OOP & the Data Model

Everything in Python is an object

Python is a fully object-oriented language: integers, strings, functions, modules — every value is an object with an identity, a type, and a value. A class is the blueprint; an object (or instance) is what you create from that blueprint with ClassName(...).

x = 42
print(type(x))    # <class 'int'>
print(id(x))      # unique memory address

The special-method protocol

Python exposes nearly every built-in operation through dunder methods (double-underscore methods like __init__, __repr__, __add__). When you write a + b, Python calls a.__add__(b). When you write len(obj), Python calls obj.__len__(). When you write repr(obj), Python calls obj.__repr__().

This protocol is called the Python Data Model — it lets your own classes integrate seamlessly with built-in syntax and standard-library functions.

A minimal class

class Point:
    def __init__(self, x: float, y: float) -> None:
        self.x = x
        self.y = y

    def __repr__(self) -> str:
        return f"Point({self.x}, {self.y})"

• __init__ is the initialiser — it runs right after the object is created and sets up instance attributes.
• __repr__ gives the object a developer-friendly string representation.

What this module covers

Lesson	Topic
1. Classes	class, __init__, instance vs class attributes, inheritance
2. Dunder methods	__repr__, __eq__, __len__, __getitem__, __add__
3. Properties	@property, computed attributes, @classmethod, @staticmethod
4. Dataclasses	@dataclass, defaults, field, frozen=True

Runnable demo

Run ▸ Open in an online REPL

class Point:
    def __init__(self, x: float, y: float) -> None:
        self.x = x
        self.y = y

    def __repr__(self) -> str:
        return f"Point({self.x}, {self.y})"

    def __add__(self, other: "Point") -> "Point":
        return Point(self.x + other.x, self.y + other.y)

p1 = Point(1, 2)
p2 = Point(3, 4)
print(p1)
print(p1 + p2)
print(type(p1))
print(isinstance(p1, Point))

🐍 Classes integrate with Python syntax via dunder methods

You do not need to call __add__ directly — you write p1 + p2. Python looks up __add__ on the left operand and calls it for you. Every operator, built-in function, and language construct has a corresponding dunder method your class can implement.

Which dunder method runs when you write `obj = MyClass(1, 2)`?

__new__ __init__ __create__ __call__

What does Python call internally when you write `len(my_list)`?

my_list.length() my_list.__len__() len.__call__(my_list) my_list.size()

What is the term for the system of dunder methods that integrates classes with Python syntax?

The magic system The protocol buffer The Python Data Model The operator registry

Which statement about Python objects is true?

Only class instances are objects; primitives like int are not Every value in Python is an object with an identity, type, and value Functions are not objects Objects only exist at runtime after explicit allocation

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