Classes

A class is a blueprint; an instance (or object) is a concrete thing built from that blueprint, with its own independent state.

class Dog:
    def __init__(self, name: str):
        self.name = name
        self.tricks: list[str] = []

    def learn_trick(self, trick: str) -> None:
        self.tricks.append(trick)

rex = Dog("Rex")
rex.learn_trick("sit")

fido = Dog("Fido")
print(rex.tricks)   # ['sit']
print(fido.tricks)  # [] — completely separate state

__init__ (highlighted, line 2) is the constructor — it runs once per instance and sets up that instance’s own attributes. learn_trick (line 6) is an instance method: the first parameter, self, is always a reference to the specific object the method was called on.

Classes exist in other languages too

The shape is the same everywhere: a constructor, instance state, instance methods.

class Dog:
    def __init__(self, name: str):
        self.name = name

    def bark(self) -> str:
        return f"{self.name} says woof!"

public class Dog {
    private final String name;

    public Dog(String name) {
        this.name = name;
    }

    public String bark() {
        return name + " says woof!";
    }
}

Class attributes vs. instance attributes

class Dog:
    species = "Canis familiaris"  # class attribute — shared by every instance

    def __init__(self, name: str):
        self.name = name          # instance attribute — unique per instance

print(Dog("Rex").species)  # "Canis familiaris"
print(Dog("Fido").species) # "Canis familiaris" — same value, shared on the class

Mutable class attributes (lists, dicts) are a classic gotcha: since they’re shared, mutating one through an instance affects every instance. Prefer setting mutable defaults inside __init__.

Where this leads

Classes are the foundation for the rest of Python OOP — inheritance, @property, dunder methods, and dataclasses all build directly on the instance/class-attribute model shown here.