NotesPythonObject-Oriented Programming

Classes & Objects

The blueprint-and-instance model — __init__, self, and instance methods.

A class is a blueprint; an object (or instance) is a specific thing built from that blueprint. __init__ runs automatically when you create an instance, and self refers to the instance itself — it must be the first parameter of every instance method.

class YouTuber:
    def __init__(self, channel_name, subscribers=0):
        self.channel_name = channel_name
        self.subscribers = subscribers

    def upload_video(self):
        self.subscribers += 100
        return f"{self.channel_name} uploaded a video and gained subscribers!"

    def describe(self):
        return f"{self.channel_name} has {self.subscribers:,} subscribers"

munna = YouTuber("CodeWithMunnaX", 250_000)
print(munna.describe())
print(munna.upload_video())
print(munna.describe())   # subscribers went up — self.subscribers persists on the instance

Every attribute set on self (like self.channel_name) belongs to that specific instance — creating a second YouTuber gives it its own independent subscribers count.

harry = YouTuber("CodeWithHarry", 4_000_000)
print(harry.describe())
print(munna.describe())   # completely independent from harry's data

Class attributes vs. instance attributes

A class attribute is shared by every instance unless a specific instance overrides it; an instance attribute (set via self.x = ... in __init__) belongs to just that one object.

class YouTuber:
    platform = "YouTube"   # class attribute — shared by all instances

    def __init__(self, channel_name):
        self.channel_name = channel_name   # instance attribute — unique per object

print(YouTuber("A").platform, YouTuber("B").platform)   # YouTube YouTube

Methods vs. plain functions

A method is just a function defined inside a class — the only real difference is that Python automatically passes the instance as the first argument (self) when you call it via instance.method().

Key points to remember
  • •__init__ is the constructor — it runs automatically when you create a new instance of a class.
  • •self refers to the specific instance a method was called on, and must be the first parameter of every instance method.
  • •Attributes set via self.x in __init__ are unique per instance; class-level attributes (defined directly in the class body) are shared across all instances unless overridden.
  • •Two instances of the same class are completely independent — changing one's attributes never affects the other.

A YouTuber class with independent instances

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DecoratorsInheritance & Polymorphism