Programming Fundamentals › Collections

Mutable vs Immutable

Whether a value can be changed after it's created.

Also known as: mutability, immutable, mutable

A mutable value can be changed in place after it’s created. An immutable value can’t: any “change” produces a new value and leaves the original alone.

name = "ana"
name.upper()         # returns "ANA"; name is still "ana"

items = [1, 2]
items.append(3)      # items itself is now [1, 2, 3]

Why it matters: shared references

When two variables point to the same mutable value, changing it through one changes it for the other:

a = [1, 2]
b = a              # not a copy: both names point to the same list
b.append(3)
print(a)           # [1, 2, 3]

This causes bugs in two classic places:

• Function arguments. A function that modifies a list you passed in changes your list too. See pass by value vs reference.
• Python’s mutable default argument. In def add(x, items=[]), the same list is created once and shared by every call. Use items=None and create a new list inside.

const is not immutability

In JavaScript, const stops you reassigning the variable, not changing the object:

const user = { name: "Ana" };
user.name = "Bo";      // allowed
user = {};             // TypeError

Object.freeze blocks changes, but only at the top level.

Working with immutable data

In UI code especially, prefer making a new value over editing the old one:

const next = { ...user, name: "Bo" };      // new object
const added = [...items, newItem];         // new array

Frameworks like React rely on spotting a new object to know something changed. See immutability for the design idea, and shallow vs deep copy for what copying does and doesn’t protect.