Programming Fundamentals › Object-Oriented Programming

Class

A blueprint for creating objects.

Also known as: classes, class definition, blueprint

A class is a blueprint for creating objects. It defines what data each object holds (fields) and what it can do (methods).

class Account:
    def __init__(self, owner, balance=0):     # constructor
        self.owner = owner                    # fields
        self.balance = balance

    def deposit(self, amount):                # method
        self.balance += amount

a = Account("Ana")          # create an object (an instance)
a.deposit(50)
print(a.balance)            # 50

class Account {
  constructor(owner) { this.owner = owner; this.balance = 0; }
  deposit(amount) { this.balance += amount; }
}

The class is the design. Each Account(...) call builds one separate object from it, with its own data.

Why use classes

• Bundle data with the behavior that goes with it, so rules live next to what they protect (encapsulation).
• Create many similar things without repeating code.
• Model the domain: Order, User, Invoice.
• Share and extend behavior (inheritance).

Remember

• Not everything needs a class. A plain function or a dictionary is often simpler. Python and JavaScript don’t force you to wrap everything.
• Class-level vs instance data. Something declared on the class is shared by all objects, which causes bugs with mutable values like lists.
• self / this is the object the method was called on (this and self).
• Keep classes focused: one clear job, not a bag of unrelated methods (single responsibility).