self</span>.number, <span class="st">"to"</span>, other_number)</a>
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<a class="sourceLine" id="cb1-10" data-line-number="10"> <span class="bu">print</span>(msg)</a>
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<a class="sourceLine" id="cb1-11" data-line-number="11"></a>
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<a class="sourceLine" id="cb1-12" data-line-number="12">test_phone <span class="op">=</span> Phone(<span class="dv">5214</span>)</a>
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<a class="sourceLine" id="cb1-13" data-line-number="13">test_phone.call(<span class="dv">515</span>)</a>
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<a class="sourceLine" id="cb1-14" data-line-number="14">test_phone.text(<span class="dv">51121</span>, <span class="st">"Hi!"</span>)</a></code></pre></div>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Have them start a new file and do this with you!</li>
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<li>Make sure students follow along, but don’t discourage copying and pasting from the slide — it’s a lot to type up!</li>
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<li>Classes are still pretty new — talk through each line as you write it.</li>
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</ul>
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</aside>
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<hr />
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</section>
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<section id="we-do-iphone-class" class="level2">
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<h2>We Do: <code>IPhone</code> Class</h2>
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<p>Underneath the <code>Phone</code> class definition, let’s create the <code>IPhone</code> class.</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb2-1" data-line-number="1"><span class="kw">class</span> IPhone(Phone):</a>
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<a class="sourceLine" id="cb2-2" data-line-number="2"> <span class="co"># Class definitions accept a parameter specifying what class they inherit from.</span></a>
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<a class="sourceLine" id="cb2-3" data-line-number="3"> <span class="kw">def</span> <span class="fu">__init__</span>(<span class="va">self</span>, phone_number):</a>
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<a class="sourceLine" id="cb2-4" data-line-number="4"> <span class="co"># super()` calls the `init` defined in the parent class.</span></a>
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<a class="sourceLine" id="cb2-5" data-line-number="5"> <span class="bu">super</span>().<span class="fu">__init__</span>(phone_number)</a>
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<a class="sourceLine" id="cb2-6" data-line-number="6"> <span class="co"># Now self.number is set, because that's what happens in the parent Phone _init_.</span></a></code></pre></div>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Walk through this line by line. When you get to super, scroll up to point out <code>Phone</code>’s super.</li>
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</ul>
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<p><strong>Talking Points:</strong></p>
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<ul>
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<li>After we define <code>Phone</code>, we can create classes that <em>inherit</em> from the <code>Phone</code> class and add their own properties and functionality.</li>
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<li>Let’s define two new classes that inherit from the <code>Phone</code> class.</li>
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<li>Notice how the <code>IPhone</code> class doesn’t repeat the code that attaches the <code>phone_number</code> passed to the <code>__init__</code> method to the <code>self</code> reference.</li>
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<li>The <code>IPhone</code> class calls the parent constructor through the super method and allows the parent class to execute that default behavior.</li>
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</ul>
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</aside>
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<hr />
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</section>
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<section id="we-do-iphone-class-1" class="level2">
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<h2>We Do: <code>IPhone</code> Class</h2>
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<p>iPhones uniquely:</p>
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<ul>
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<li>Have an <code>unlock</code> method that accepts a fingerprint.</li>
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<li>Have a <code>set_fingerprint</code> method that accepts a fingerprint.</li>
