Classes & Objects

Introduction

Python classes use class + __init__. Instance state lives on self. Methods are functions whose first arg is the instance.

Understanding the topic

Core concepts to understand:

• __init__ is the constructor.
• self is explicit (not implicit).
• Class vs instance attributes.
• @classmethod, @staticmethod.

Syntax reference

Visual flow / code:

class User:
    species = "human"  # class attribute

    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

    def greet(self) -> str:
        return f"Hi, I'm {self.name}"

    @classmethod
    def anonymous(cls):
        return cls("Anon", 0)

    @staticmethod
    def is_adult(age: int) -> bool:
        return age >= 18

u = User("Alice", 30)
print(u.greet())

Execution workflow

Real-world use

Most production Python uses dataclasses instead of hand-written __init__ for plain data containers — less boilerplate, more correct.

Best practices

• Prefer dataclasses for data containers.
• Use type hints on attributes.
• Keep classes small and focused.

Common mistakes

• Class attributes shared across instances — beware mutable defaults.

Hands-on exercise

Interview preparation — practice these questions:

• Class vs instance attribute?
• @classmethod vs @staticmethod?
• Why explicit self?

Summary

In summary: self is the first arg. dataclasses for data, classes for behavior.