What is "is" in Python?

The operators is and is not test for object identity. x is y is true if and only if x and y are the same object (object location in memory is same). Object identity is determined using the id() function. x is not y yields the inverse truth value.

It’s easy to understand if I compare this operator with equality operator.

• is and is not compares the object reference. Check for identity.

• == and != compares the object value. Check for equality.

For example, if you consider integer objects (excluding integers from -5 to 256),

>>> A=9999 

>>> B=9999 

>>> A == B, A is B

(True, False) 

>>> A, B

(9999, 9999) 

>>> id(A), id(B)

(4452685328, 4452686992)

Python stores integer objects as single object between range -5 to 256 so the identity is same.

>>> A=99

>>> B=99 

>>> A == B, A is B

(True, True) 

>>> A, B

(99, 99) 

>>> id(A), id(B)

(4404392064, 4404392064)

Lets see behavior of other immutable objects like int & float, string, tuples and boolean:

>>> 1 == 1.0, 1 is 1.0

(True, False) 

>>> 1 == 1, 1 is 1

(True, True) 

>>> 'A' == 'A', 'A' is 'A'

(True, True) 

>>> (1,2) == (1,2), (1,2) is (1,2)

(True, True) 

>>> True == True, True is True

(True, True)

What about mutable objects - list, set, dict? Behavior is totally different.

>>> [1,2] == [1,2], [1,2] is [1,2]

(True, False) 

>>> {1,2} == {1,2}, {1,2} is {1,2}

(True, False) 

>>> {'k1':1} == {'k1':1}, {'k1':1} is {'k1':1}

(True, False)

is operator is used only when you want to compare the object identity, for regular comparison equality operator is used.