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</ul>
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<div class="sourceCode" id="cb3"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb3-1" data-line-number="1"><span class="kw">class</span> IPhone(Phone):</a>
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<a class="sourceLine" id="cb3-2" data-line-number="2"> <span class="kw">def</span> <span class="fu">__init__</span>(<span class="va">self</span>, phone_number):</a>
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<a class="sourceLine" id="cb3-3" data-line-number="3"> <span class="bu">super</span>().<span class="fu">__init__</span>(phone_number)</a>
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<a class="sourceLine" id="cb3-4" data-line-number="4"></a>
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<a class="sourceLine" id="cb3-5" data-line-number="5"> <span class="co"># Under the call to super, we can define unique IPhone variables.</span></a>
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<a class="sourceLine" id="cb3-6" data-line-number="6"> <span class="co"># Regular Phone objects won't have this!</span></a>
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<a class="sourceLine" id="cb3-7" data-line-number="7"> <span class="va">self</span>.fingerprint <span class="op">=</span> <span class="va">None</span></a>
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<a class="sourceLine" id="cb3-8" data-line-number="8"></a>
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<a class="sourceLine" id="cb3-9" data-line-number="9"> <span class="co"># Here are methods unique to IPhone objects:</span></a>
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<a class="sourceLine" id="cb3-10" data-line-number="10"> <span class="kw">def</span> set_fingerprint(<span class="va">self</span>, fingerprint):</a>
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<a class="sourceLine" id="cb3-11" data-line-number="11"> <span class="va">self</span>.fingerprint <span class="op">=</span> fingerprint</a>
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<a class="sourceLine" id="cb3-12" data-line-number="12"></a>
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<a class="sourceLine" id="cb3-13" data-line-number="13"> <span class="kw">def</span> unlock(<span class="va">self</span>, fingerprint<span class="op">=</span><span class="va">None</span>):</a>
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<a class="sourceLine" id="cb3-14" data-line-number="14"> <span class="cf">if</span> (fingerprint <span class="op">==</span> <span class="va">self</span>.fingerprint):</a>
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<a class="sourceLine" id="cb3-15" data-line-number="15"> <span class="bu">print</span>(<span class="st">"Phone unlocked. Fingerprint matches."</span>)</a>
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<a class="sourceLine" id="cb3-16" data-line-number="16"> <span class="cf">else</span>:</a>
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<a class="sourceLine" id="cb3-17" data-line-number="17"> <span class="bu">print</span>(<span class="st">"Phone locked. Fingerprint doesn't match."</span>)</a></code></pre></div>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Make sure students are still following along, conceptually and in their file.<br />
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</li>
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<li>Walk through this carefully. Recap that methods are just class functions.</li>
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</ul>
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</aside>
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<hr />
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</section>
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<section id="side-discussion-edge-cases" class="level2">
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<h2>Side Discussion: Edge Cases</h2>
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<p>Look at:</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb4-1" data-line-number="1"><span class="kw">def</span> unlock(<span class="va">self</span>, fingerprint<span class="op">=</span><span class="va">None</span>):</a>
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<a class="sourceLine" id="cb4-2" data-line-number="2"> <span class="cf">if</span> (fingerprint <span class="op">==</span> <span class="va">self</span>.fingerprint):</a>
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<a class="sourceLine" id="cb4-3" data-line-number="3"> <span class="bu">print</span>(<span class="st">"Phone unlocked. Fingerprint matches."</span>)</a>
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<a class="sourceLine" id="cb4-4" data-line-number="4"> <span class="cf">else</span>:</a>
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<a class="sourceLine" id="cb4-5" data-line-number="5"> <span class="bu">print</span>(<span class="st">"Phone locked. Fingerprint doesn't match."</span>)</a></code></pre></div>
