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<liclass="toctree-l1"><aclass="reference internal" href="../intro.html">Python Like You Mean It</a></li>
<liclass="toctree-l1"><aclass="reference internal" href="../module_1.html">Module 1: Getting Started with Python</a></li>
<liclass="toctree-l1"><aclass="reference internal" href="../module_2.html">Module 2: The Essentials of Python</a></li>
<liclass="toctree-l1"><aclass="reference internal" href="../module_2_problems.html">Module 2: Problems</a></li>
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<liclass="toctree-l2"><aclass="reference internal" href="ClassDefinition.html">Defining a New Class of Object</a></li>
<liclass="toctree-l2 current"><aclass="current reference internal" href="#">Instances of a Class</a><ul>
<liclass="toctree-l3"><aclass="reference internal" href="#Object-Identity-and-Creating-an-Instance">Object Identity and Creating an Instance</a></li>
<liclass="toctree-l3"><aclass="reference internal" href="#Defining-Instance-Level-Attributes:-the-__init__-Method">Defining Instance-Level Attributes: the <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> Method</a></li>
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<divclass="section" id="Instances-of-a-Class">
<h1>Instances of a Class<aclass="headerlink" href="#Instances-of-a-Class" title="Permalink to this headline"></a></h1>
<p>Thus far we have learned about the syntax for defining a new class of object, specifying its name, attributes, and methods (which are attributes that are functions). Once we leave the scope of the class definition, a class object is formed - the resulting <em>class object</em> is the singular object that encapsulates our class definition. We seldom will want to pass around or manipulate this class object once it is created. Rather, we will want to use it to create individual <em>instances</em> of that class.
To be more concrete, <codeclass="docutils literal notranslate"><spanclass="pre">list</span></code> is a class object (remember that “class” and “type” are synonymous) - it is the same sort of object that is produced when a <codeclass="docutils literal notranslate"><spanclass="pre">class</span></code> definition is executed. As you saw in <aclass="reference external" href="http://www.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Basic_Objects.html#Lists">Module 2</a>, we can use this class object to create individual <em>instances</em> of the list class, each one containing its own sequence of items.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># using the class object `list` to create list-instances</span>
<spanclass="o">>>></span><spanclass="nb">list</span><spanclass="p">()</span><spanclass="c1"># create a `list`-instance that is empty</span>
<spanclass="p">[]</span>
<spanclass="o">>>></span><spanclass="nb">list</span><spanclass="p">((</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">))</span><spanclass="c1"># create a `list`-instance containing 1, 2, and 3</span>
<spanclass="p">[</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">]</span>
<spanclass="c1"># `a` and `b` are distinct instances of the list class/type</span>
<spanclass="c1"># even though they contain the same sequence of integers</span>
<spanclass="o">>>></span><spanclass="n">a</span><spanclass="o">=</span><spanclass="nb">list</span><spanclass="p">((</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">))</span>
<spanclass="o">>>></span><spanclass="n">b</span><spanclass="o">=</span><spanclass="nb">list</span><spanclass="p">((</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">))</span>
<spanclass="o">>>></span><spanclass="n">a</span><spanclass="ow">is</span><spanclass="n">b</span>
<spanclass="kc">False</span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">a</span><spanclass="p">,</span><spanclass="nb">list</span><spanclass="p">)</span>
<spanclass="kc">True</span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">b</span><spanclass="p">,</span><spanclass="nb">list</span><spanclass="p">)</span>
<spanclass="kc">True</span>
<spanclass="c1"># Calling the append method on `a` only affects that particular</span>
<spanclass="c1"># list-instance.</span>
<spanclass="o">>>></span><spanclass="n">a</span><spanclass="o">.</span><spanclass="n">append</span><spanclass="p">(</span><spanclass="o">-</span><spanclass="mi">1</span><spanclass="p">)</span>
<spanclass="o">>>></span><spanclass="n">a</span>
<spanclass="p">[</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">,</span><spanclass="o">-</span><spanclass="mi">1</span><spanclass="p">]</span>
<spanclass="o">>>></span><spanclass="n">b</span>
<spanclass="p">[</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">]</span>
</pre></div>
</div>
