Classes and Objects

What it means

A class is a blueprint for creating objects — it bundles data (attributes) and behavior (methods) together. An object (or instance) is a concrete thing built from that blueprint. __init__ is the constructor, called automatically when you create a new instance, and self refers to the specific instance a method is operating on.

Examples

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

    def bark(self):
        return f"{self.name} says Woof!"

fido = Dog("Fido", "Labrador")
print(fido.bark())        # Fido says Woof!
print(fido.name)          # Fido

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1

c = Counter()
c.increment()
c.increment()
print(c.count)             # 2

Common mistake

Forgetting self as the first parameter of an instance method, or forgetting to prefix attribute access with self. inside the class — both cause NameError or TypeError.

class Broken:
    def __init__(name):        # missing self -- 'name' silently becomes self
        name = name             # this is a local variable, not an attribute!

# Fixed:
class Fixed:
    def __init__(self, name):
        self.name = name        # self.name is the actual instance attribute

Quick practice

What does self represent inside a method?

Answer
The specific instance the method is being called on — Python passes it automatically as the first argument.
What is __init__ called for?

Answer
It's the constructor — Python calls it automatically right after a new instance is created, to set up initial attribute values.
What happens if you assign name = name inside __init__ instead of self.name = name?

Answer
It creates/reassigns a local variable that disappears when the method returns — the instance never gets a name attribute.