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<p>What if <code>self.fingerprint</code> is currently <code>None</code>? We need to account for this!</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb5-1" data-line-number="1"><span class="kw">def</span> unlock(<span class="va">self</span>, fingerprint<span class="op">=</span><span class="va">None</span>):</a>
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<a class="sourceLine" id="cb5-2" data-line-number="2"> <span class="cf">if</span> (<span class="va">self</span>.fingerprint <span class="op">==</span> <span class="va">None</span>):</a>
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<a class="sourceLine" id="cb5-3" data-line-number="3"> <span class="bu">print</span>(<span class="st">"Phone unlocked. No fingerprint needed."</span>)</a>
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<a class="sourceLine" id="cb5-4" data-line-number="4"> <span class="cf">elif</span> (fingerprint <span class="op">==</span> <span class="va">self</span>.fingerprint):</a>
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<a class="sourceLine" id="cb5-5" data-line-number="5"> <span class="bu">print</span>(<span class="st">"Phone unlocked. Fingerprint matches."</span>)</a>
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<a class="sourceLine" id="cb5-6" data-line-number="6"> <span class="cf">else</span>:</a>
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<a class="sourceLine" id="cb5-7" data-line-number="7"> <span class="bu">print</span>(<span class="st">"Phone locked. Fingerprint doesn't match."</span>)</a></code></pre></div>
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<p>When programming, always watch for <strong>edge cases</strong>. This isn’t specific to classes!</p>
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<aside class="notes">
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<p><strong>Teaching Tips:</strong></p>
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<ul>
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<li>Be very clear that this isn’t specific to classes — it’s something to always watch out for, in functions, <code>for</code> loops, and anything at all. Note that this is a reason why pseudocode is so important.</li>
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</ul>
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</aside>
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<hr />
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</section>
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<section id="we-do-testing-iphone" class="level2">
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<h2>We Do: Testing <code>IPhone</code></h2>
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<p>Add some test lines at the bottom:</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb6-1" data-line-number="1">my_iphone <span class="op">=</span> IPhone(<span class="dv">151</span>)</a>
|
|
|
<a class="sourceLine" id="cb6-2" data-line-number="2">my_iphone.unlock()</a>
|
|
|
<a class="sourceLine" id="cb6-3" data-line-number="3">my_iphone.set_fingerprint(<span class="st">"Jory's Fingerprint"</span>)</a>
|
|
|
<a class="sourceLine" id="cb6-4" data-line-number="4">my_iphone.unlock()</a>
|
|
|
<a class="sourceLine" id="cb6-5" data-line-number="5">my_iphone.unlock(<span class="st">"Jory's Fingerprint"</span>)</a>
|
|
|
<a class="sourceLine" id="cb6-6" data-line-number="6"></a>
|
|
|
<a class="sourceLine" id="cb6-7" data-line-number="7"><span class="co"># And we can call the Phone methods:</span></a>
|
|
|
<a class="sourceLine" id="cb6-8" data-line-number="8">my_iphone.call(<span class="dv">515</span>)</a>
|
|
|
<a class="sourceLine" id="cb6-9" data-line-number="9">my_iphone.text(<span class="dv">51121</span>, <span class="st">"Hi!"</span>)</a></code></pre></div>
|
|
|
<p>Try it! Then, try this. Why does it fail?</p>
|
|
|
<div class="sourceCode" id="cb7"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb7-1" data-line-number="1"><span class="co"># Let's try a Phone object on an iPhone method.</span></a>
|
|
|
<a class="sourceLine" id="cb7-2" data-line-number="2">test_phone.unlock()</a></code></pre></div>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Talking Points:</strong></p>
|
|
|
<ul>
|
|
|
<li>We can create <code>IPhone</code>s, <code>AndroidPhone</code>s, and still regular <code>Phone</code>s. <code>Phone</code> is being used as a parent class, but it’s still a class!</li>
|
|
|
<li>Inheritance is an extremely powerful feature of classes.</li>
|
|
|
<li>It allows us to create “generic” parent classes, such as the <code>Phone()</code> class, and then create child classes like <code>IPhone()</code> that represent subsets of the parent class.</li>
|
|
|
<li>We change the characteristics of the object when we define the <code>IPhone()</code> class.</li>
|
|
|
<li>Because it inherits from <code>Phone()</code>, we’re still able to use the parent methods <code>call()</code> and <code>text()</code>.
