Object-oriented programming

A class is a blueprint and an object is a concrete instance built from it. For example, the Account class produces a separate object for each customer. The __init__ method runs when an object is created, and self is the object itself.

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

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

    def show(self):
        print(f"{self.owner}: {self.balance} AZN")

a = Account("Rena", 50)
a.deposit(25)
a.show()   # Rena: 75 AZN

• Attributes hold the state of the object.
• Methods are what the object can do.

Inheritance lets a new class take over what an existing class offers. The subclass gets the parent's methods, may override them, or call the parent through super().

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

class SavingsAccount(Account):
    def __init__(self, owner, balance=0, rate=0.05):
        super().__init__(owner, balance)
        self.rate = rate

    def add_interest(self):
        self.balance *= 1 + self.rate

s = SavingsAccount("Tural", 200)
s.add_interest()
print(s.balance)   # 210.0

__str__ decides how an object looks when printed, and __len__ makes it work with len(). Double underscores mark special language hooks.

class Playlist:
    def __init__(self, songs):
        self.songs = songs
    def __len__(self):
        return len(self.songs)
    def __str__(self):
        return "Playlist: " + ", ".join(self.songs)

p = Playlist(["A", "B"])
print(p, len(p))

Advice: not everything needs to be a class. A function is enough for a simple calculation; a class pays off when state and behaviour belong together.