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POBruce Eckel's code snippet from Design Pattern: I'm confused on how it works
 

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  1. POBruce Eckel's code snippet from Design Pattern: I'm confused on how it works
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    <p>I've been reading <a href="http://www.mindview.net/Books/TIPython" rel="nofollow noreferrer">Thinking in python</a> by Bruce Eckel. Currently, I'm reading the <em>Pattern Concept</em> chapter. In this chapter, Eckel shows the different implementations of Singletons in python. But I have an unclear understanding of Alex Martelli's code of Singleton (utilizing inheritance, instead of privated nested class). </p> <p>This is my understanding of the code so far: </p> <ul> <li>All <em>Singleton</em> objects are subclasses of <em>Borg</em></li> <li><em>_shared_state</em> is initially an empty dictionary</li> <li><em>_shared_state</em> is a global variable; Any objects utilizing <em>Borg</em> will have the same <em>_shared_state</em> value</li> </ul> <p>My confusion so far:</p> <ul> <li>What's the purpose of this line: <code>self.__dict__ = self._shared_state</code> ; or the purpose of the dictionary</li> <li>How did all the objects of <em>Singletons</em> eventually have the same <em>val</em>, even though they are all different instances of the class. </li> <li>In overall, I don't know how <em>Borg</em> works</li> </ul> <p>Many Many thanks in advance!</p> <p>-Tri</p> <p>*Update: What is stored in <em>_shared_state</em> after each <em>Singleton</em> object creation?</p> <pre><code>#: Alex' Martelli's Singleton in Python class Borg: _shared_state = {} def __init__(self): self.__dict__ = self._shared_state class Singleton(Borg): def __init__(self, arg): Borg.__init__(self) self.val = arg def __str__(self): return self.val x = Singleton('sausage') print x y = Singleton('eggs') print y z = Singleton('spam') print z print x print y print ´x´ print ´y´ print ´z´ output = ''' sausage eggs spam spam spam &lt;__main__.Singleton instance at 0079EF2C&gt; &lt;__main__.Singleton instance at 0079E10C&gt; &lt;__main__.Singleton instance at 00798F9C&gt; ''' </code></pre>
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