<p>Each of these instances share the common attributes <codeclass="docutils literal notranslate"><spanclass="pre">append</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">count</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">reverse</span></code>, and so on, as specified in the definition of Python’s list class, which is encapsulated by the <codeclass="docutils literal notranslate"><spanclass="pre">list</span></code> class object. That being said, the specific content of any given list is an attribute of that particular list instance; that is, the content of a particular list is an <em>instance attribute</em> rather than a class attribute. Thus far, we do not have the ability to create instance-level attributes. Let’s change
that.</p>
<p>Suppose that we want to make our own <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> class. Each person should have her/his own name, thus the name should be an instance-level attribute. We will learn to define a special initialization method that allows us to define and set instance-level attributes. In the context of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code>, this will allow us to give each person their own name:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="k">class</span><spanclass="nc">Person</span><spanclass="p">:</span>
<spanclass="gp">... </span><spanclass="k">def</span><spanclass="fm">__init__</span><spanclass="p">(</span><spanclass="bp">self</span><spanclass="p">,</span><spanclass="n">name</span><spanclass="p">):</span>
<spanclass="gp">... </span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">name</span><spanclass="o">=</span><spanclass="n">name</span>
<spanclass="gp">>>> </span><spanclass="n">emmy</span><spanclass="o">=</span><spanclass="n">Person</span><spanclass="p">(</span><spanclass="s2">"Emmy"</span><spanclass="p">)</span>
<spanclass="gp">>>> </span><spanclass="n">niels</span><spanclass="o">=</span><spanclass="n">Person</span><spanclass="p">(</span><spanclass="s2">"Niels"</span><spanclass="p">)</span>
<spanclass="gp">>>> </span><spanclass="n">emmy</span><spanclass="o">.</span><spanclass="n">name</span>
<spanclass="go">'Emmy'</span>
<spanclass="gp">>>> </span><spanclass="n">niels</span><spanclass="o">.</span><spanclass="n">name</span>
<spanclass="go">'Niels'</span>
<spanclass="gp">>>> </span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">emmy</span><spanclass="p">,</span><spanclass="n">Person</span><spanclass="p">)</span>
<spanclass="go">True</span>
</pre></div>
</div>
<p>We will learn about the <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> method and that peculiar <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code> argument momentarily. First, we will learn about creating an instance object from a class object.</p>
<divclass="section" id="Object-Identity-and-Creating-an-Instance">
<h2>Object Identity and Creating an Instance<aclass="headerlink" href="#Object-Identity-and-Creating-an-Instance" title="Permalink to this headline"></a></h2>
<p>Here we will learn the basic mechanism for creating an instance of a class. Consider the following trivial class definition:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="nc">Dummy</span><spanclass="p">:</span>
<spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">1</span>
</pre></div>
</div>
<p>We can use the “call” syntax on <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">Dummy()</span></code>, to create individual instances of this class:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># create an object that is an instance of our Dummy class</span>
<spanclass="o">>>></span><spanclass="n">d1</span><spanclass="o">=</span><spanclass="n">Dummy</span><spanclass="p">()</span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">d1</span><spanclass="p">,</span><spanclass="n">Dummy</span><spanclass="p">)</span>
<spanclass="kc">True</span>
</pre></div>
</div>
<p>Recall that the <codeclass="docutils literal notranslate"><spanclass="pre">is</span></code> operator checks to see if two items reference the exact same object in your computer’s memory. Also recall that the built-in <codeclass="docutils literal notranslate"><spanclass="pre">isinstance</span></code> function checks to see if an object is an instance of a class/type. These will help us understand the relationship between class objects, their instances, and references to objects.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># `Dummy` is the class object that encapsulates</span>
<spanclass="c1"># our class definition</span>
<spanclass="o">>>></span><spanclass="n">Dummy</span>
<spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span>
<spanclass="c1"># `d1` is an object that is an instance of our Dummy class.</span>
