Identity operators (is, is not) in Python

Introduction

Identity operators check if two things are actually the same object in memory, not just if they look the same.

When you want to know if two variables point to the exact same thing.

When comparing objects where being the same object matters, like singletons.

When checking if a variable is None using 'is' for clarity and correctness.

Syntax

Python

variable1 is variable2
variable1 is not variable2

is returns True if both variables point to the same object.

is not returns True if they point to different objects.

Examples

Both variables point to the same list, so is returns True.

Python

a = [1, 2, 3]
b = a
print(a is b)  # True

Both lists look the same but are different objects, so is returns False.

Python

a = [1, 2, 3]
b = [1, 2, 3]
print(a is b)  # False

Use is to check if a variable is None.

Python

x = None
print(x is None)  # True

Since x and y are different objects, is not returns True.

Python

x = 5
y = 10
print(x is not y)  # True

Sample Program

This program shows how is and is not work by comparing lists and checking for None.

Python

a = [1, 2, 3]
b = a
c = [1, 2, 3]

print('a is b:', a is b)
print('a is c:', a is c)
print('a is not c:', a is not c)

x = None
print('x is None:', x is None)

Important Notes

Use is only to check if two variables are the same object, not to compare values.

For value comparison, use == instead.

Checking for None should always use is None or is not None for clarity.

Summary

is checks if two variables point to the same object.

is not checks if they point to different objects.

Use these operators when object identity matters, like checking for None.