定义`__eq__`的类型是不可更改的？ [英] Types that define `__eq__` are unhashable?

问题描述

将功能移植到我的程序的Python 3.1分支时，出现了一个奇怪的错误。我将其缩小到以下假设：

与Python 2.x相反，在Python 3.x中，如果对象具有 __ eq __ 方法它自动不可用。

这是真的吗？

Python 3.1：

 >>> class O（object）：
 ... def __eq __（self，other）：
 ... return'whatever'
 ... 
>>> o = O（）
>>> d = {o：0} 
 Traceback（最近一次调用最后一次）：
在< module>中，第1行的文件< pyshell＃16> 
d = {o：0} 
 TypeError：不可用类型：'O'
  

后续问题是，我如何解决我的个人问题？我有一个对象 ChangeTracker ，它存储了一个指向几个对象的 WeakKeyDictionary ，为每个pickle dump在过去的某个时间点。无论何时检入现有对象，更改跟踪器都会说明它的新pickle是否与旧的pickle相同，因此说明对象是否在此期间发生了更改。问题是，现在我甚至无法检查给定的对象是否在库中，因为它使得它引发了一个关于不可对象的异常。 （原因是它有一个 __ eq __ 方法。）我该如何解决这个问题？

解决方案

是的，如果你定义了 __ eq __ ，默认的 __ hash __ （即，散列对象的地址记忆）消失。这很重要，因为散列需要与等式一致：相同的对象需要散列相同的结果。

解决方法很简单：只需定义 __ hash __ 以及定义 __ eq __ 。

I had a strange bug when porting a feature to the Python 3.1 fork of my program. I narrowed it down to the following hypothesis:

In contrast to Python 2.x, in Python 3.x if an object has an __eq__ method it is automatically unhashable.

Is this true?

Here's what happens in Python 3.1:

>>> class O(object):
...     def __eq__(self, other):
...         return 'whatever'
...
>>> o = O()
>>> d = {o: 0}
Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    d = {o: 0}
TypeError: unhashable type: 'O'

The follow-up question is, how do I solve my personal problem? I have an object ChangeTracker which stores a WeakKeyDictionary that points to several objects, giving for each the value of their pickle dump at a certain time point in the past. Whenever an existing object is checked in, the change tracker says whether its new pickle is identical to its old one, therefore saying whether the object has changed in the meantime. Problem is, now I can't even check if the given object is in the library, because it makes it raise an exception about the object being unhashable. (Cause it has a __eq__ method.) How can I work around this?

解决方案

Yes, if you define __eq__, the default __hash__ (namely, hashing the address of the object in memory) goes away. This is important because hashing needs to be consistent with equality: equal objects need to hash the same.

The solution is simple: just define __hash__ along with defining __eq__.

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