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<div class="section" id="properties">
<span id="id1"></span><h1>Properties<a class="headerlink" href="#properties" title="Permalink to this headline"></a></h1>
<p><a class="reference external" href="https://en.wikipedia.org/wiki/Property_%28programming%29#Python">https://en.wikipedia.org/wiki/Property_%28programming%29#Python</a></p>
<div class="section" id="attributes-are-clear-and-concise">
<h2>Attributes are clear and concise<a class="headerlink" href="#attributes-are-clear-and-concise" title="Permalink to this headline"></a></h2>
<div class="docutils container">
<p>One of the strengths of Python is lack of clutter.</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [5]: </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="go"> def __init__(self):</span>
<span class="go"> self.x = 5</span>
<span class="gp">In [6]: </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="gp">In [7]: </span><span class="n">c</span><span class="o">.</span><span class="n">x</span>
<span class="gh">Out[7]: </span><span class="go">5</span>
<span class="gp">In [8]: </span><span class="n">c</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">8</span>
<span class="gp">In [9]: </span><span class="n">c</span><span class="o">.</span><span class="n">x</span>
<span class="gh">Out[9]: </span><span class="go">8</span>
</pre></div>
</div>
</div>
<p>And we want to maintain this clarity as we develop our programs.</p>
</div>
<div class="section" id="getter-and-setters">
<h2>Getter and Setters<a class="headerlink" href="#getter-and-setters" title="Permalink to this headline"></a></h2>
<p>But what if you need to add behavior later?</p>
<ul class="simple">
<li><p>do some calculation</p></li>
<li><p>check data validity</p></li>
<li><p>keep things in sync</p></li>
</ul>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [5]: </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="gp"> ...: </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="gp"> ...: </span> <span class="bp">self</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="gp"> ...: </span> <span class="k">def</span> <span class="nf">get_x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">x</span>
<span class="gp"> ...: </span> <span class="k">def</span> <span class="nf">set_x</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="gp"> ...: </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="gp"> ...:</span>
<span class="gp">In [6]: </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="gp">In [7]: </span><span class="n">c</span><span class="o">.</span><span class="n">get_x</span><span class="p">()</span>
<span class="gh">Out[7]: </span><span class="go">5</span>
<span class="gp">In [8]: </span><span class="n">c</span><span class="o">.</span><span class="n">set_x</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span>
<span class="gp">In [9]: </span><span class="n">c</span><span class="o">.</span><span class="n">get_x</span><span class="p">()</span>
<span class="gh">Out[9]: </span><span class="go">8</span>
</pre></div>
</div>
<p>This is verbose – <a class="reference external" href="http://dirtsimple.org/2004/12/python-is-not-java.html">Java</a>?</p>
</div>
<div class="section" id="id2">
<h2>Properties<a class="headerlink" href="#id2" title="Permalink to this headline"></a></h2>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [27]: </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">_x</span> <span class="o">=</span> <span class="kc">None</span>
<span class="gp"> ....: </span> <span class="nd">@property</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="gp"> ....: </span> <span class="nd">@x</span><span class="o">.</span><span class="n">setter</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="n">value</span>
<span class="gp">In [28]: </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="gp">In [30]: </span><span class="n">c</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="gp">In [31]: </span><span class="nb">print</span><span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">x</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
<p>Now the interface is like simple attribute access!</p>
</div>
<div class="section" id="decorators">
<h2>Decorators<a class="headerlink" href="#decorators" title="Permalink to this headline"></a></h2>
<p>What’s up with the “@” symbols?</p>
<p>Those are “decorations”. It is a syntax for wrapping functions up with something special.</p>
<p>We will cover decorators in detail in another part of the program, but for now just copy the syntax.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nd">@property</span>
<span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
</pre></div>
</div>
<p>means: make a property called x with this as the “getter”.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nd">@x</span><span class="o">.</span><span class="n">setter</span>
<span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
</pre></div>
</div>
<p>means: make the “setter” of the ‘x’ property this new function.</p>
</div>
<div class="section" id="read-only-attributes">
<h2>Read Only Attributes<a class="headerlink" href="#read-only-attributes" title="Permalink to this headline"></a></h2>
<p>You do not need to define a setter. If you don’t, you get a “read only” attribute:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="k">class</span> <span class="nc">D</span><span class="p">():</span>
<span class="gp"> ....: </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="o">=</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> ....: </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="gp"> ....: </span> <span class="nd">@property</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="sd">"""I am read only"""</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="gp"> ....:</span>
<span class="gp">In [12]: </span><span class="n">d</span> <span class="o">=</span> <span class="n">D</span><span class="p">()</span>
<span class="gp">In [13]: </span><span class="n">d</span><span class="o">.</span><span class="n">x</span>
<span class="gh">Out[13]: </span><span class="go">5</span>
<span class="gp">In [14]: </span><span class="n">d</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">6</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">AttributeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-14-c83386d97be3></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">d</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">6</span>
<span class="ne">AttributeError</span>: can't set attribute
</pre></div>
</div>
</div>
<div class="section" id="deleters">
<h2>Deleters<a class="headerlink" href="#deleters" title="Permalink to this headline"></a></h2>
<p>If you want to do something special when a property is deleted, you can define a deleter as well:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="k">class</span> <span class="nc">D</span><span class="p">():</span>
<span class="gp"> ....: </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="o">=</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="gp"> ....: </span> <span class="nd">@property</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
<span class="gp"> ....: </span> <span class="nd">@x</span><span class="o">.</span><span class="n">deleter</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">_x</span>
</pre></div>
</div>
<p>If you leave this out, the property can’t be deleted, which is usually
what you want.</p>
<p>Play around with some properties code:</p>
<p><a class="reference download internal" download="" href="../_downloads/d3ffb34a48b3ade73addcdb3de293a0d/properties_example.py"><code class="xref download docutils literal notranslate"><span class="pre">properties_example.py</span></code></a></p>
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