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无需手动缓存长度 - 雪地
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<h1>无需手动缓存长度</h1>
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<span class="date">2016/7/27 17:13 下午</span>
<span>posted in </span>
<span class="posted-in"><a href='Python.html'>Python</a></span>
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<p>今天在阅读《机器学习实战》的时候，看到了这样一句描述</p>
<p><img src="media/14696108056583/IMG_0629.png" alt="IMG_0629"/></p>
<p>其中数据集是一个List。看到这里说到为了提高代码效率，特地开了一个变量来保存其长度。</p>
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<p>我回忆起以前调试bug的时候，总是能看到List对象里面有一个函数<code>__len__</code>，想必Python对于这种常用函数会有所优化，对于这种集合类型的对象，如果对其长度进行缓存，这样多次调用<code>len</code>函数，就不会重复进行遍历计算了。这么简单的优化，Python必然是有做的。</p>
<p>于是去Google，原本准备搜len函数的具体实现代码，结果直接找到stackoverflow有<a href="http://stackoverflow.com/questions/29057153/does-the-len-built-in-function-iterates-through-the-collection-to-calculate-it">相关问题</a>，明确的验证了我的想法。</p>
<p><img src="media/14696108056583/%E5%B1%8F%E5%B9%95%E5%BF%AB%E7%85%A7%202016-07-27%2018.44.21.png" alt="屏幕快照 2016-07-27 18.44.21"/></p>
<p>对于这种类型，Python都会对其长度进行缓存。也就是说，对于len函数，只在第一次调用的时候复杂度为<code>O(N)</code>，后续调用的复杂度都是常数级的。所以，我们平时写代码的时候，可以不需要多此一举的再开一个单独的变量来缓存了。也就是说，以下代码在没有特殊需求的情况下，是多此一举的：</p>
<pre><code class="language-python">l = len(some_list)
</code></pre>
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