Why is the cmp parameter removed from sort/sorted in Python3.0?

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For two objects a and b, __cmp__ requires that one of a < b, a == b, and a > b is true. But that might not be the case: consider sets, where it’s very common that none of those are true, e.g. {1, 2, 3} vs {4, 5, 6}.

So __lt__ and friends were introduced. But that left Python with two separate ordering mechanisms, which is kind of ridiculous, so the less flexible one was removed in Python 3.

You don’t actually have to implement all six comparison methods. You can use the @total_ordering decorator and only implement __lt__ and __eq__.

edit: Also note that, in the case of sorting, key functions can be more efficient than cmp: in the example you gave, Python may have to call your Python comparison function O(n²) times. But a key function only needs to be called O(n) times, and if the return value is then a builtin type (as it very often is), the O(n²) pairwise comparisons go through C.

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