|
|
|
<ul>
|
|
|
<li>We don’t need to rewrite these methods in the child class.</li>
|
|
|
</ul></li>
|
|
|
<li>Using inheritance, you can easily create hierarchies of functionality. This keeps your code clean and intuitive.</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="quick-recap-inheritance" class="level2">
|
|
|
<h2>Quick Recap: Inheritance</h2>
|
|
|
<ul>
|
|
|
<li>A class can inherit from another class — a parent class and a child class.</li>
|
|
|
<li>The child class can declare its own variables and methods, but it also has access to all the parents’.</li>
|
|
|
</ul>
|
|
|
<div class="sourceCode" id="cb8"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb8-1" data-line-number="1"><span class="co">## Parent class: A regular class ##</span></a>
|
|
|
<a class="sourceLine" id="cb8-2" data-line-number="2"><span class="kw">class</span> Phone:</a>
|
|
|
<a class="sourceLine" id="cb8-3" data-line-number="3"> <span class="kw">def</span> <span class="fu">__init__</span>(<span class="va">self</span>, phone_number):</a>
|
|
|
<a class="sourceLine" id="cb8-4" data-line-number="4"> <span class="va">self</span>.number <span class="op">=</span> phone_number</a>
|
|
|
<a class="sourceLine" id="cb8-5" data-line-number="5"></a>
|
|
|
<a class="sourceLine" id="cb8-6" data-line-number="6"> <span class="kw">def</span> call(<span class="va">self</span>, other_number):</a>
|
|
|
<a class="sourceLine" id="cb8-7" data-line-number="7"> <span class="bu">print</span>(<span class="st">"Calling from"</span>, <span class="va">self</span>.number, <span class="st">"to"</span>, other_number)</a>
|
|
|
<a class="sourceLine" id="cb8-8" data-line-number="8"></a>
|
|
|
<a class="sourceLine" id="cb8-9" data-line-number="9">test_phone <span class="op">=</span> Phone(<span class="dv">5214</span>) <span class="co"># It's a regular class!</span></a>
|
|
|
<a class="sourceLine" id="cb8-10" data-line-number="10">test_phone.call(<span class="dv">515</span>)</a>
|
|
|
<a class="sourceLine" id="cb8-11" data-line-number="11"></a>
|
|
|
<a class="sourceLine" id="cb8-12" data-line-number="12"><span class="co">## Child class: Pass in the parent class and call super() ##</span></a>
|
|
|
<a class="sourceLine" id="cb8-13" data-line-number="13"><span class="kw">class</span> IPhone(Phone):</a>
|
|
|
<a class="sourceLine" id="cb8-14" data-line-number="14"> <span class="kw">def</span> <span class="fu">__init__</span>(<span class="va">self</span>, phone_number):</a>
|
|
|
<a class="sourceLine" id="cb8-15" data-line-number="15"> <span class="bu">super</span>().<span class="fu">__init__</span>(phone_number)</a>
|
|
|
<a class="sourceLine" id="cb8-16" data-line-number="16"></a>
|
|
|
<a class="sourceLine" id="cb8-17" data-line-number="17"> <span class="co"># Under the call to super, define unique child class variables and methods.</span></a>
|
|
|
<a class="sourceLine" id="cb8-18" data-line-number="18"> <span class="co"># Parent class objects won't have this!</span></a>
|
|
|
<a class="sourceLine" id="cb8-19" data-line-number="19"> <span class="va">self</span>.fingerprint <span class="op">=</span> <span class="va">None</span></a>
|
|
|
<a class="sourceLine" id="cb8-20" data-line-number="20"></a>
|
|
|
<a class="sourceLine" id="cb8-21" data-line-number="21"> <span class="kw">def</span> set_fingerprint(<span class="va">self</span>, fingerprint):</a>
|
|
|
<a class="sourceLine" id="cb8-22" data-line-number="22"> <span class="va">self</span>.fingerprint <span class="op">=</span> fingerprint</a>
|
|
|
<a class="sourceLine" id="cb8-23" data-line-number="23"></a>
|
|
|
<a class="sourceLine" id="cb8-24" data-line-number="24">my_iphone <span class="op">=</span> IPhone(<span class="dv">151</span>) <span class="co"># Create an object as usual.</span></a>
|
|
|
<a class="sourceLine" id="cb8-25" data-line-number="25">my_iphone.set_fingerprint(<span class="st">"Jory's Fingerprint"</span>) <span class="co"># Call a method.</span></a>
|
|
|
<a class="sourceLine" id="cb8-26" data-line-number="26">my_iphone.call(<span class="dv">515</span>) <span class="co"># Call a method from the parent class.</span></a></code></pre></div>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="i-do-overwriting-attributes" class="level2">
