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“ is None”和“ == None”有什么区别？

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问题：“ is None”和“ == None”有什么区别？
回答 0
回答 1
回答 2
回答 3
回答 4

问题：“ is None”和“ == None”有什么区别？

我最近遇到了这种语法，我不知道有什么区别。

如果有人可以告诉我与众不同，我将不胜感激。

点击查看英文原文

I recently came across this syntax, I am unaware of the difference.

I would appreciate it if someone could tell me the difference.

回答 0

答案在这里解释。

报价：

一个类可以自由选择以任何方式实现比较，并且可以选择与None进行比较意味着某种意义（这实际上是有道理的；如果有人告诉您从头开始实现None对象，那么您将如何获得它来比较True？反对自己？）。

实际上，由于自定义比较运算符很少见，因此差异不大。但是您应该使用is None一般规则。

点击查看英文原文

The answer is explained here.

To quote:

A class is free to implement comparison any way it chooses, and it can choose to make comparison against None mean something (which actually makes sense; if someone told you to implement the None object from scratch, how else would you get it to compare True against itself?).

Practically-speaking, there is not much difference since custom comparison operators are rare. But you should use is None as a general rule.

回答 1

class Foo:
    def __eq__(self,other):
        return True
foo=Foo()

print(foo==None)
# True

print(foo is None)
# False
点击查看英文原文 
class Foo:
    def __eq__(self,other):
        return True
foo=Foo()

print(foo==None)
# True

print(foo is None)
# False

回答 2

在这种情况下，它们是相同的。None是单例对象（只有一个存在None）。

is 检查对象是否是同一对象，而==只是检查它们是否等效。

例如：

p = [1]
q = [1]
p is q # False because they are not the same actual object
p == q # True because they are equivalent

但是由于只有一个None，它们将始终相同，is并将返回True。

p = None
q = None
p is q # True because they are both pointing to the same "None"
点击查看英文原文

In this case, they are the same. None is a singleton object (there only ever exists one None).

is checks to see if the object is the same object, while == just checks if they are equivalent.

For example:

p = [1]
q = [1]
p is q # False because they are not the same actual object
p == q # True because they are equivalent

But since there is only one None, they will always be the same, and is will return True.

p = None
q = None
p is q # True because they are both pointing to the same "None"

回答 3

如果您使用numpy，

if np.zeros(3)==None: pass

numpy进行元素比较时会给你错误

点击查看英文原文

If you use numpy,

if np.zeros(3)==None: pass

will give you error when numpy does elementwise comparison

回答 4

这取决于您要与“无”进行比较。一些类具有自定义比较方法，这些方法与== None区别对待is None。

特别是的输出a == None 甚至不必是布尔值！-错误的常见原因。

对于特定示例，请使用numpy数组，其中==逐元素实现比较：

import numpy as np
a = np.zeros(3) # now a is array([0., 0., 0.])
a == None #compares elementwise, outputs array([False, False, False]), i.e. not boolean!!!
a is None #compares object to object, outputs False
点击查看英文原文

It depends on what you are comparing to None. Some classes have custom comparison methods that treat == None differently from is None.

In particular the output of a == None does not even have to be boolean !! – a frequent cause of bugs.

For a specific example take a numpy array where the == comparison is implemented elementwise:

import numpy as np
a = np.zeros(3) # now a is array([0., 0., 0.])
a == None #compares elementwise, outputs array([False, False, False]), i.e. not boolean!!!
a is None #compares object to object, outputs False