<spanclass="c1"># this instance resides at some memory address (0x2ae8f68f2e8)</span>
<spanclass="o">>>></span><spanclass="n">d1</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x2ae8f68f2e8</span><spanclass="o">></span>
<spanclass="c1"># d1 is not Dummy; it is an instance of Dummy</span>
<spanclass="o">>>></span><spanclass="n">d1</span><spanclass="ow">is</span><spanclass="n">Dummy</span>
<spanclass="kc">False</span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">d1</span><spanclass="p">,</span><spanclass="n">Dummy</span><spanclass="p">)</span>
<spanclass="kc">True</span>
</pre></div>
</div>
<p>See that <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> is to <codeclass="docutils literal notranslate"><spanclass="pre">d1</span></code> as <codeclass="docutils literal notranslate"><spanclass="pre">list</span></code> is to <codeclass="docutils literal notranslate"><spanclass="pre">[1,</span><spanclass="pre">4,</span><spanclass="pre">"a"]</span></code></p>
<p>Let’s create another instance of <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code>. It is important to understand that this new instance is <em>distinct</em> from the one that we already created.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># `d2` is a new instance of our Dummy class.</span>
<spanclass="c1"># It resides at a distinct memory address (0x2ae8f666f60)</span>
<spanclass="o">>>></span><spanclass="n">d2</span><spanclass="o">=</span><spanclass="n">Dummy</span><spanclass="p">()</span>
<spanclass="o">>>></span><spanclass="n">d2</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x2ae8f666f60</span><spanclass="o">></span>
<spanclass="o">>>></span><spanclass="n">d2</span><spanclass="ow">is</span><spanclass="n">d1</span><spanclass="c1"># `d2` and `d1` are distinct instances of `Dummy`</span>
<spanclass="kc">False</span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">d2</span><spanclass="p">,</span><spanclass="n">Dummy</span><spanclass="p">)</span>
<spanclass="kc">True</span>
</pre></div>
</div>
<p>Python’s <aclass="reference external" href="http://www.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Variables_and_Assignment.html">rules for referencing objects with variables</a> still apply here: assigning an object to a variable, be it a class object or an instance, does not create a distinct copy of that object. The variable merely references that object, serving only as an alias for it.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># `A` references `Dummy`</span>
<spanclass="o">>>></span><spanclass="n">A</span><spanclass="o">=</span><spanclass="n">Dummy</span>
<spanclass="o">>>></span><spanclass="n">A</span><spanclass="ow">is</span><spanclass="n">Dummy</span>
<spanclass="kc">True</span>
<spanclass="c1"># creates an instance of `Dummy`, using `A`</span>
<spanclass="o">>>></span><spanclass="n">dummy_instance</span><spanclass="o">=</span><spanclass="n">A</span><spanclass="p">()</span>
<spanclass="o">>>></span><spanclass="n">dummy_instance</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x2ae8f65fcf8</span><spanclass="o">></span>
<spanclass="o">>>></span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">dummy_instance</span><spanclass="p">,</span><spanclass="n">A</span><spanclass="p">)</span><spanclass="c1"># equivalent to `isinstance(dummy_instance, Dummy)`</span>
<spanclass="kc">True</span>
<spanclass="c1"># `var` references the Dummy-instance `dummy_instance`</span>
<spanclass="o">>>></span><spanclass="n">var</span><spanclass="o">=</span><spanclass="n">dummy_instance</span>
<spanclass="o">>>></span><spanclass="n">var</span><spanclass="ow">is</span><spanclass="n">dummy_instance</span>
<spanclass="kc">True</span>
<spanclass="c1"># setting a new value to `var.x` is equivalent to</span>
<spanclass="c1"># setting that value to `dummy_instance.x`</span>
<spanclass="o">>>></span><spanclass="n">var</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">22</span>
<spanclass="o">>>></span><spanclass="n">dummy_instance</span><spanclass="o">.</span><spanclass="n">x</span>
<spanclass="mi">22</span>
</pre></div>
</div>
<divclass="admonition note">
<pclass="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: Class Initialization</strong></p>
<p>Using the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class defined above, create a list consisting of 10 <em>distinct</em> instances of this type. Write code to explicitly verify that each entry is distinct from the other, and that each entry is an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class.</p>
<p>Then, create a tuple that contains a <em>single</em> instance of <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> stored ten times. Write code to explicitly verify that the entries all reference the exact same object, and that each entry is an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class.</p>
