OOP Deep Dive

Python Intermediate

Course 2 · Chapter 2 · OOP Deep Dive

🔗 OOP Deep Dive

Chapter 1 covered defining a single class. This chapter covers three ways classes work together and with the rest of the language: inheritance (building a class on top of another), polymorphism (different classes responding to the same method call), and magic/dunder methods — the hooks that let your own objects work with print(), ==, and other built-in behavior.

🧬 Inheritance

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        return f"{self.name} makes a sound"

class Dog(Animal):            # Dog inherits from Animal
    def __init__(self, name, breed):
        super().__init__(name)   # call Animal's __init__ first
        self.breed = breed

    def speak(self):             # overriding Animal's speak()
        return f"{self.name} says Woof!"

rex = Dog("Rex", "Labrador")
print(rex.speak())   # Rex says Woof! — Dog's own version

class Dog(Animal): makes Dog a subclass of Animal, inheriting its attributes and methods. super().__init__(name) calls the parent class's constructor, so Dog doesn't have to duplicate the logic for setting self.name.

⚠ Forgetting super().__init__() Silently Skips Parent Setup

If Dog.__init__ sets self.breed but never calls super().__init__(name), the object never gets a self.name attribute at all — calling rex.name later raises an AttributeError, and the failure happens far away from the actual mistake, making it a confusing bug to track down. Whenever a subclass defines its own __init__, call super().__init__(...) first unless you deliberately want to skip the parent's setup.

🎭 Polymorphism

Different classes can implement the same method name, and calling code can treat them uniformly without checking which specific class it's dealing with:

class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow!"

animals = [Dog("Rex", "Labrador"), Cat("Whiskers")]

for animal in animals:
    print(animal.speak())   # each object uses its OWN speak(), automatically
# Rex says Woof!
# Whiskers says Meow!

The loop doesn't need an if isinstance(animal, Dog) check anywhere — calling .speak() automatically runs whichever version belongs to that object's actual class. This is polymorphism: one interface (.speak()), many implementations.

✨ Magic / Dunder Methods

"Dunder" (double underscore) methods let your class hook into Python's built-in behavior. __init__ is one you already know — here are two more of the most common:

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __str__(self):
        return f"({self.x}, {self.y})"

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

p1 = Point(1, 2)
p2 = Point(1, 2)

print(p1)          # (1, 2) — uses __str__, not "<__main__.Point object at 0x...>"
print(p1 == p2)   # True — uses __eq__, compares VALUES not identity

⚠ Without __eq__, == Compares Identity, Not Values

Without a custom __eq__, p1 == p2 would be False even with identical x/y values — the default == checks whether both names point to the exact same object in memory, the same identity-vs-equality distinction that matters for strings and numbers too, but is easy to forget once you're writing your own classes.

Similarly, without __str__, print(p1) would show something unhelpful like <__main__.Point object at 0x000001A2B3C4D5E6> — the default representation, which just identifies the object's type and memory address rather than anything about its actual data.

Inheritance & Object Representation: Go vs Kotlin vs Python

Feature	Go	Kotlin	Python
Inheritance	No classes/inheritance — struct embedding approximates it	Classes are `final` by default; must mark `open class` to allow subclassing	Every class is inheritable by default, no keyword needed
Custom string form	Implement the `Stringer` interface's `String()` method	Override `toString()`	Define `__str__`
Custom equality	Manual field-by-field comparison (or reflect.DeepEqual)	`data class` generates `equals()` automatically	Define `__eq__` manually

Kotlin's open class requirement is a real, easy-to-forget difference coming from Python — Python never blocks subclassing by default, where Kotlin does unless you opt in.

Inheritance

class Child(Parent):, with super() to call the parent's methods.

Overriding

Redefining a parent's method in the subclass to change its behavior.

Polymorphism

Calling code uses one method name; each object's own class decides what actually runs.

Dunder methods

__str__ for print(), __eq__ for == — hooks into built-in behavior.

💻 Coding Challenges

Challenge 1: Shape Hierarchy

Write a base class Shape with a method area() that returns 0, then subclasses Square(side) and Circle(radius) that each override area() with their correct formula. Loop over a list of one of each and print each shape's area.

Goal: Practice inheritance and overriding together, plus polymorphic iteration.

→ Solution

Challenge 2: A Printable Fraction Class

Write a Fraction class with numerator and denominator instance attributes, and a __str__ method that prints it as "numerator/denominator" (e.g. "3/4").

Goal: Practice writing a __str__ method and seeing it kick in automatically with print().

→ Solution

Challenge 3: Value-Based Equality

Add an __eq__ method to the Fraction class from Challenge 2, so that two Fraction objects with the same numerator and denominator compare as equal with ==. Prove it works by comparing two separately created but equal fractions.

Goal: Practice writing __eq__ and understand why it's needed for value-based comparison.

→ Solution

🎯 What's Next

Next chapter: Exception Handling — try/except/else/finally, raising exceptions, and custom exceptions.