How do you define a class in Python, and what is the role of __init__?

A class is a blueprint for objects. Define it with the class keyword. The special method __init__ (the constructor) is called automatically when you create an instance and is where you set up the object's initial state by assigning to self.attribute.

class BankAccount:
    interest_rate = 0.03   # class attribute — shared by all instances

    def __init__(self, owner, balance=0):
        self.owner   = owner    # instance attributes
        self.balance = balance

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError('Deposit amount must be positive')
        self.balance += amount

    def __repr__(self):
        return f'BankAccount({self.owner!r}, balance={self.balance})'

acc = BankAccount('Alice', 1000)
acc.deposit(500)
print(acc)          # BankAccount('Alice', balance=1500)
print(acc.interest_rate)  # 0.03

self is a reference to the current instance; it is not a keyword but the universal convention. Every instance method receives it as the first parameter. Class attributes are defined directly in the class body and shared across all instances; instance attributes are set with self.attr = value inside methods and belong to each object individually.

Important dunder methods to know: __str__ (readable string for end users, called by print), __repr__ (unambiguous string for developers, called in the REPL), __len__, __eq__, __lt__, and __enter__/__exit__ for context managers.

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