问题:自定义类型的对象作为字典键

如何将自定义类型的对象用作Python字典中的键(我不希望“对象id”用作键),例如

class MyThing:
    def __init__(self,name,location,length):
            self.name = name
            self.location = location
            self.length = length

如果名称和位置相同,我想将MyThing用作相同的键。从C#/ Java开始,我习惯于重写并提供一个equals和hashcode方法,并保证不会突变该hashcode依赖的任何内容。

我必须在Python中做什么才能做到这一点?我应该吗?

(在一个简单的例子中,就像这里一样,也许最好将一个(名称,位置)元组放置为键-但考虑到我希望键成为一个对象)

What must I do to use my objects of a custom type as keys in a Python dictionary (where I don’t want the “object id” to act as the key) , e.g.

class MyThing:
    def __init__(self,name,location,length):
            self.name = name
            self.location = location
            self.length = length

I’d want to use MyThing’s as keys that are considered the same if name and location are the same. From C#/Java I’m used to having to override and provide an equals and hashcode method, and promise not to mutate anything the hashcode depends on.

What must I do in Python to accomplish this ? Should I even ?

(In a simple case, like here, perhaps it’d be better to just place a (name,location) tuple as key – but consider I’d want the key to be an object)


回答 0

您需要添加2种方法,注意__hash____eq__

class MyThing:
    def __init__(self,name,location,length):
        self.name = name
        self.location = location
        self.length = length

    def __hash__(self):
        return hash((self.name, self.location))

    def __eq__(self, other):
        return (self.name, self.location) == (other.name, other.location)

    def __ne__(self, other):
        # Not strictly necessary, but to avoid having both x==y and x!=y
        # True at the same time
        return not(self == other)

Python dict文档对关键对象定义了这些要求,即它们必须是可哈希的

You need to add 2 methods, note __hash__ and __eq__:

class MyThing:
    def __init__(self,name,location,length):
        self.name = name
        self.location = location
        self.length = length

    def __hash__(self):
        return hash((self.name, self.location))

    def __eq__(self, other):
        return (self.name, self.location) == (other.name, other.location)

    def __ne__(self, other):
        # Not strictly necessary, but to avoid having both x==y and x!=y
        # True at the same time
        return not(self == other)

The Python dict documentation defines these requirements on key objects, i.e. they must be hashable.


回答 1

使用python 2.6或更高版本的替代collections.namedtuple()方法-它可以节省编写任何特殊方法的时间:

from collections import namedtuple
MyThingBase = namedtuple("MyThingBase", ["name", "location"])
class MyThing(MyThingBase):
    def __new__(cls, name, location, length):
        obj = MyThingBase.__new__(cls, name, location)
        obj.length = length
        return obj

a = MyThing("a", "here", 10)
b = MyThing("a", "here", 20)
c = MyThing("c", "there", 10)
a == b
# True
hash(a) == hash(b)
# True
a == c
# False

An alternative in Python 2.6 or above is to use collections.namedtuple() — it saves you writing any special methods:

from collections import namedtuple
MyThingBase = namedtuple("MyThingBase", ["name", "location"])
class MyThing(MyThingBase):
    def __new__(cls, name, location, length):
        obj = MyThingBase.__new__(cls, name, location)
        obj.length = length
        return obj

a = MyThing("a", "here", 10)
b = MyThing("a", "here", 20)
c = MyThing("c", "there", 10)
a == b
# True
hash(a) == hash(b)
# True
a == c
# False

回答 2

__hash__如果需要特殊的哈希语义,则可以覆盖,__cmp__或者__eq__为了使您的类可用作键。比较相等的对象需要具有相同的哈希值。

Python期望__hash__返回一个整数,Banana()不建议返回:)

如您所述,用户定义的类在__hash__默认情况下会调用id(self)

文档中还有一些其他技巧:

__hash__() 从父类继承方法但更改的含义__cmp__()__eq__() 使得返回的哈希值不再合适的类(例如,通过切换到基于值的相等性概念,而不是基于默认的基于身份的相等性),这些类可以显式地标记为可以通过__hash__ = None 在类定义中进行设置来取消哈希。这样做意味着,在程序尝试检索其哈希值时,该类的实例不仅会引发适当的TypeError,而且在检查时它们也将被正确标识为不可哈希 isinstance(obj, collections.Hashable) (与定义自己__hash__()明确地引发TypeError的类不同 )。

You override __hash__ if you want special hash-semantics, and __cmp__ or __eq__ in order to make your class usable as a key. Objects who compare equal need to have the same hash value.

Python expects __hash__ to return an integer, returning Banana() is not recommended :)

User defined classes have __hash__ by default that calls id(self), as you noted.

There is some extra tips from the documentation.:

Classes which inherit a __hash__() method from a parent class but change the meaning of __cmp__() or __eq__() such that the hash value returned is no longer appropriate (e.g. by switching to a value-based concept of equality instead of the default identity based equality) can explicitly flag themselves as being unhashable by setting __hash__ = None in the class definition. Doing so means that not only will instances of the class raise an appropriate TypeError when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable) (unlike classes which define their own __hash__() to explicitly raise TypeError).


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