设置为"in"接线员:使用平等还是同一性? [英] Set "in" operator: uses equality or identity?

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?而且都没有输入运算符 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

您需要定义 __hash__ .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])

将按预期工作.

通常，只要您实现 __cmp__ 应该实现__hash__，以便对所有x和y都实现，使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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