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    <p>To support arbitrary attribute assignment, an object needs a <code>__dict__</code>: a dict associated with the object, where arbitrary attributes can be stored. Otherwise, there's nowhere to <em>put</em> new attributes.</p> <p>An instance of <code>object</code> does <strong>not</strong> carry around a <code>__dict__</code> -- if it did, before the horrible circular dependence problem (since <code>dict</code>, like most everything else, inherits from <code>object</code>;-), this would saddle <em>every</em> object in Python with a dict, which would mean an overhead of <em>many</em> bytes per object that currently doesn't have or need a dict (essentially, all objects that don't have arbitrarily assignable attributes don't have or need a dict).</p> <p>For example, using the excellent <code>pympler</code> project (you can get it via svn from <a href="http://code.google.com/p/pympler/source/checkout" rel="noreferrer">here</a>), we can do some measurements...:</p> <pre><code>&gt;&gt;&gt; from pympler import asizeof &gt;&gt;&gt; asizeof.asizeof({}) 144 &gt;&gt;&gt; asizeof.asizeof(23) 16 </code></pre> <p>You wouldn't want every <code>int</code> to take up 144 bytes instead of just 16, right?-)</p> <p>Now, when you make a class (inheriting from whatever), things change...:</p> <pre><code>&gt;&gt;&gt; class dint(int): pass ... &gt;&gt;&gt; asizeof.asizeof(dint(23)) 184 </code></pre> <p>...the <code>__dict__</code> <em>is</em> now added (plus, a little more overhead) -- so a <code>dint</code> instance can have arbitrary attributes, but you pay quite a space cost for that flexibility.</p> <p>So what if you wanted <code>int</code>s with just <em>one</em> extra attribute <code>foobar</code>...? It's a rare need, but Python does offer a special mechanism for the purpose...</p> <pre><code>&gt;&gt;&gt; class fint(int): ... __slots__ = 'foobar', ... def __init__(self, x): self.foobar=x+100 ... &gt;&gt;&gt; asizeof.asizeof(fint(23)) 80 </code></pre> <p>...not <em>quite</em> as tiny as an <code>int</code>, mind you! (or even the two <code>int</code>s, one the <code>self</code> and one the <code>self.foobar</code> -- the second one can be reassigned), but surely much better than a <code>dint</code>.</p> <p>When the class has the <code>__slots__</code> special attribute (a sequence of strings), then the <code>class</code> statement (more precisely, the default metaclass, <code>type</code>) does <strong>not</strong> equip every instance of that class with a <code>__dict__</code> (and therefore the ability to have arbitrary attributes), just a finite, rigid set of "slots" (basically places which can each hold one reference to some object) with the given names.</p> <p>In exchange for the lost flexibility, you gain a lot of bytes per instance (probably meaningful only if you have zillions of instances gallivanting around, but, there <em>are</em> use cases for that).</p>
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