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    <p>If you want to know if an object is <strong>sortable</strong>, you must check if it implements the necessary methods of comparison.</p> <p>In <strong>Python 2.X</strong> there were two different ways to implement those methods:</p> <ol> <li><p>cmp method (equivalent of compareTo in Java per example)</p> <p><code>__cmp__(self, other)</code>: returns >0, 0 or &lt;0 wether self is more, equal or less than <em>other</em></p></li> <li><p>rich comparison methods</p> <p><code>__lt__, __gt__, __eq__, __le__, __ge__, __ne__</code></p> <p>The sort() functions call this method to make the necessary comparisons between instances (actually sort only needs the <code>__lt__</code> or <code>__gt__</code> methods but it's recommended to implement all of them)</p></li> </ol> <p>In <strong>Python 3.X</strong> the <code>__cmp__</code> was removed in favor of the rich comparison methods as having more than one way to do the same thing is really against Python's "laws".</p> <p>So, you basically need a function that check if these methods are implemented by a class:</p> <pre><code># Python 2.X def is_sortable(obj): return hasattr(obj, "__cmp__") or \ hasattr(obj, "__lt__") or \ hasattr(obj, "__gt__") # Python 3.X def is_sortable(obj): cls = obj.__class__ return cls.__lt__ != object.__lt__ or \ cls.__gt__ != object.__gt__ </code></pre> <p>Different functions are needed for Python 2 and 3 because a lot of other things also change about unbound methods, method-wrappers and other internal things in Python 3.</p> <p>Read this links you want better understanding of the sortable objects in Python:</p> <ul> <li><p><a href="http://python3porting.com/problems.html#unorderable-types-cmp-and-cmp" rel="nofollow">http://python3porting.com/problems.html#unorderable-types-cmp-and-cmp</a></p></li> <li><p><a href="http://docs.python.org/2/howto/sorting.html#the-old-way-using-the-cmp-parameter" rel="nofollow">http://docs.python.org/2/howto/sorting.html#the-old-way-using-the-cmp-parameter</a></p></li> </ul> <p>PS: this was a complete re-edit of my first answer, but it was needed as I investigated the problem better and had a cleaner idea about it :)</p>
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    1. POCheck if an object is order-able in python?
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    1. COThis seems like an appropriate use case for `is not` instead of `!=`, otherwise, good answer. Thank you.
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    2. CO@sortfiend I completely reedited my answer. I may be breaking some rules at stackoverflow or something :p but I investigated be problem and got a much cleared idea about the subject. I think the answer is much cleaner now.
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    3. COoh and one more thing: the function that I suggested in the first answer wasn't really working at all. These ones seem to cover all cases now.
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