[python] What is the difference between "is None" and "== None"

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

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

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

print(foo==None)
# True

print(foo is None)
# False

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"

If you use numpy,

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

will give you error when numpy does elementwise comparison

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