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href="/python/call-python-from-c.html" class="sidebar-link">Call Python from C#</a></li><li><a href="/python/ctypes.html" class="sidebar-link">ctypes</a></li><li><a href="/python/writing-extensions.html" class="sidebar-link">Writing extensions</a></li><li><a href="/python/python-lex-yacc.html" class="sidebar-link">Python Lex-Yacc</a></li><li><a href="/python/unit-testing.html" class="sidebar-link">Unit Testing</a></li><li><a href="/python/py-test.html" class="sidebar-link">py.test</a></li><li><a href="/python/profiling.html" class="sidebar-link">Profiling</a></li><li><a href="/python/python-speed-of-program.html" class="sidebar-link">Python speed of program</a></li><li><a href="/python/performance-optimization.html" class="sidebar-link">Performance optimization</a></li><li><a href="/python/security-and-cryptography.html" class="sidebar-link">Security and Cryptography</a></li><li><a href="/python/secure-shell-connection-in-python.html" class="sidebar-link">Secure Shell Connection in Python</a></li><li><a href="/python/python-anti-patterns.html" class="sidebar-link">Python Anti-Patterns</a></li><li><a href="/python/common-pitfalls.html" class="sidebar-link">Common Pitfalls</a></li><li><a href="/python/hidden-features.html" class="sidebar-link">Hidden Features</a></li><li><a href="/python/date-formatting.html" class="sidebar-link">Date Formatting</a></li><li><a href="/python/ijson.html" class="sidebar-link">ijson</a></li><li><a href="/python/django.html" class="sidebar-link">Django</a></li><li><a href="/python/code-blocks-execution-frames-and-namespaces.html" class="sidebar-link">Code blocks, execution frames, and namespaces</a></li><li><a href="/python/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="descriptor"><a href="#descriptor" class="header-anchor">#</a> Descriptor</h1> <h2 id="simple-descriptor"><a href="#simple-descriptor" class="header-anchor">#</a> Simple descriptor</h2> <p>There are two different types of descriptors. Data descriptors are defined as objects that define both a <code>__get__()</code> and a <code>__set__()</code> method, whereas non-data descriptors only define a <code>__get__()</code> method. This distinction is important when considering overrides and the namespace of an instance's dictionary. If a data descriptor and an entry in an instance's dictionary share the same name, the data descriptor will take precedence. However, if instead a non-data descriptor and an entry in an instance's dictionary share the same name, the instance dictionary's entry will take precedence.</p> <p>To make a read-only data descriptor, define both <strong>get</strong>() and <strong>set</strong>() with the <strong>set</strong>() raising an AttributeError when called. Defining the <strong>set</strong>() method with an exception raising placeholder is enough to make it a data descriptor.</p> <div class="language-py extra-class"><pre class="language-py"><code>descr<span class="token punctuation">.</span>__get__<span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> <span class="token builtin">type</span><span class="token operator">=</span><span class="token boolean">None</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">></span> value
descr<span class="token punctuation">.</span>__set__<span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> value<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">></span> <span class="token boolean">None</span>
descr<span class="token punctuation">.</span>__delete__<span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">></span> <span class="token boolean">None</span>
</code></pre></div><p>An implemented example:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">DescPrinter</span><span class="token punctuation">(</span><span class="token builtin">object</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token triple-quoted-string string">"""A data descriptor that logs activity."""</span>
_val <span class="token operator">=</span> <span class="token number">7</span>
<span class="token keyword">def</span> <span class="token function">__get__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> objtype<span class="token operator">=</span><span class="token boolean">None</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">print</span><span class="token punctuation">(</span><span class="token string">'Getting ...'</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> self<span class="token punctuation">.</span>_val
<span class="token keyword">def</span> <span class="token function">__set__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> val<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">print</span><span class="token punctuation">(</span><span class="token string">'Setting'</span><span class="token punctuation">,</span> val<span class="token punctuation">)</span>
self<span class="token punctuation">.</span>_val <span class="token operator">=</span> val
<span class="token keyword">def</span> <span class="token function">__delete__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> obj<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">print</span><span class="token punctuation">(</span><span class="token string">'Deleting ...'</span><span class="token punctuation">)</span>
<span class="token keyword">del</span> self<span class="token punctuation">.</span>_val
