3开始没这个函数了，官方文档是这么写的
The cmp() function should be treated as gone, and the __cmp__() special method is no longer supported. Use __lt__() for sorting, __eq__() with __hash__(), and other rich comparisons as needed. (If you really need the cmp() functionality, you could use the expression (a > b) - (a < b) as the equivalent for cmp(a, b).)
大意就是cmp()函数已经“离开”了，如果你真的需要cmp()函数，你可以用表达式(a > b) - (a < b)代替cmp(a,b)
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