</div>
<divclass="admonition note">
<pclass="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: Terminology</strong></p>
<p>Given:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="k">class</span><spanclass="nc">Cat</span><spanclass="p">:</span>
<spanclass="gp">... </span><spanclass="k">pass</span>
<spanclass="gp">>>> </span><spanclass="n">x</span><spanclass="o">=</span><spanclass="n">Cat</span>
<spanclass="gp">>>> </span><spanclass="n">y</span><spanclass="o">=</span><spanclass="n">Cat</span><spanclass="p">()</span>
<spanclass="gp">>>> </span><spanclass="n">x1</span><spanclass="o">=</span><spanclass="n">x</span>
<spanclass="gp">>>> </span><spanclass="n">x2</span><spanclass="o">=</span><spanclass="n">x</span><spanclass="p">()</span>
<spanclass="gp">>>> </span><spanclass="n">y1</span><spanclass="o">=</span><spanclass="n">y</span>
</pre></div>
</div>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> share?</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> share?</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> share?</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x2</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> share?</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y1</span></code> share?</p>
<p>Next, identify each of the following objects as a class object or an instance (and if instance, an instance of what)</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">"hello</span><spanclass="pre">world"</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">True</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">int</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">{"a"</span><spanclass="pre">:</span><spanclass="pre">22}</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">tuple</span></code></p></li>
</ul>
</div>
<p>Now that we know the basics of how to create an instance of a class, understand the relationship between class objects and instances, and understand the distinction between instances that are independent from one another, we can move on to learning about creating instance-level attributes for our class.</p>
</div>
<divclass="section" id="Defining-Instance-Level-Attributes:-the-__init__-Method">
<h2>Defining Instance-Level Attributes: the <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> Method<aclass="headerlink" href="#Defining-Instance-Level-Attributes:-the-__init__-Method" title="Permalink to this headline"></a></h2>
<p>As demonstrated in the <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> class that we defined earlier in this section, and the <codeclass="docutils literal notranslate"><spanclass="pre">Rectangle</span></code> class that we defined in this module’s introduction section, there is a special method, <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code>, that allows us to define instance-level attributes for our class. This is a critically-important method, which we will leverage often. Note that the name of this is: “underscore-underscore-init-underscore-underscore”, which can be pronounced as “dunder-init” (where “dunder” stands for
double-underscore).</p>
<p>Consider the slightly-modified definition of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code>, which also includes the class-attribute <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code>:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="nc">Person</span><spanclass="p">:</span>
<spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">1</span><spanclass="c1"># this sets a class-level attribute, common to all instances of `Person`</span>
<spanclass="k">def</span><spanclass="fm">__init__</span><spanclass="p">(</span><spanclass="bp">self</span><spanclass="p">,</span><spanclass="n">name</span><spanclass="p">):</span>
<spanclass="sd">""" This method is executed every time we create a new `Person` instance.</span>
<spanclass="sd"> `self` is the object instance being created."""</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">name</span><spanclass="o">=</span><spanclass="n">name</span><spanclass="c1"># set the attribute `name` to the Person-instance `self`</span>
<spanclass="c1"># __init__ cannot not return any value other than `None`. Its sole purpose is to affect</span>
<spanclass="c1"># `self`, the instance of `Person` that is being created.</span>
</pre></div>
</div>
