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  1. PO
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    <p>A basic approach will use builtin data types. If I get your drift, the <code>Letter</code> object should be created by a factory with a <code>dict</code> cache to keep previously generated <code>Letter</code> objects. The factory will create only one <code>Letter</code> object for each key.</p> <p>A <code>Number</code> object can be a sub-class of <code>list</code> that will hold the <code>Letter</code> objects, so that <code>append()</code> can be used to add a child. A <code>list</code> is easy to navigate.</p> <p>A crude outline of a caching factory:</p> <pre><code>&gt;&gt;&gt; class Letters(object): ... def __init__(self): ... self.cache = {} ... def create(self, v): ... l = self.cache.get(v, None) ... if l: ... return l ... l = self.cache[v] = Letter(v) ... return l &gt;&gt;&gt; factory=Letters() &gt;&gt;&gt; factory.cache {} &gt;&gt;&gt; factory.create('a') &lt;__main__.Letter object at 0x00EF2950&gt; &gt;&gt;&gt; factory.create('a') &lt;__main__.Letter object at 0x00EF2950&gt; &gt;&gt;&gt; </code></pre> <p>To fulfill requirement 6 (constructor), here is a more contrived example, using <code>__new__</code>, of a caching constructor. This is similar to <a href="http://code.activestate.com/recipes/413717/" rel="nofollow noreferrer">Recipe 413717: Caching object creation </a>.</p> <pre><code>class Letter(object): cache = {} def __new__(cls, v): o = cls.cache.get(v, None) if o: return o else: o = cls.cache[v] = object.__new__(cls) return o def __init__(self, v): self.v = v self.refcount = 0 def addAsChild(self, chain): if self.refcount &gt; 0: return False self.refcount += 1 chain.append(self) return True </code></pre> <p>Testing the cache functionality</p> <pre><code>&gt;&gt;&gt; l1 = Letter('a') &gt;&gt;&gt; l2 = Letter('a') &gt;&gt;&gt; l1 is l2 True &gt;&gt;&gt; </code></pre> <p>For enforcing a single parent, you'll need a method on <code>Letter</code> objects (not <code>Number</code>) - with a reference counter. When called to perform the addition it will refuse addition if the counter is greater than zero.</p> <pre><code>l1.addAsChild(num4) </code></pre>
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    1. POHierarchy / Flyweight / Instancing Problem in Python
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    1. USgimel
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    1. COThis is a good start, but it doesn't satisfy 3 or 6.
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      1. USDan
    2. COfor 6, you can easily hide a factory behind any callable. Are you sure about the constructor requirement?
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    3. COCool, that's more like how I've ended up implementing it, so sounds like I'm on the right lines. One thing I found though - the __init__ is always executed after the __new__ (regardless if it returns an existing object), which is fine in this instance, but what if the __init__ does a lot more?
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