When can I rely on identity tests with the is operator in Python?

Example

The is operator is an identity operator in Python. It tests for object identity. Let us see an example −

x = ["Paul","Mark"]
y = ["Paul","Mark"]
z = x

# Python IS operator
print(x is z)

Output

True

Let’s say we consider another example, wherein the test a is b is equivalent to −

id(a) == id(b)

The key property of an identity test is that an object is always identical to itself, a is a always returns True. Identity tests are usually faster than equality tests. And unlike equality tests, identity tests are guaranteed to return a boolean True or False.

However, identity tests can only be substituted for equality tests when object identity is assured. Generally, there are three circumstances where identity is guaranteed −

• Assignments create new names but do not change object identity.

  After the assignment new = old, it is guaranteed that new is old.

• Putting an object in a container that stores object references does not change object identity.

  After the list assignment s[0] = x, it is guaranteed that s[0] is x.

• If an object is a singleton, it means that only one instance of that object can exist. After the assignments a = None and b = None, it is guaranteed that a is b because None is a singleton.

Remember, identity tests should not be used to check constants such as int and str which aren’t guaranteed to be singletons. In the below examples, we only get False −

Identity test on int

Example

Let us test on integer elements −

a = 1000
b = 500
c = b + 500
print(a is c)

Output

False

Identity test on strings

Example

An example for identity tests on strings −

a = 'Amit'
b = 'Am'
c = b + 'it'
print(a is c)

Output

False

Identity test on Mutable Type

Example

In Python, Lists are mutable. The new instances of mutable containers are never identical; therefore identity test returns False −

a = [10, 20, 30]
b = [10, 20, 30]
print(a is b)

Output

False