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

The is operator in Python tests for object identity, not equality. You can rely on identity tests in specific circumstances where object identity is guaranteed.

Understanding the is Operator

The is operator compares object identities using their memory addresses. The test a is b is equivalent to id(a) == id(b).

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

# Identity test
print("x is z:", x is z)
print("x is y:", x is y)

# Verify with id()
print("id(x):", id(x))
print("id(z):", id(z))
print("id(y):", id(y))

x is z: True
x is y: False
id(x): 140234567890432
id(z): 140234567890432
id(y): 140234567890816

When You Can Rely on Identity Tests

Identity tests are guaranteed to work reliably in three specific circumstances:

1. Variable Assignments

Assignments create new names but preserve object identity ?

original = [1, 2, 3]
new_name = original

print("new_name is original:", new_name is original)
print("Always guaranteed: True")

new_name is original: True
Always guaranteed: True

2. Container References

Objects stored in containers maintain their identity ?

my_object = "hello"
container = [None]
container[0] = my_object

print("container[0] is my_object:", container[0] is my_object)
print("Always guaranteed: True")

container[0] is my_object: True
Always guaranteed: True

3. Singleton Objects

Singletons like None, True, and False have guaranteed identity ?

a = None
b = None
print("a is b:", a is b)

x = True
y = True
print("x is y:", x is y)

a is b: True
x is y: True

When NOT to Use Identity Tests

Large Integers

Python doesn't guarantee identity for large integers ?

a = 1000
b = 500 + 500
print("a is b:", a is b)
print("a == b:", a == b)

a is b: False
a == b: True

String Concatenation

Dynamically created strings may not have the same identity ?

a = 'Amit'
b = 'Am' + 'it'
print("a is b:", a is b)
print("a == b:", a == b)

a is b: False
a == b: True

Mutable Objects

New instances of mutable containers are never identical ?

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

a is b: False
a == b: True

Best Practices

Conclusion

Use is for comparing with singletons like None, checking variable assignments, and container references. For value comparisons, always use == to ensure reliable results regardless of object identity.