<span class="token keyword">class</span> <span class="token class-name">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
x <span class="token operator">=</span> DescPrinter<span class="token punctuation">(</span><span class="token punctuation">)</span>
i <span class="token operator">=</span> Foo<span class="token punctuation">(</span><span class="token punctuation">)</span>
i<span class="token punctuation">.</span>x
<span class="token comment"># Getting ...</span>
<span class="token comment"># 7</span>
i<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">100</span>
<span class="token comment"># Setting 100</span>
i<span class="token punctuation">.</span>x
<span class="token comment"># Getting ...</span>
<span class="token comment"># 100</span>
<span class="token keyword">del</span> i<span class="token punctuation">.</span>x
<span class="token comment"># Deleting ...</span>
i<span class="token punctuation">.</span>x
<span class="token comment"># Getting ...</span>
<span class="token comment"># 7</span>
</code></pre></div><h2 id="two-way-conversions"><a href="#two-way-conversions" class="header-anchor">#</a> Two-way conversions</h2> <p>Descriptor objects can allow related object attributes to react to changes automatically.</p> <p>Suppose we want to model an oscillator with a given frequency (in Hertz) and period (in seconds). When we update the frequency we want the period to update, and when we update the period we want the frequency to update:</p> <div class="language- extra-class"><pre class="language-text"><code>
>>> oscillator = Oscillator(freq=100.0) # Set frequency to 100.0 Hz
>>> oscillator.period # Period is 1 / frequency, i.e. 0.01 seconds
0.01
>>> oscillator.period = 0.02 # Set period to 0.02 seconds
>>> oscillator.freq # The frequency is automatically adjusted
50.0
>>> oscillator.freq = 200.0 # Set the frequency to 200.0 Hz
>>> oscillator.period # The period is automatically adjusted
0.005
</code></pre></div><p>We pick one of the values (frequency, in Hertz) as the "anchor," i.e. the one that can be set with no conversion, and write a descriptor class for it:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">Hertz</span><span class="token punctuation">(</span><span class="token builtin">object</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">__get__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> instance<span class="token punctuation">,</span> owner<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span> self<span class="token punctuation">.</span>value
<span class="token keyword">def</span> <span class="token function">__set__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> instance<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">:</span>
self<span class="token punctuation">.</span>value <span class="token operator">=</span> <span class="token builtin">float</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
</code></pre></div><p>The "other" value (period, in seconds) is defined in terms of the anchor. We write a descriptor class that does our conversions:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">Second</span><span class="token punctuation">(</span><span class="token builtin">object</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">__get__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> instance<span class="token punctuation">,</span> owner<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token comment"># When reading period, convert from frequency</span>
<span class="token keyword">return</span> <span class="token number">1</span> <span class="token operator">/</span> instance<span class="token punctuation">.</span>freq
<span class="token keyword">def</span> <span class="token function">__set__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> instance<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token comment"># When setting period, update the frequency</span>
instance<span class="token punctuation">.</span>freq <span class="token operator">=</span> <span class="token number">1</span> <span class="token operator">/</span> <span class="token builtin">float</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
</code></pre></div><p>Now we can write the Oscillator class:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">Oscillator</span><span class="token punctuation">(</span><span class="token builtin">object</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
period <span class="token operator">=</span> Second<span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment"># Set the other value as a class attribute</span>
<span class="token keyword">def</span> <span class="token function">__init__</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> freq<span class="token punctuation">)</span><span class="token punctuation">:</span>
self<span class="token punctuation">.</span>freq <span class="token operator">=</span> Hertz<span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment"># Set the anchor value as an instance attribute</span>
self<span class="token punctuation">.</span>freq <span class="token operator">=</span> freq <span class="token comment"># Assign the passed value - self.period will be adjusted</span>
</code></pre></div></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/python/descriptor.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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