<p>Invoking <codeclass="docutils literal notranslate"><spanclass="pre">Person()</span></code> actually calls <codeclass="docutils literal notranslate"><spanclass="pre">__init__()</span></code> “under the hood”, and any argument that we feed to <codeclass="docutils literal notranslate"><spanclass="pre">Person()</span></code> gets passed to <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code>. Looking at our definition of <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> it looks like we must pass two values to this method: <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">name</span></code>. This first argument, <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code>, actually represents the object instance of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> that is being created. Python will pass the appropriate object for <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code> to <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> automatically, thus we need only worry about passing a
value for <codeclass="docutils literal notranslate"><spanclass="pre">name</span></code>.</p>
<p>Let’s make an instance of our <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> class, passing the string <codeclass="docutils literal notranslate"><spanclass="pre">"Kamasi"</span></code> as the name:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># Creates the instance `self`, passes it</span>
<spanclass="c1"># and `"Kamasi"` to `Person.__init__`, and then</span>
<spanclass="c1"># returns the instance-object that was created</span>
<spanclass="o">>>></span><spanclass="n">p</span><spanclass="o">=</span><spanclass="n">Person</span><spanclass="p">(</span><spanclass="s2">"Kamasi"</span><spanclass="p">)</span>
<spanclass="o">>>></span><spanclass="n">p</span><spanclass="o">.</span><spanclass="n">name</span><spanclass="c1"># access the instance-attribute `name`</span>
<spanclass="s1">'Kamasi'</span>
<spanclass="o">>>></span><spanclass="n">p</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="c1"># access the class-attribute `x`</span>
<spanclass="mi">1</span>
</pre></div>
</div>
<p>Here is what is going on “under the hood” when we create this instance of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> (<strong>this is very important</strong>):</p>
<ulclass="simple">
<li><p>Invoking <codeclass="docutils literal notranslate"><spanclass="pre">Person("Kamasi")</span></code> first creates an instance of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> as if there was no <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> method specified. The resulting object does not yet have a <codeclass="docutils literal notranslate"><spanclass="pre">name</span></code> attribute. It only has the class-level attribute <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code>.</p></li>
<li><p>Next, that instance of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> is passed to <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> as the argument <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code>, and <codeclass="docutils literal notranslate"><spanclass="pre">"Kamasi"</span></code>, which we provided explicitly, is passed as the argument <codeclass="docutils literal notranslate"><spanclass="pre">name</span></code>.</p></li>
<li><p>With these arguments, <codeclass="docutils literal notranslate"><spanclass="pre">Person.__init__(self,</span><spanclass="pre">"Kamasi")</span></code> executes its body of instructions. Specifically, <codeclass="docutils literal notranslate"><spanclass="pre">self.name</span><spanclass="pre">=</span><spanclass="pre">name</span></code> sets the attribute <codeclass="docutils literal notranslate"><spanclass="pre">name</span></code> on <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code>, using the value <codeclass="docutils literal notranslate"><spanclass="pre">"Kamasi"</span></code>.</p></li>
<li><p>Having finished executing the <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> method, <codeclass="docutils literal notranslate"><spanclass="pre">Person("Kamasi")</span></code> resolves by returning the instance-object that was created.</p></li>
</ul>
<p>We now have the ability to define and set attributes on an instance-level! Understanding this process is critical to mastering object oriented programming in Python. Let’s create several <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code>-instances, all stored in a list:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># creating several instances of `Person`</span>
<spanclass="o">>>></span><spanclass="n">list_of_people</span><spanclass="o">=</span><spanclass="p">[</span><spanclass="n">Person</span><spanclass="p">(</span><spanclass="n">n</span><spanclass="p">)</span><spanclass="k">for</span><spanclass="n">n</span><spanclass="ow">in</span><spanclass="p">(</span><spanclass="s2">"Fermi"</span><spanclass="p">,</span><spanclass="s2">"Noether"</span><spanclass="p">,</span><spanclass="s2">"Euler"</span><spanclass="p">)]</span>
