在python中-集合用于测试对象是否在集合中的运算符 [英] In python - the operator which a set uses for test if an object is in the set

问题描述

如果我有对象列表，则可以使用__cmp__方法覆盖比较对象.这会影响==运算符以及item in list函数的工作方式.但是，它似乎并没有影响item in set函数-我想知道如何更改MyClass对象，以便可以覆盖集合比较项目的行为.

If I have a list of objects, I can use the __cmp__ method to override objects are compared. This affects how the == operator works, and the item in list function. However, it doesn't seem to affect the item in set function - I'm wondering how I can change the MyClass object so that I can override the behaviour how the set compares items.

例如，我想创建一个在底部的三个打印语句中返回True的对象.此刻，最后一个打印语句返回False.

For example, I would like to create an object that returns True in the three print statements at the bottom. At the moment, the last print statement returns False.

class MyClass(object):
    def __init__(self, s):
        self.s = s
    def __cmp__(self, other):
        return cmp(self.s, other.s)

instance1, instance2 = MyClass("a"), MyClass("a")

print instance2==instance1             # True
print instance2 in [instance1]         # True
print instance2 in set([instance1])    # False

推荐答案

set使用__hash__进行比较.改写它，你会很好的:

set uses __hash__ for comparison. Override that, and you'll be good:

class MyClass(object):
    def __init__(self, s):
        self.s = s
    def __cmp__(self, other):
        return cmp(self.s, other.s)
    def __hash__(self):
        return hash(self.s) # Use default hash for 'self.s'

instance1, instance2 = MyClass("a"), MyClass("a")
instance3 = MyClass("b")

print instance2==instance1             # True
print instance2 in [instance1]         # True
print instance2 in set([instance1])    # True

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