|
|
|
<h2>I Do: Overwriting Attributes</h2>
|
|
|
<p><strong>Next up: Overwriting attributes!</strong></p>
|
|
|
<p>Let’s switch to a new example. You don’t need to follow along.</p>
|
|
|
<p>Here’s a regular <code>Building</code> class:</p>
|
|
|
<div class="sourceCode" id="cb9"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb9-1" data-line-number="1"><span class="kw">class</span> Building(<span class="bu">object</span>):</a>
|
|
|
<a class="sourceLine" id="cb9-2" data-line-number="2"> <span class="co"># Class variables</span></a>
|
|
|
<a class="sourceLine" id="cb9-3" data-line-number="3"> avg_sqft <span class="op">=</span> <span class="dv">12500</span></a>
|
|
|
<a class="sourceLine" id="cb9-4" data-line-number="4"> avg_bedrooms <span class="op">=</span> <span class="dv">3</span></a>
|
|
|
<a class="sourceLine" id="cb9-5" data-line-number="5"></a>
|
|
|
<a class="sourceLine" id="cb9-6" data-line-number="6"> <span class="co"># No __init__ - there are no instance variables to declare!</span></a>
|
|
|
<a class="sourceLine" id="cb9-7" data-line-number="7"> <span class="co"># This is possible in any class, not just inheritance. (Building is a normal class.)</span></a>
|
|
|
<a class="sourceLine" id="cb9-8" data-line-number="8"></a>
|
|
|
<a class="sourceLine" id="cb9-9" data-line-number="9"> <span class="kw">def</span> describe_building(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb9-10" data-line-number="10"> <span class="bu">print</span>(<span class="st">'Avg. Beds:'</span>, <span class="va">self</span>.avg_bedrooms)</a>
|
|
|
<a class="sourceLine" id="cb9-11" data-line-number="11"> <span class="bu">print</span>(<span class="st">'Avg. Sq. Ft.:'</span>, <span class="va">self</span>.avg_sqft)</a>
|
|
|
<a class="sourceLine" id="cb9-12" data-line-number="12"></a>
|
|
|
<a class="sourceLine" id="cb9-13" data-line-number="13"> <span class="kw">def</span> get_avg_price(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb9-14" data-line-number="14"> price <span class="op">=</span> <span class="va">self</span>.avg_sqft <span class="op">*</span> <span class="dv">5</span> <span class="op">+</span> <span class="va">self</span>.avg_bedrooms <span class="op">*</span> <span class="dv">15000</span></a>
|
|
|
<a class="sourceLine" id="cb9-15" data-line-number="15"> <span class="cf">return</span> price</a>
|
|
|
<a class="sourceLine" id="cb9-16" data-line-number="16"></a>
|
|
|
<a class="sourceLine" id="cb9-17" data-line-number="17">my_building <span class="op">=</span> Building()</a>
|
|
|
<a class="sourceLine" id="cb9-18" data-line-number="18">my_building.describe_building()</a></code></pre></div>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>This is run as an I Do to save time and typing. Run it as a We Do if you think students need the practice typing everything out (and you have time). Because it introduces new concepts, don’t run it as a You Do.</li>
|
|
|
<li>There’s no <code>init</code>! This is new. Note that this isn’t unique to inheritance — right now, this is a regular class.</li>
|
|
|
<li>Call out the order of operations for the price math.</li>
|
|
|
<li>This is a regular class, but walk through each line.</li>
|
|
|
</ul>
|
|
|
<p><strong>Talking Points:</strong></p>
|
|
|
<ul>
|
|
|
<li>Here’s a new class.</li>
|
|
|
<li>In it, we have some attributes for the average square feet, bedrooms, and bathrooms of a building. We also have two functions: one to print the attributes out in <code>describe_building()</code> and one to calculate the average price and return it based on those attributes in <code>get_avg_price()</code>.</li>
|
|
|
<li><em>Note:</em> When we define our <code>Building()</code> class and specify <code>Building(object)</code> in the definition, we’re indicating that the <code>Building()</code> class we’re creating <em>inherits</em> characteristics from the <code>object</code> class.</li>
|
|
|
<li>The built-in <code>object</code> class in Python is the most basic “parent class” we could have.</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="i-do-inheriting-building" class="level2">
|
|
|
<h2>I Do: Inheriting Building</h2>
|
|
|
