设置“入"运算符:使用相等还是恒等?

Set quot;inquot; operator: uses equality or identity?(设置“入运算符:使用相等还是恒等?)

问题描述

class A(object):
    def __cmp__(self):
        print '__cmp__'
        return object.__cmp__(self)

    def __eq__(self, rhs):
        print '__eq__'
        return True
a1 = A()
a2 = A()
print a1 in set([a1])
print a1 in set([a2])

为什么第一行打印 True，而第二行打印 False?并且两者都没有进入运算符 eq?

Why does first line prints True, but second prints False? And neither enters operator eq?

我正在使用 Python 2.6

I am using Python 2.6

推荐答案

需要定义__hash__ 也是.例如

You need to define __hash__ too. For example

class A(object):
    def __hash__(self):
        print '__hash__'
        return 42

    def __cmp__(self, other):
        print '__cmp__'
        return object.__cmp__(self, other)

    def __eq__(self, rhs):
        print '__eq__'
        return True

a1 = A()
a2 = A()
print a1 in set([a1])
print a1 in set([a2])

会按预期工作.

作为一般规则，只要您实施 __cmp__ 你应该实现一个 __hash__ 使得对于所有 x 和 y 使得 x == y, x.__hash__() == y.__hash__().

As a general rule, any time you implement __cmp__ you should implement a __hash__ such that for all x and y such that x == y, x.__hash__() == y.__hash__().

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