<spanclass="o">>>></span><spanclass="k">for</span><spanclass="n">person</span><spanclass="ow">in</span><spanclass="n">list_of_people</span><spanclass="p">:</span>
<spanclass="o">...</span><spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">person</span><spanclass="o">.</span><spanclass="n">name</span><spanclass="p">)</span>
<spanclass="o">...</span><spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">person</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="p">)</span>
<spanclass="n">Fermi</span>
<spanclass="mi">1</span>
<spanclass="n">Noether</span>
<spanclass="mi">1</span>
<spanclass="n">Euler</span>
<spanclass="mi">1</span>
</pre></div>
</div>
<p>Updating the class-level attribute <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code> affects all instances of <codeclass="docutils literal notranslate"><spanclass="pre">Person</span></code>:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># setting a new value to the class-attribute `x`</span>
<spanclass="o">>>></span><spanclass="n">Person</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">22</span>
<spanclass="c1"># this affects all instances of `Person`</span>
<spanclass="o">>>></span><spanclass="k">for</span><spanclass="n">person</span><spanclass="ow">in</span><spanclass="n">list_of_people</span><spanclass="p">:</span>
<spanclass="o">...</span><spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">person</span><spanclass="o">.</span><spanclass="n">name</span><spanclass="p">)</span>
<spanclass="o">...</span><spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">person</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="p">)</span>
<spanclass="n">Fermi</span>
<spanclass="mi">22</span>
<spanclass="n">Noether</span>
<spanclass="mi">22</span>
<spanclass="n">Euler</span>
<spanclass="mi">22</span>
</pre></div>
</div>
<divclass="admonition note">
<pclass="admonition-title fa fa-exclamation-circle"><strong>Reading Comprehension: Instance Attributes</strong></p>
<p>Define a class, <codeclass="docutils literal notranslate"><spanclass="pre">Tmp</span></code>, that has three instance attributes: <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code>, and <codeclass="docutils literal notranslate"><spanclass="pre">z</span></code>. <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> should be numbers that are set according to values passed to the instance creation, and <codeclass="docutils literal notranslate"><spanclass="pre">z</span></code> should be the product of these two values.</p>
<p>For example:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="n">tmp</span><spanclass="o">=</span><spanclass="n">Tmp</span><spanclass="p">(</span><spanclass="mf">2.1</span><spanclass="p">,</span><spanclass="mf">3.0</span><spanclass="p">)</span>
<spanclass="gp">>>> </span><spanclass="n">tmp</span><spanclass="o">.</span><spanclass="n">x</span>
<spanclass="go">2.1</span>
<spanclass="gp">>>> </span><spanclass="n">tmp</span><spanclass="o">.</span><spanclass="n">y</span>
<spanclass="go">3.0</span>
<spanclass="gp">>>> </span><spanclass="n">tmp</span><spanclass="o">.</span><spanclass="n">z</span>
<spanclass="go">6.3</span>
</pre></div>
</div>
</div>
<p>You should now have a grasp of how the special <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> method can be used to define and set instance-level attributes for your classes. Furthermore, the basic process by which invoking class instantiation produces an instance object which then automatically gets passed to <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code> as the <codeclass="docutils literal notranslate"><spanclass="pre">self</span></code> argument, should be salient. In the following section, we will encounter three varieties of methods: instance methods, class methods, and static methods. Additionally, we will encounter even
more so-called “special methods”, similar to <codeclass="docutils literal notranslate"><spanclass="pre">__init__</span></code>, which can be used to more broadly specify how your class behaves and interacts with Python’s operators.</p>
</div>
<divclass="section" id="Reading-Comprehension-Solutions">
<h2>Reading Comprehension Solutions<aclass="headerlink" href="#Reading-Comprehension-Solutions" title="Permalink to this headline"></a></h2>
<p><strong>Solution: Class Initialization</strong></p>
<p>Using the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class defined above, create a list consisting of 10 <em>distinct</em> instances of this type</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># will call `Dummy()` once for each iteration</span>
<spanclass="o">>>></span><spanclass="n">list_of_dummies</span><spanclass="o">=</span><spanclass="p">[</span><spanclass="n">Dummy</span><spanclass="p">()</span><spanclass="k">for</span><spanclass="n">i</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">10</span><spanclass="p">)]</span>
