(a > b) - (a < b)，相當於cmp(a,b)
Ordering Comparisons
Python 3.0 has simplified the rules for ordering comparisons:
- The ordering comparison operators (<, <=, >=, >) raise a TypeError exception when the operands don’t have a meaningful natural ordering. Thus, expressions like 1 < '', 0 > None or len <= len are no longer valid, and e.g. None < None raises TypeError instead of returning False. A corollary is that sorting a heterogeneous list no longer makes sense – all the elements must be comparable to each other. Note that this does not apply to the == and != operators: objects of different incomparable types always compare unequal to each other.
- builtin.sorted() and list.sort() no longer accept the cmp argument providing a comparison function. Use the key argument instead. N.B. the key and reverse arguments are now “keyword-only”.
- 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).)
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