<p>Inheriting from <code>Building</code>, we can create a <code>Mansion</code> class.</p>
|
|
|
<div class="sourceCode" id="cb10"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb10-1" data-line-number="1"><span class="co"># Call in the parent, Building, to the class definition.</span></a>
|
|
|
<a class="sourceLine" id="cb10-2" data-line-number="2"><span class="kw">class</span> Mansion(Building):</a>
|
|
|
<a class="sourceLine" id="cb10-3" data-line-number="3"> <span class="co"># Our child class definition goes here.</span></a>
|
|
|
<a class="sourceLine" id="cb10-4" data-line-number="4"> <span class="co"># Will have the same class variables, instance variables, and methods as Mansion objects.</span></a></code></pre></div>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="overwriting-variables" class="level2">
|
|
|
<h2>Overwriting Variables</h2>
|
|
|
<p>What if we want the class variables to have different values? We can set new ones. Remember, child classes do not affect the parent class.</p>
|
|
|
<div class="sourceCode" id="cb11"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb11-1" data-line-number="1"><span class="kw">class</span> Mansion(Building):</a>
|
|
|
<a class="sourceLine" id="cb11-2" data-line-number="2"> <span class="co"># Overwrite the class variables.</span></a>
|
|
|
<a class="sourceLine" id="cb11-3" data-line-number="3"> avg_sqft <span class="op">=</span> <span class="dv">6</span></a>
|
|
|
<a class="sourceLine" id="cb11-4" data-line-number="4"> avg_bedrooms <span class="op">=</span> <span class="dv">1</span></a>
|
|
|
<a class="sourceLine" id="cb11-5" data-line-number="5"></a>
|
|
|
<a class="sourceLine" id="cb11-6" data-line-number="6"> <span class="co"># We don't have a call to super __init__. Why?</span></a>
|
|
|
<a class="sourceLine" id="cb11-7" data-line-number="7"> <span class="co"># There's no __init__ in the parent to call!</span></a>
|
|
|
<a class="sourceLine" id="cb11-8" data-line-number="8"></a>
|
|
|
<a class="sourceLine" id="cb11-9" data-line-number="9"><span class="co">### Now, let's try it out. </span><span class="al">###</span></a>
|
|
|
<a class="sourceLine" id="cb11-10" data-line-number="10"><span class="co"># This still has the old values.</span></a>
|
|
|
<a class="sourceLine" id="cb11-11" data-line-number="11">my_building <span class="op">=</span> Building()</a>
|
|
|
<a class="sourceLine" id="cb11-12" data-line-number="12">my_building.describe_building()</a>
|
|
|
<a class="sourceLine" id="cb11-13" data-line-number="13"></a>
|
|
|
<a class="sourceLine" id="cb11-14" data-line-number="14"><span class="co"># The mansion object has the new class variables!</span></a>
|
|
|
<a class="sourceLine" id="cb11-15" data-line-number="15">avg_mansion <span class="op">=</span> Mansion()</a>
|
|
|
<a class="sourceLine" id="cb11-16" data-line-number="16">avg_mansion.describe_building()</a></code></pre></div>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Run this!</li>
|
|
|
<li>Do this in the same file so students can see the different class variables on the screen. Scroll up and point them out.</li>
|
|
|
<li>Note that <code>Mansion</code> is an entire class! Classes don’t need to be fancy.</li>
|
|
|
</ul>
|
|
|
<p><strong>Talking Points:</strong></p>
|
|
|
<ul>
|
|
|
<li>In this class definition, the average square feet, bedrooms, and bathrooms have been changed, but nothing else has been done.</li>
|
|
|
<li>Because the <code>Mansion()</code> class <em>inherits</em> from the <code>Building()</code> parent class, it has access to the class methods we defined for <code>Building()</code>.</li>
|
|
|
<li>It is important to note that child classes do not affect the functionality of the parent class. Below, we’ve added a function, <code>how_big()</code>, to the <code>Mansion()</code> class.</li>
|
|
|
<li>We don’t have super because there’s no <code>init</code> in the parent!</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="discussion-child-class-methods" class="level2">
|
|
|
<h2>Discussion: Child Class Methods</h2>
|
|
|
<p>In the <code>Building</code> class, we have:</p>
|
|
|