<spanclass="c1"># note the distinct memory addresses</span>
<spanclass="o">>>></span><spanclass="n">list_of_dummies</span>
<spanclass="p">[</span><spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89940</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de896d8</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de897b8</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89a20</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89ac8</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89a58</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de899e8</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89a90</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89b00</span><spanclass="o">></span><spanclass="p">,</span>
<spanclass="o"><</span><spanclass="n">__main__</span><spanclass="o">.</span><spanclass="n">Dummy</span><spanclass="n">at</span><spanclass="mh">0x1d50de89b38</span><spanclass="o">></span><spanclass="p">]</span>
</pre></div>
</div>
<p>Write code to explicitly verify that each entry is distinct from the other, and that each entry is an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="kn">from</span><spanclass="nn">itertools</span><spanclass="kn">import</span><spanclass="n">combinations</span>
<spanclass="go"># `combinations(list_of_dummies, 2)` loops over all pairs of entries</span>
<spanclass="go"># in `list_of_dummies`</span>
<spanclass="gp">>>> </span><spanclass="nb">all</span><spanclass="p">(</span><spanclass="n">a</span><spanclass="ow">is</span><spanclass="ow">not</span><spanclass="n">b</span><spanclass="k">for</span><spanclass="n">a</span><spanclass="p">,</span><spanclass="n">b</span><spanclass="ow">in</span><spanclass="n">combinations</span><spanclass="p">(</span><spanclass="n">list_of_dummies</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">))</span>
<spanclass="go">True</span>
<spanclass="gp">>>> </span><spanclass="nb">all</span><spanclass="p">(</span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">a</span><spanclass="p">,</span><spanclass="n">Dummy</span><spanclass="p">)</span><spanclass="k">for</span><spanclass="n">a</span><spanclass="ow">in</span><spanclass="n">list_of_dummies</span><spanclass="p">)</span>
<spanclass="go">True</span>
</pre></div>
</div>
<p>Create a tuple contains a <em>single</em> instance of <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> ten times. Note here that we initialize <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> once, and that the tuple-comprehension merely populates the tuple with that same instance ten times.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="n">dummy</span><spanclass="o">=</span><spanclass="n">Dummy</span><spanclass="p">()</span><spanclass="c1"># a single instance of `Dummy`</span>
<spanclass="gp">>>> </span><spanclass="n">tuple_of_dummy</span><spanclass="o">=</span><spanclass="nb">tuple</span><spanclass="p">(</span><spanclass="n">dummy</span><spanclass="k">for</span><spanclass="n">i</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">10</span><spanclass="p">))</span>
<spanclass="go"># note that the memory addresses are identical</span>
<spanclass="gp">>>> </span><spanclass="n">tuple_of_dummy</span>
<spanclass="go">(<__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>,</span>
<spanclass="go"> <__main__.Dummy at 0x1d50de887b8>)</span>
</pre></div>
</div>
<p>Write code to explicitly verify that the entries all reference the exact same object, and that each entry is an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">Dummy</span></code> class.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="nb">all</span><spanclass="p">(</span><spanclass="n">dummy</span><spanclass="ow">is</span><spanclass="n">i</span><spanclass="k">for</span><spanclass="n">i</span><spanclass="ow">in</span><spanclass="n">tuple_of_dummy</span><spanclass="p">)</span>
<spanclass="go">True</span>
<spanclass="gp">>>> </span><spanclass="nb">all</span><spanclass="p">(</span><spanclass="nb">isinstance</span><spanclass="p">(</span><spanclass="n">a</span><spanclass="p">,</span><spanclass="n">Dummy</span><spanclass="p">)</span><spanclass="k">for</span><spanclass="n">a</span><spanclass="ow">in</span><spanclass="n">tuple_of_dummy</span><spanclass="p">)</span>
<spanclass="go">True</span>
</pre></div>
</div>
<p><strong>Reading Comprehension: Terminology</strong></p>
<p>Given:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="k">class</span><spanclass="nc">Cat</span><spanclass="p">:</span>
<spanclass="gp">... </span><spanclass="k">pass</span>
<spanclass="gp">>>> </span><spanclass="n">x</span><spanclass="o">=</span><spanclass="n">Cat</span>
