Python OOP — Classes, Inheritance, and Design Patterns

Classes and Objects

class Dog:
    species = "Canis familiaris"
    
    def __init__(self, name, age, breed):
        self.name = name
        self.age = age
        self.breed = breed
    
    def bark(self):
        return f"{self.name} says Woof!"
    
    def __str__(self):
        return f"{self.name} ({self.breed}, {self.age})"

rex = Dog("Rex", 5, "German Shepherd")
print(rex.bark())  # Rex says Woof!

Inheritance

class Animal:
    def __init__(self, name, sound):
        self.name = name
        self.sound = sound
    def speak(self):
        return f"{self.name} says {self.sound}!"

class Cat(Animal):
    def __init__(self, name, indoor=True):
        super().__init__(name, "Meow")
        self.indoor = indoor
    def purr(self):
        return f"{self.name} purrs..."

cat = Cat("Whiskers")
print(cat.speak())   # Whiskers says Meow!
print(cat.purr())    # Whiskers purrs...

Polymorphism

class Shape:
    def area(self):
        raise NotImplementedError

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    def area(self):
        return 3.14159 * self.radius ** 2

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height
    def area(self):
        return self.width * self.height

shapes = [Circle(5), Rectangle(4, 6)]
for s in shapes:
    print(f"{s.__class__.__name__}: {s.area():.2f}")

Properties and Encapsulation

class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self._balance = balance
    
    @property
    def balance(self):
        return self._balance
    
    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("Must be positive")
        self._balance += amount
        return self._balance