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    <p>This function already exists in the <a href="http://docs.python.org/3/library/itertools.html#recipes" rel="nofollow"><code>itertools</code> recipes</a>, as <code>unique_everseen</code>. You can copy and paste it from there, or read it to see how it works, or install the third-party package <code>more-itertools</code> and use it from there.</p> <p>Here's a simplified version of the code:</p> <pre><code>def unique_everseen(iterable): seen = set() for element in iterable: if element not in seen: seen.add(element) yield element </code></pre> <hr> <p>The version in the recipes allows for a <code>key</code> function, which you don't need, and it has two optimizations. But first understand the simple version:</p> <p><code>seen</code> is a set of all values seen so far. For each value, we check whether it's in <code>seen</code>. If so, we skip it. Otherwise, we add it to the set and <code>yield</code> it. So, we <code>yield</code> each element only the first time it's seen.</p> <hr> <p>The first optimization in the recipe version is simple: looking up the <code>seen.add</code> method isn't quite free, so we do it once instead of N times, by doing <code>seen_add = seen.add</code>. This makes a sizable difference when benchmarking trivial cases, like a list of small integers; it may not make much difference in real use cases with values that are more expensive to hash.</p> <p>The second optimization is to use <code>ifilterfalse</code> instead of an <code>if</code> to skip over the elements that have already been seen. Basically this means that if you have N elements and M unique elements, you only do M iterations in Python and N in the optimized C code inside <code>ifilterfalse</code>, instead of doing N in Python. Since iterating in C is much faster, this is worth it unless almost all of your elements are unique.</p> <hr> <p>To make it work with a <code>key</code> function, all you have to do is keep a set of <code>key(element)</code> values seen so far, instead of <code>element</code> values seen so far. This makes the <code>ifilterfalse</code> optimization a little harder to do and much less effective, so it isn't done.</p> <hr> <p>If you're only dealing with sequences, not arbitrary iterables, and you can count on Python 2.7+, there's another way to do this which is almost as efficient, and even simpler:</p> <pre><code>def unique(a): return OrderedDict.fromkeys(a).keys() </code></pre>
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