【转】实习小记－python中可哈希对象是个啥？what is hashable object in python?

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废话不多说直接祭上python3.3x的文档：（原文链接）

object.__hash__(self)
Called by built-in function hash() and for operations on members of hashed collections including set, frozenset, and dict. __hash__() should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to somehow mix together (e.g. using exclusive or) the hash values for the components of the object that also play a part in comparison of objects.

可哈希对象是对象拥有__hash__(self)内置函数的对象。对于可哈希的对象执行这个函数将会返回一个整数。可哈希对象判断相等的唯一条件就是两者的哈希值相等。如果我们的对象有多个属性值，我们会使用一种方法（比方说逻辑运算异或）来将其属性值结合在一起做比较。（如果不对，麻烦一定告诉我，谢谢！）

If a class does not define an __eq__() method it should not define a __hash__() operation either; if it defines __eq__() but not __hash__(), its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an __eq__() method, it should not implement __hash__(), since the implementation of hashable collections requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket).

如果一个类型定义没有定义__eq__()函数，那么它也不应该定义__hash__()；因为如果它定义了__eq__而没有定义__hash__，那么它的实例在某个可哈希集合中将会无效（比方说在字典／集合这类类型中）。如果一个类型定义了一个可变对象而且定义了__eq__方法，那么它不应该去定义__hash__方法，因为在哈希集合中要求其中元素的哈希值是不变的（如果某个对象实例的哈希值发生了改变，那么他将会到错误的哈希表中。。）。

User-defined classes have __eq__() and __hash__() methods by default; with them, all objects compare unequal (except with themselves) and x.__hash__() returns an appropriate value such that x == y implies both that x is y and hash(x) == hash(y).

用户定义的类中都有默认的__eq__和__hash__方法；有了它，所有的对象实例都是不等的（除非是自己和自己比较），在做 x == y 比较时是和这个等价的 hash(x) == hash(y)。

A class that overrides __eq__() and does not define __hash__() will have its __hash__() implicitly set to None. When the __hash__() method of a class is None, instances of the class will raise an appropriate TypeError when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable).

这句话的意思是，如果我们定义了一个类型，该类型只定义了__eq__而没有定义__hash__的话，那么他的__hash__()会隐式的设置为None，如果某个情况下需要这个类型的哈希值，那么程序将会报错。具体请看下面代码：

>>> class A:
... def __eq__(self, other):
... return True
...
>>> a = A()
>>> import collections
>>> isinstance(a, collections.Hashable) # 发现它不是可哈希对象
False
>>> a.__hash__
>>> a.__hash__()
Traceback (most recent call last):
File "<ipython-input-20-0fddd0562e01>", line 1, in <module>
a.__hash__()
TypeError: 'NoneType' object is not callable

然后往下看文档

If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__.

如果某个重写了__eq__方法的对象需要保留__hash__属性，那么我们需要在类型设置中添加该语句__hash__ = <ParentClass>.__hash__

请看代码

>>> class A:
... __hash__ = object.__hash__
... def __eq__(self, other):
... return True
...
>>> a = A()
>>> a.__hash__
<method-wrapper '__hash__' of A object at 0x7f21029cfa10>
>>> set([a,2,3])
{<__main__.A object at 0x7f21029cfa10>, 2, 3}

If a class that does not override __eq__() wishes to suppress hash support, it should include __hash__ = None in the class definition. A class which defines its own __hash__() that explicitly raises a TypeError would be incorrectly identified as hashable by an isinstance(obj, collections.Hashable) call.

如果某个类型定义需要将__hash__属性删掉，那么我们可以在类变量中这样写__hash__ = None

看完了还是有点小激动的，今天因为一个偶然原因，接触到了这么多的python知识。真的是相当高兴阿！

然而我也发现__eq__ __hash__这两个方法不能随意动，如果我么需要改写其中一个的话一定要仔细考虑可能的情况，以免出现问题。