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Equals in python

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  • == 和 is 的区别
  • 怎么使得 Object 使用 == 比较相等
  • Set 集合中判断相等

== Vs is

== 用来判断值相等，is 用来判断引用相等

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a = [1, 2, 3]
b = a

a == b # true
a is b # true

b = a[:]
a == b # true
a is b # false

怎么使得 Object 使用 == 比较相等

你需要重写 class 的 eq 方法

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class Person:
    def __init__(self, id, name):
        self.id = id
        self.name = name

    def __eq__(self, other):
        if not isinstance(other, Person):
            # don't attempt to compare against unrelated types
            return NotImplemented
        return self.id == other.id

p1 = Person(1, 'a')
p2 = Person(2, 'b')
p3 = Person(1, 'c')

p1 == p2 # false
p1 == p3 # true

Note: 重写 eq 将会使对象变为 unhashable，在存到 Set， Map 等集合中会有影响，你可以重写 hash 来定制

Set 集合中判断相等

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class Person:
    def __init__(self, id, name):
        self.id = id
        self.name = name

    def __eq__(self, other):
        if not isinstance(other, Person):
            # don't attempt to compare against unrelated types
            return NotImplemented
        return self.id == other.id

    def __hash__(self):
        # necessary for instances to behave sanely in dicts and sets.
        return hash(self.id)

set([p1, p2, p3]) # only  p1, p2 will be stored

set 中并没有使用新的 object 代替旧的的方法，所以如果想要更新的话只能 remove + add 了