Inheritance and Polymorphism

# Inheritance and Polymorphism

Welcome to Chapter 19! Inheritance lets you create new classes based on existing ones, and polymorphism lets different classes share a common interface.

---

1. Learning Objectives #

• Implement single and multiple inheritance.

• Override methods and use super().

• Apply polymorphism, encapsulation, and abstraction.

• Use abstract base classes (ABC).

---

2. Inheritance Basics #

```python id="py19_ex1" class Animal: def __init__(self, name, sound): self.name = name self.sound = sound def speak(self): return f"{self.name} says {self.sound}!" def info(self): return f"Animal: {self.name}"

class Dog(Animal): # Dog inherits from Animal def __init__(self, name, breed): super().__init__(name, "Woof") # Call parent constructor self.breed = breed def fetch(self): return f"{self.name} fetches the ball! 🎾"

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

dog = Dog("Buddy", "Labrador") cat = Cat("Luna")

print(dog.speak()) # Buddy says Woof! print(dog.fetch()) # Buddy fetches the ball! print(cat.speak()) # Luna says Meow! print(isinstance(dog, Animal)) # True

---

## 3. Method Overriding

python id="py19_ex2" class Shape: def area(self): return 0 def describe(self): return f"Shape with area: {self.area()}"

class Circle(Shape): def __init__(self, radius): self.radius = radius def area(self): # Override parent method 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), Circle(3)] for shape in shapes: print(shape.describe())

---

## 4. Multiple Inheritance

python id="py19_ex3" class Flyable: def fly(self): return "I can fly! ✈️"

class Swimmable: def swim(self): return "I can swim! 🏊"

class Duck(Animal, Flyable, Swimmable): def __init__(self, name): super().__init__(name, "Quack")

duck = Duck("Donald") print(duck.speak()) # Donald says Quack! print(duck.fly()) # I can fly! print(duck.swim()) # I can swim!

# Method Resolution Order (MRO) print(Duck.__mro__)

---

## 5. Encapsulation

python id="py19_ex4" class BankAccount: def __init__(self, owner, balance): self.owner = owner # Public self._account_id = "ACC123" # Protected (convention) self.__balance = balance # Private (name mangling) def get_balance(self): return self.__balance def deposit(self, amount): if amount > 0: self.__balance += amount

acc = BankAccount("Alice", 1000) print(acc.owner) # Alice — public, OK print(acc._account_id) # ACC123 — works but discouraged # print(acc.__balance) # ❌ AttributeError! print(acc.get_balance()) # 1000 — access through method

---

## 6. Abstraction with ABC

python id="py19_ex5" from abc import ABC, abstractmethod

class Vehicle(ABC): def __init__(self, brand): self.brand = brand @abstractmethod def start(self): pass # Must be implemented by subclasses @abstractmethod def stop(self): pass

class Car(Vehicle): def start(self): return f"{self.brand} car engine started! 🚗" def stop(self): return f"{self.brand} car stopped."

class Motorcycle(Vehicle): def start(self): return f"{self.brand} motorcycle revving! 🏍️" def stop(self): return f"{self.brand} motorcycle stopped."

# vehicle = Vehicle("Generic") # ❌ Can't instantiate abstract class! car = Car("Toyota") moto = Motorcycle("Harley") print(car.start()) print(moto.start())

---

## 7. Polymorphism

python id="py19_ex6" # Same interface, different behavior def print_area(shape): print(f"Area: {shape.area():.2f}")

print_area(Circle(5)) # Area: 78.54 print_area(Rectangle(4, 6)) # Area: 24.00

# Duck typing — Python doesn't care about type, only behavior class Triangle: def __init__(self, base, height): self.base = base self.height = height def area(self): return 0.5 * self.base * self.height

print_area(Triangle(6, 4)) # Area: 12.00 — works without inheritance! ``

Q1: super() calls: A) Static method B) Parent class method C) Global function D) Class method Answer: B

Q2: __variable in Python is: A) Public B) Private (name mangled) C) Protected D) Constant Answer: B

Q6: isinstance(obj, Class) checks: A) Type only B) Type and inheritance C) Memory D) Value Answer: B

Q8: @abstractmethod requires: A) No implementation B) Implementation in subclass C) Both D) Neither Answer: B

Q10: issubclass(Dog, Animal) returns: A) True if Dog inherits Animal B) Always True C) Always False D) Error Answer: A

• super() calls parent class methods.

• Encapsulation hides internal state (_protected, __private`).

• Abstraction with ABC defines interfaces that subclasses must implement.

• Polymorphism lets different classes be used through a common interface.

---

11. Next Chapter Recommendation #

In Chapter 20: Python Iterators and Generators, you'll learn lazy evaluation and memory-efficient data processing! 🚀