What is the difference between "is None" and "== None"
PythonPython Problem Overview
I recently came across this syntax, I am unaware of the difference.
I would appreciate it if someone could tell me the difference.
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
class Foo:
def __eq__(self,other):
return True
foo=Foo()
print(foo==None)
# True
print(foo is None)
# False
Solution 3 - Python
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"
Solution 4 - Python
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
Solution 5 - Python
If you use numpy,
if np.zeros(3)==None: pass
will give you error when numpy does elementwise comparison