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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Classes in Python</h1><h2 id="general-points">General points</h2><ul><li>We don’t need to use <code>new</code> when instantiating an instance of a class.</li><li>All properties and methods must be defined in the constructor. This is in contrast to JavaScript where the properties in the constructor are those we want to initialise when the object is created and where there may be other properties and methods that are used outside of the instantiation process.</li><li>In contrast to TypeScript and other OOP languages it is not necessary to declare the properties you wish to define in the constructor, outside of the constructor. As long as they are defined in the constructor they are accessible.</li></ul><h2 id="basic-syntax">Basic syntax</h2><pre><code class="py language-py">class Person:
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""" An example class to hold a persons name and age"""
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def __init__(self, name, age):
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self.name = name
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self.age = age
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def birthday(self):
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print('Happy birthday you were', self.age)
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self.age += 1
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print('You are now', self.age)
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p1 = Person('John', 36)
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p2 = Person('Thomas', 34)
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print(p1)
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print(p2)
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print(p1.name)
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# John</code></pre><p>Key points to note:</p><ul><li>The <code>__init__</code> method is the constructor function and must exist on every class to define the properties of the class</li><li><code>self</code> is a reference to the class itself and the object it will create, akin to <code>this</code> in other languages</li><li>You must pass <code>self</code> as a parameter to every method (this is a difference from JS)</li><li>As with functions, we can use docstrings to document the class. What you write here will show up in Intellisense etc</li></ul><h2 id="more-complex-class">More complex class</h2><pre><code class="py language-py">class Person:
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""" An example class to hold a persons name and age"""
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def __init__(self, name, age):
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self.name = name
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self.age = age
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def __str__(self):
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return self.name + ' is ' + str(self.age)
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def birthday(self):
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print ('Happy birthday you were', self.age)
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self.age += 1
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print('You are now', self.age)
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def calculate_pay(self, hours_worked):
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rate_of_pay = 7.50
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if self.age >= 21:
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rate_of_pay += 2.50
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return hours_worked * rate_of_pay
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def is_teenager(self):
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return self.age < 20</code></pre><h2 id="object-references">Object references</h2><p>When you log a class you get a reference to its hexadecimal <span class="zettel-link-container errors"><span class="zettel-link" title="Wiki-link does not refer to any existing zettel"><a>Memory</a></span></span> reference.</p><pre><code class="py language-py">p1 = Person('John', 36)
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p2 = Person('Thomas', 34)
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print(p1)
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print(p2)
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# <__main__.Person object at 0x102e75510>
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# <__main__.Person object at 0x102e75511></code></pre><p>This shows each object is unique. You can also generate a specific ID with the <code>id()</code> method:</p><pre><code class="py language-py">print(id(p1))
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print(id(p2))
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# 4379088272
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# 4379088656</code></pre><h2 id="copying-objects">Copying objects</h2><p>The same principle that applies to <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Functions in Python"><a href="Functions_in_Python.html">copying functions</a></span></span> applies to copying objects created through classes: redeclaration results in a duplicate entity. Thus changes to the duplicate will affect the original.</p><p>This becomes obvious when you use <code>id()</code> but otherwise might not be apparent. To copy you should therefore declare a new instance of the class.</p><h3 id="the-str-method">The <code>str</code> method</h3><p>The memory reference isn’t very helpful for viewing the data contents. To get around this we can use the builtin <code>__string__</code> method which automatically logs whatever properties you put in there:</p><pre><code class="py language-py">class Person:
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""" An example class to hold a persons name and age"""
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def __init__(self, name, age):
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self.name = name
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self.age = age
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def __str__(self):
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return self.name + ' is ' + str(self.age)
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p3 = Person('Thomas', 34)
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print(p3)
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# Thomas is 34</code></pre><h2 id="deleting-objects">Deleting objects</h2><p>You might want to delete an object reference because:</p><ul><li>the variable referencing the object goes out of scope</li><li>the variable is set to <code>None</code></li></ul><p>After the <code>del</code> statement is applied to a variable that holds an object, the object will no longer be available and any attempt to reference it will result in an error.</p><pre><code class="py language-py">p1 = Person('J-Man', 76)
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del p1</code></pre></div></article><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">OOP</span><span class="ui basic label zettel-tag" title="Tag">python</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html> |