實習小記－python中可雜湊對象是個啥？what is hashable object in python?

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廢話不多說直接祭上python3.3x的文檔：（原文連結）

 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.

可雜湊對象是對象擁有__hash__(self)內建函數的對象。對於可雜湊的對象執行這個函數將會返回一個整數。可雜湊對象判斷相等的唯一條件就是兩者 的雜湊值相等。如果我們的對象有多個屬性值，我們會使用一種方法（比方說邏輯運算異或）來將其屬性值結合在一起做比較。（如果不對，麻煩一定告訴我，謝謝！）

 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).

如果一個類型定義沒有定義__eq__()函數，那麼它也不應該定義__hash__()；因為如果它定義了__eq__而沒有定義__hash__，那麼它的執行個體在某個可雜湊集合中將會無效（比方說在字典／集合這類類型中）。如果一個類型定義了一個可變對象而且定義了__eq__方法，那麼它不應該去定義__hash__方法，因為在雜湊集合中要求其中元素的雜湊值是不變的（如果某個對象執行個體的雜湊值發生了改變，那麼他將會到錯誤的雜湊表中。。）。

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).

使用者定義的類中都有預設的__eq__和__hash__方法；有了它，所有的對象執行個體都是不等的（除非是自己和自己比較），在做 x == y 比較時是和這個等價的 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).

這句話的意思是，如果我們定義了一個類型，該類型只定義了__eq__而沒有定義__hash__的話，那麼他的__hash__()會隱式的設定為None，如果某個情況下需要這個類型的雜湊值，那麼程式將會報錯。具體請看下面代碼：

>>> class A:...     def __eq__(self, other):...         return True...     >>> a = A()>>> import collections>>> isinstance(a, collections.Hashable)  # 發現它不是可雜湊對象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

然後往下看文檔

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__.

如果某個重寫了__eq__方法的對象需要保留__hash__屬性，那麼我們需要在類型設定中添加該語句　__hash__ = <ParentClass>.__hash__

請看代碼

 

>>> 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.

如果某個類型定義需要將__hash__屬性刪掉，那麼我們可以在類變數中這樣寫　__hash__ = None

 

看完了還是有點小激動的，今天因為一個偶然原因，接觸到了這麼多的python知識。真的是相當高興阿！

然而我也發現__eq__ __hash__這兩個方法不能隨意動，如果我麼需要改寫其中一個的話一定要仔細考慮可能的情況，以免出現問題。

 

實習小記－python中可雜湊對象是個啥？what is hashable object in python?