<spanclass="gp">>>> </span><spanclass="n">y</span><spanclass="o">=</span><spanclass="n">Cat</span><spanclass="p">()</span>
<spanclass="gp">>>> </span><spanclass="n">x1</span><spanclass="o">=</span><spanclass="n">x</span>
<spanclass="gp">>>> </span><spanclass="n">x2</span><spanclass="o">=</span><spanclass="n">x</span><spanclass="p">()</span>
<spanclass="gp">>>> </span><spanclass="n">y1</span><spanclass="o">=</span><spanclass="n">y</span>
</pre></div>
</div>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> share?: <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> reference the same class object.</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> share?: <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> is an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code> class.</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> share?: <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> references <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code>, and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> is an instance of <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code>. Thus <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> is an instance of <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code>.</p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">x2</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> share?: They are independent instances of <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code></p>
<p>What relationship do <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y1</span></code> share?: They reference the same instance of <codeclass="docutils literal notranslate"><spanclass="pre">Cat</span></code>.</p>
<p>Identify each of the following objects as a class object or an instance (and if so, an instance of what)</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">"hello</span><spanclass="pre">world"</span></code>: An instance of the <codeclass="docutils literal notranslate"><spanclass="pre">str</span></code> type (a.k.a class)</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">True</span></code>: an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">bool</span></code> type</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">int</span></code>: a class object describing integers</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">{"a"</span><spanclass="pre">:</span><spanclass="pre">22}</span></code>: an instance of the <codeclass="docutils literal notranslate"><spanclass="pre">dict</span></code> type</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">tuple</span></code>: a class object describing tuples</p></li>
</ul>
<p><strong>Reading Comprehension: Instance Attributes</strong></p>
<p>Define a class, <codeclass="docutils literal notranslate"><spanclass="pre">Tmp</span></code>, that has three instance attributes: <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code>, and <codeclass="docutils literal notranslate"><spanclass="pre">z</span></code>. <codeclass="docutils literal notranslate"><spanclass="pre">x</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">y</span></code> should be numbers that are set according to values passed to the instance creation, and <codeclass="docutils literal notranslate"><spanclass="pre">z</span></code> should be the product of these two values.</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="nc">Tmp</span><spanclass="p">:</span>
<spanclass="k">def</span><spanclass="fm">__init__</span><spanclass="p">(</span><spanclass="bp">self</span><spanclass="p">,</span><spanclass="n">x</span><spanclass="p">,</span><spanclass="n">y</span><spanclass="p">):</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">x</span><spanclass="o">=</span><spanclass="n">x</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">y</span><spanclass="o">=</span><spanclass="n">y</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">z</span><spanclass="o">=</span><spanclass="n">x</span><spanclass="o">*</span><spanclass="n">y</span>
</pre></div>
</div>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">>>> </span><spanclass="n">test</span><spanclass="o">=</span><spanclass="n">Tmp</span><spanclass="p">(</span><spanclass="mi">8</span><spanclass="p">,</span><spanclass="mi">5</span><spanclass="p">)</span>
<spanclass="gp">>>> </span><spanclass="n">test</span><spanclass="o">.</span><spanclass="n">x</span>
<spanclass="go">8</span>
<spanclass="gp">>>> </span><spanclass="n">test</span><spanclass="o">.</span><spanclass="n">y</span>
<spanclass="go">5</span>
<spanclass="gp">>>> </span><spanclass="n">test</span><spanclass="o">.</span><spanclass="n">z</span>
<spanclass="go">40</span>
</pre></div>
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