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Classes and Objects

A class is a blueprint; an object is an instance of it.

classes.py
class Account:
    bank = "PrepVerse Bank"          # class attribute

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

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("amount must be positive")
        self._balance += amount

    @property
    def balance(self):
        return self._balance

    def __repr__(self):
        return f"Account({self.owner!r}, {self._balance})"


acct = Account("Asha", 100)
acct.deposit(50)
print(acct.balance)   # 150
print(acct)           # Account('Asha', 150)

Key ideas​

  • __init__ initialises a new instance; self is the instance.
  • A leading underscore (_balance) is a convention for "internal".
  • @property exposes a computed or read-only attribute.
  • @classmethod receives the class, @staticmethod receives neither.
methods.py
class Temperature:
    def __init__(self, celsius):
        self.celsius = celsius

    @classmethod
    def from_fahrenheit(cls, f):
        return cls((f - 32) * 5 / 9)

    @staticmethod
    def is_valid(c):
        return c >= -273.15
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Python OOP
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Inheritance and Polymorphism
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