<div class="sourceCode" id="cb12"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb12-1" data-line-number="1"><span class="kw">def</span> get_avg_price(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb12-2" data-line-number="2"> price <span class="op">=</span> <span class="va">self</span>.avg_sqft <span class="op">*</span> <span class="dv">5</span> <span class="op">+</span> <span class="va">self</span>.avg_bedrooms <span class="op">*</span> <span class="dv">15000</span></a>
|
|
|
<a class="sourceLine" id="cb12-3" data-line-number="3"> <span class="cf">return</span> price</a></code></pre></div>
|
|
|
<p>What if a <code>Mansion</code>’s price calculation is different? What do you think we can do?</p>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Start a discussion. See if students can guess that, like overwriting class variables, we can overwrite methods.</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="overwriting-methods" class="level2">
|
|
|
<h2>Overwriting Methods</h2>
|
|
|
<p>We know that we can overwrite variables. Turns out, we can also overwrite methods!</p>
|
|
|
<div class="sourceCode" id="cb13"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb13-1" data-line-number="1"><span class="kw">class</span> Mansion(Building):</a>
|
|
|
<a class="sourceLine" id="cb13-2" data-line-number="2"></a>
|
|
|
<a class="sourceLine" id="cb13-3" data-line-number="3"> <span class="kw">def</span> get_avg_price(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb13-4" data-line-number="4"> <span class="cf">return</span> <span class="dv">1000000</span></a>
|
|
|
<a class="sourceLine" id="cb13-5" data-line-number="5"></a>
|
|
|
<a class="sourceLine" id="cb13-6" data-line-number="6">mans <span class="op">=</span> Mansion()</a>
|
|
|
<a class="sourceLine" id="cb13-7" data-line-number="7">bldg <span class="op">=</span> Building()</a>
|
|
|
<a class="sourceLine" id="cb13-8" data-line-number="8"></a>
|
|
|
<a class="sourceLine" id="cb13-9" data-line-number="9">bldg.get_avg_price()</a>
|
|
|
<a class="sourceLine" id="cb13-10" data-line-number="10"><span class="co"># # returns `self.avg_sqft * 5 + self.avg_bedrooms * 15000`</span></a>
|
|
|
<a class="sourceLine" id="cb13-11" data-line-number="11"></a>
|
|
|
<a class="sourceLine" id="cb13-12" data-line-number="12">mans.get_avg_price()</a>
|
|
|
<a class="sourceLine" id="cb13-13" data-line-number="13"><span class="co"># Returns 1000000</span></a></code></pre></div>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Run this!</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="quick-review" class="level2">
|
|
|
<h2>Quick Review</h2>
|
|
|
<p>When we make child classes, we can overwrite class variables and methods.</p>
|
|
|
<div class="sourceCode" id="cb14"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb14-1" data-line-number="1"><span class="kw">class</span> Building(<span class="bu">object</span>):</a>
|
|
|
<a class="sourceLine" id="cb14-2" data-line-number="2"> <span class="co"># Class variables</span></a>
|
|
|
<a class="sourceLine" id="cb14-3" data-line-number="3"> avg_sqft <span class="op">=</span> <span class="dv">12500</span></a>
|
|
|
<a class="sourceLine" id="cb14-4" data-line-number="4"> avg_bedrooms <span class="op">=</span> <span class="dv">3</span></a>
|
|
|
<a class="sourceLine" id="cb14-5" data-line-number="5"></a>
|
|
|
<a class="sourceLine" id="cb14-6" data-line-number="6"> <span class="kw">def</span> get_avg_price(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb14-7" data-line-number="7"> price <span class="op">=</span> <span class="va">self</span>.avg_sqft <span class="op">*</span> <span class="dv">5</span> <span class="op">+</span> <span class="va">self</span>.avg_bedrooms <span class="op">*</span> <span class="dv">15000</span></a>
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<a class="sourceLine" id="cb14-8" data-line-number="8"> <span class="cf">return</span> price</a>
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<a class="sourceLine" id="cb14-9" data-line-number="9"></a>
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|
<a class="sourceLine" id="cb14-10" data-line-number="10"></a>
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<a class="sourceLine" id="cb14-11" data-line-number="11"><span class="kw">class</span> Mansion(Building):</a>
