Programming Fundamentals › Object-Oriented Programming

Object

An instance of a class holding its own state.

Also known as: instance, objects, object instance

An object is a thing in your program that holds its own data (attributes) and has behavior (methods). Objects are made from a class, and each one is an instance of it.

class Account:
    def __init__(self, owner):
        self.owner = owner
        self.balance = 0
    def deposit(self, amount):
        self.balance += amount

a = Account("Ana")      # one object
b = Account("Bo")       # another, with its own balance
a.deposit(100)
a.balance               # 100
b.balance               # 0

Each object has its own state, and they share only the class’s code.

Objects in other forms

In JavaScript, you can write an object directly, without a class:

const user = { name: "Ana", greet() { return "Hi " + this.name; } };

Python’s dictionaries, lists, even numbers and functions are all objects, with methods you can call.

Things to know

• Variables hold references. Two names can point to the same object, and changing it through one shows up through the other (equality vs identity, pass by value vs reference).
• Most objects are mutable, so copies need care (shallow vs deep copy).
• Identity vs equality. Two objects with the same contents aren’t automatically the same object.
• Lifetime. Objects live while something refers to them, and are then cleaned up by the garbage collector.
• Design objects around responsibilities, with clear methods, rather than as bags of public fields (encapsulation).