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<title>Python Classes — PythonCert 4.0 documentation</title>
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<div class="section" id="python-classes">
<h1>Python Classes<a class="headerlink" href="#python-classes" title="Permalink to this headline">¶</a></h1>
<p class="left">The <code class="docutils literal"><span class="pre">class</span></code> statement</p>
<p class="left"><code class="docutils literal"><span class="pre">class</span></code> creates a new type object:</p>
<div class="left highlight-ipython"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="go"> pass</span>
<span class="gp"> ...:</span>
<span class="gp">In [5]: </span><span class="nb">type</span><span class="p">(</span><span class="n">C</span><span class="p">)</span>
<span class="gh">Out[5]: </span><span class="go">type</span>
</pre></div>
</div>
<p class="left">A class is a type – interesting!</p>
<p class="left">It is created when the statement is run – much like <code class="docutils literal"><span class="pre">def</span></code></p>
<p>A simple class</p>
<p>About the simplest class you can write</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="gp">... </span> <span class="n">y</span> <span class="o">=</span> <span class="mi">2</span>
<span class="gp">>>> </span><span class="n">Point</span>
<span class="go"><class __main__.Point at 0x2bf928></span>
<span class="gp">>>> </span><span class="n">Point</span><span class="o">.</span><span class="n">x</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Point</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span>
<span class="go"><__main__.Point instance at 0x2de918></span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">x</span>
<span class="go">1</span>
</pre></div>
</div>
<div class="section" id="basic-structure-of-a-class">
<h2>Basic Structure of a class<a class="headerlink" href="#basic-structure-of-a-class" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span>
<span class="c1"># everything defined in here is in the class namespace</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span>
<span class="bp">self</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="n">y</span>
<span class="c1">## create an instance of the class</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">Point</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">)</span>
<span class="c1">## access the attributes</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"p.x is:"</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">x</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s2">"p.y is:"</span><span class="p">,</span> <span class="n">p</span><span class="o">.</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
<p>see: <code class="docutils literal"><span class="pre">Examples/Session07/simple_classes.py</span></code></p>
</div>
<div class="section" id="the-initializer">
<h2>The Initializer<a class="headerlink" href="#the-initializer" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal"><span class="pre">__init__</span></code> special method is called when a new instance of a class is created.</p>
<p>You can use it to do any set-up you need</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Point</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span>
<span class="bp">self</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="n">y</span>
</pre></div>
</div>
<p>It gets the arguments passed when you call the class object:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">Point</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="self">
<h2>Self<a class="headerlink" href="#self" title="Permalink to this headline">¶</a></h2>
<p>What is this <code class="docutils literal"><span class="pre">self</span></code> thing?</p>
<p>The instance of the class is passed as the first parameter for every method.</p>
<p>“<code class="docutils literal"><span class="pre">self</span></code>” is only a convention – but you DO want to use it.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">a_function</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Does this look familiar from C-style procedural programming?</p>
<p>Anything assigned to a <code class="docutils literal"><span class="pre">self.</span></code> attribute is kept in the instance
name space – <code class="docutils literal"><span class="pre">self</span></code> <em>is</em> the instance.</p>
<p>That’s where all the instance-specific data is.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Point</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="n">size</span> <span class="o">=</span> <span class="mi">4</span>
<span class="n">color</span><span class="o">=</span> <span class="s2">"red"</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span>
<span class="bp">self</span><span class="o">.</span><span class="n">y</span> <span class="o">=</span> <span class="n">y</span>
</pre></div>
</div>
</div>
<div class="section" id="class-attributes">
<h2>Class Attributes<a class="headerlink" href="#class-attributes" title="Permalink to this headline">¶</a></h2>
<p>Anything assigned in the class scope is a class attribute – every
instance of the class shares the same one.</p>
<p>Note: the methods defined by <code class="docutils literal"><span class="pre">def</span></code> are class attributes as well.</p>
<p>The class is one namespace, the instance is another.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Point</span><span class="p">:</span>
<span class="n">size</span> <span class="o">=</span> <span class="mi">4</span>
<span class="n">color</span><span class="o">=</span> <span class="s2">"red"</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">get_color</span><span class="p">():</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">color</span>
<span class="o">>>></span> <span class="n">p3</span><span class="o">.</span><span class="n">get_color</span><span class="p">()</span>
<span class="s1">'red'</span>
</pre></div>
</div>
<p>class attributes are accessed with <code class="docutils literal"><span class="pre">self</span></code> also.</p>
</div>
<div class="section" id="typical-methods">
<h2>Typical methods<a class="headerlink" href="#typical-methods" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Circle</span><span class="p">:</span>
<span class="n">color</span> <span class="o">=</span> <span class="s2">"red"</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">diameter</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">diameter</span> <span class="o">=</span> <span class="n">diameter</span>
<span class="k">def</span> <span class="nf">grow</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">factor</span><span class="o">=</span><span class="mi">2</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">diameter</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">diameter</span> <span class="o">*</span> <span class="n">factor</span>
</pre></div>
</div>
<p>Methods take some parameters, manipulate the attributes in <code class="docutils literal"><span class="pre">self</span></code>.</p>
<p>They may or may not return something useful.</p>
</div>
<div class="section" id="arity-gotcha">
<h2>Arity Gotcha<a class="headerlink" href="#arity-gotcha" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="o">...</span>
<span class="k">def</span> <span class="nf">grow</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">factor</span><span class="o">=</span><span class="mi">2</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">diameter</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">diameter</span> <span class="o">*</span> <span class="n">factor</span>
<span class="o">...</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">205</span><span class="p">]:</span> <span class="n">C</span> <span class="o">=</span> <span class="n">Circle</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">206</span><span class="p">]:</span> <span class="n">C</span><span class="o">.</span><span class="n">grow</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="n">grow</span><span class="p">()</span> <span class="n">takes</span> <span class="n">at</span> <span class="n">most</span> <span class="mi">2</span> <span class="n">arguments</span> <span class="p">(</span><span class="mi">3</span> <span class="n">given</span><span class="p">)</span>
</pre></div>
</div>
<p>Huh???? I only gave 2</p>
<p><code class="docutils literal"><span class="pre">self</span></code> is implicitly passed in for you by python.</p>
</div>
<div class="section" id="functions-methods-are-first-class">
<h2>Functions (methods) are First Class<a class="headerlink" href="#functions-methods-are-first-class" title="Permalink to this headline">¶</a></h2>
<p class="center">Note that in python, functions are first class objects, so a method <em>is</em> an attribute</p>
</div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Python Classes</a><ul>
<li><a class="reference internal" href="#basic-structure-of-a-class">Basic Structure of a class</a></li>
<li><a class="reference internal" href="#the-initializer">The Initializer</a></li>
<li><a class="reference internal" href="#self">Self</a></li>
<li><a class="reference internal" href="#class-attributes">Class Attributes</a></li>
<li><a class="reference internal" href="#typical-methods">Typical methods</a></li>
<li><a class="reference internal" href="#arity-gotcha">Arity Gotcha</a></li>
<li><a class="reference internal" href="#functions-methods-are-first-class">Functions (methods) are First Class</a></li>
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