7.8 Identity Operators

This section covers is/is not specifically as comparison operators. The underlying concept — a variable being a reference to an object, not the object itself — is covered in full in 6.2 Objects and Variable References.

is

What Is It?

Tests whether two names reference the exact same object in memory (same identity), not merely equal values.

>>> a = [1, 2, 3]
>>> b = a
>>> c = [1, 2, 3]
>>> a is b      # same object
True
>>> a is c      # equal value, but a DIFFERENT object
False

is not

>>> a is not c
True

Identity vs Equality

The Rule of Thumb

Use == to compare values (almost always what you want), and reserve is specifically for identity checks — most commonly x is None, since None is a singleton.

>>> a == c    # equal value
True

id()

Directly inspecting the identities being compared — see also 5.10 Type Checking:

>>> id(a), id(c)
(140475706460992, 140475706462784)    # different -- confirms a is c is False

Quick Interview Answer

“is tests identity — whether two names point at the exact same object — while == tests value equality. Two lists can hold identical values (== is True) while being two entirely separate objects (is is False). The standard rule of thumb is to use == for almost everything, and reserve is specifically for identity checks like x is None, since None is a singleton and there’s exactly one object to ever compare against.”

Common Mistakes

• Using is to compare values instead of == — see 7.14 Common Mistakes for why this can silently appear correct due to CPython’s small-integer caching and string interning.
• Writing x == None instead of the idiomatic x is None — is None is both faster and immune to a custom __eq__ on x that might override equality behavior unexpectedly.