不覆盖比较运算符的Python对象是否等于自身？ [英] Does a Python object which doesn't override comparison operators equals itself?

问题描述

class A(object):

    def __init__(self, value):
        self.value = value

x = A(1)
y = A(2)

q = [x, y]
q.remove(y)

我想从列表中删除之前添加到该对象的特定对象，但仍然可以参考。我不要平等测试。我要进行身份测试。该代码似乎在CPython和IronPython中都可以使用，但是该语言是否可以保证这种行为还是仅仅是is幸？

I want to remove from the list a specific object which was added before to it and to which I still have a reference. I do not want an equality test. I want an identity test. This code seems to work in both CPython and IronPython, but does the language guarantee this behavior or is it just a fluke?

list.remove 方法文档是这样的：与del s [s.index（x）] 相同，这意味着将执行相等性测试。

The list.remove method documentation is this: same as del s[s.index(x)], which implies that an equality test is performed.

如果您不覆盖 __ cmp __ ， __ eq __ 或 __ ne __ ？

推荐答案

是。在您的示例中， q.remove（y）将删除与 y 相等的对象的第一个匹配项。但是，定义类 A 的方式，您不应^†拥有与 y -除了也绑定到同一 y 实例的任何其他名称之外。

Yes. In your example q.remove(y) would remove the first occurrence of an object which compares equal with y. However, the way the class A is defined, you shouldn't^† ever have a variable compare equal with y - with the exception of any other names which are also bound to the same y instance.

文档的相关部分是此处：

如果没有 __ cmp __（），__ eq __（）或定义了__ne __（）操作，通过对象标识（地址）比较类
实例。

If no __cmp__(), __eq__() or __ne__() operation is defined, class instances are compared by object identity ("address").

因此，对 A 实例的比较是通过身份（在CPython中实现为内存地址）进行的。在 y 的生存期内，其他任何对象都不能具有等于 id（y）的身份，即只要您拥有对 y 的引用（如果要从列表中删除它，则必须这样做）。

So comparison for A instances is by identity (implemented as memory address in CPython). No other object can have an identity equal to id(y) within y's lifetime, i.e. for as long as you hold a reference to y (which you must, if you're going to remove it from a list!)

^† _{从技术上讲，仍然有可能在其他内存位置具有相等对象的对象- mock.ANY 就是这样一个例子。但是这些对象需要重写其比较运算符以强制执行结果。}

^† _{Technically, it is still possible to have objects at other memory locations which are comparing equal - mock.ANY is one such example. But these objects need to override their comparison operators to force the result.}

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