What is the difference between “ is None ” and “ =

问题:

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1:

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2:

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

print(foo==None)
# True

print(foo is None)
# False

回答3:

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4:

If you use numpy,

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

will give you error when numpy does elementwise comparison