Programming Fundamentals › Collections

Equality vs Identity

Same value vs same object in memory.

Also known as: == vs is, value equality, reference equality

Two names can be equal, meaning they have the same value, or identical, meaning they’re the very same object in memory.

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

a == b        # True   same contents (equality)
a is b        # False  two separate lists (identity)
a is c        # True   one list, two names

Think of two printed copies of the same book (equal) versus one book with two bookmarks (identical).

By language

[1, 2] === [1, 2]     // false: two different arrays
const x = [1, 2]; x === x   // true

JavaScript has no built-in deep equality for arrays and objects (== vs ===).

Why it matters

• Bugs from identity. Comparing strings or lists with Java’s ==, or with is in Python, can work by accident (small numbers and strings are sometimes shared), then fail.
• Shared mutable objects. If two names are identical, changing one changes the other (mutable vs immutable, shallow vs deep copy).
• Dictionary and set keys rely on value equality plus hashing.
• React and similar tools use identity checks to decide whether something changed, so mutating an object in place can hide the change.

Use is in Python mainly for None checks (x is None). Use == for values.