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<a class="sourceLine" id="cb14-12" data-line-number="12"> <span class="co"># Overwrite the class variables.</span></a>
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<a class="sourceLine" id="cb14-13" data-line-number="13"> avg_sqft <span class="op">=</span> <span class="dv">6</span></a>
|
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|
<a class="sourceLine" id="cb14-14" data-line-number="14"> avg_bedrooms <span class="op">=</span> <span class="dv">1</span></a>
|
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|
<a class="sourceLine" id="cb14-15" data-line-number="15"></a>
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<a class="sourceLine" id="cb14-16" data-line-number="16"> <span class="kw">def</span> get_avg_price(<span class="va">self</span>):</a>
|
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<a class="sourceLine" id="cb14-17" data-line-number="17"> <span class="cf">return</span> <span class="dv">1000000</span></a></code></pre></div>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Do a quick check for understanding.</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="knowledge-check" class="level2">
|
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|
<h2>Knowledge Check</h2>
|
|
|
<p>Consider the following classes:</p>
|
|
|
<div class="sourceCode" id="cb15"><pre class="sourceCode python"><code class="sourceCode python"><a class="sourceLine" id="cb15-1" data-line-number="1"><span class="kw">class</span> Animal(<span class="bu">object</span>):</a>
|
|
|
<a class="sourceLine" id="cb15-2" data-line-number="2"> <span class="kw">def</span> is_mammal(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb15-3" data-line-number="3"> <span class="cf">return</span> <span class="va">True</span></a>
|
|
|
<a class="sourceLine" id="cb15-4" data-line-number="4"> <span class="kw">def</span> is_alive(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb15-5" data-line-number="5"> <span class="cf">return</span> <span class="va">True</span></a>
|
|
|
<a class="sourceLine" id="cb15-6" data-line-number="6"></a>
|
|
|
<a class="sourceLine" id="cb15-7" data-line-number="7"><span class="kw">class</span> Grasshopper(Animal):</a>
|
|
|
<a class="sourceLine" id="cb15-8" data-line-number="8"> <span class="kw">def</span> is_small(<span class="va">self</span>):</a>
|
|
|
<a class="sourceLine" id="cb15-9" data-line-number="9"> <span class="cf">return</span> <span class="va">True</span></a></code></pre></div>
|
|
|
<p>You instantiate two objects: <code>bug = Grasshopper()</code> and <code>cat = Animal()</code>. Which of the following instance methods are available for each?</p>
|
|
|
<ol type="1">
|
|
|
<li><code>bug.is_mammal()</code></li>
|
|
|
<li><code>bug.is_alive()</code></li>
|
|
|
<li><code>bug.is_small()</code></li>
|
|
|
<li><code>bug.is_animal()</code></li>
|
|
|
</ol>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Give them a few minutes to guess.</li>
|
|
|
</ul>
|
|
|
</aside>
|
|
|
<hr />
|
|
|
</section>
|
|
|
<section id="summary-and-qa" class="level2">
|
|
|
<h2>Summary and Q&A</h2>
|
|
|
<p>Inheritance:</p>
|
|
|
<ul>
|
|
|
<li>Allows us to make classes using other classes as templates.</li>
|
|
|
<li>Has a <strong>parent</strong> class (<code>Phone</code>) and a <strong>child</strong> class (<code>IPhone</code>).
|
|
|
<ul>
|
|
|
<li>The parent class is still a usable class!</li>
|
|
|
</ul></li>
|
|
|
</ul>
|
|
|
<p>Child classes:</p>
|
|
|
<ul>
|
|
|
<li><code>inherit</code> methods and properties from a parent class.</li>
|
|
|
<li>Have access to all of the functionality of its parent.</li>
|
|
|
<li>Can have new attributes and methods.
|
|
|
<ul>
|
|
|
<li>They won’t be available to the parent.</li>
|
|
|
</ul></li>
|
|
|
<li>Can overwrite values from the parent class.</li>
|
|
|
</ul>
|
|
|
<aside class="notes">
|
|
|
<p><strong>Teaching Tips:</strong></p>
|
|
|
<ul>
|
|
|
<li>Do a quick check for understanding.</li>
|
|
|
<li>If there’s time, assign an exercise! It’s in the <code>xx-additional-exercises</code> folder. Otherwise, assign it for homework.</li>
|
|
|
</ul>
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