Python “is not” Operator — Checking Object Difference

The Python is not operator — consisting of two keywords is and not — tests if the left and right operands refer to a different object—in which case it returns True. It returns False if they refer to the same object. For example, the expression [1, 2, 3] is not [1, 2, 3] returns True because although both lists are equal, they are two different objects in memory.

Python "is not" operator

The following example shows that both lists lst_1 and lst_2 are not different lst_1 != lst_2, but when tested for negative identity lst_1 is not lst_2, they are still different objects.

>>> lst_1 = [1, 2, 3]
>>> lst_2 = [1, 2, 3]
>>> lst_1 is not lst_2
True
>>> lst_1 != lst_2
False

The difference between the ‘is not‘ and ‘!=‘ operators is that ‘is not‘ compares the object reference of two objects whereas ‘!=‘ compares the semantic equality of two objects.

Identity Operators

Python has two identity operators: is and is not. They are used to check if two values or variables reside at the same memory location, i.e., refer to the same object in memory. However, if two variables are equal, this doesn’t imply that they are identical. For example, two lists [1, 2, 3] and [1, 2, 3] may be different objects (not identical) but they’re equal in value.

OperatorMeaningExample
isReturns True if both operands refer to the same object in memory and False otherwise.x is True
is notReturns False if both operands refer to the same object in memory and True otherwise.x is not True

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