在Python中为None [英] identity versus equality for None in Python

问题描述

各种Python指南说使用 x是无而不是 x == None 。这是为什么？平等用于比较值，因此看来是否 x 具有值 None ，表示为 == 而不是是。有人可以解释为什么是是首选表单，并显示一个示例，其中两个不给出相同的答案？

Various Python guides say to use x is None instead of x == None. Why is that? Equality is used for comparing values, so it seems natural to ask if x has the value None, denoted with == and not is. Can someone explain why is is the preferred form and show an example where the two do not give the same answer?

感谢。

推荐答案

人们使用是是因为使用 == 没有优势。可以编写比较等于 None 的对象，但不常见。

The reason people use is is because there is no advantage to using ==. It is possible to write objects that compare equal to None, but it is uncommon.

class A(object):
    def __eq__(self, other):
        return True

print A() == None

输出：

True

是不要认为这个事实很重要。

The is operator is also faster, but I don't consider this fact important.

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