Introduction to Classes and Objects

Definition

How would you describe every student the same way, with their own name and marks?

A class is a blueprint that defines what data (attributes) and behaviour (methods) its objects will have. An object, or instance, is one thing made from that blueprint. The special method __init__ sets up each new object, and self refers to the particular object a method is working on.

In simple words

A class is a cookie cutter; objects are the cookies. The Student class says every student has a name and marks and can work out an average. Student("Meera", [91, 88]) makes one student. Each object keeps its own data, so changing Meera's marks doesn't touch Kabir's.

Why it matters

Every AI library is organised into classes. In scikit-learn, LinearRegression() creates a model object with fit() and predict() methods; in PyTorch, every network is a class. Understanding classes lets you read and use these libraries.

How it works

Term	Meaning	In the Student example
Class	The blueprint	class Student:
Object	One thing made from it	meera = Student("Meera", [91, 88])
Attribute	Data stored in an object	meera.name
Method	A function belonging to the class	meera.average()
__init__	Sets up each new object	Stores name and marks
self	The object the method is working on	self.marks

In code

A Student class with two objects, each keeping its own data.

student_class.py

class Student:
    def __init__(self, name, marks):
        self.name = name
        self.marks = marks

    def average(self):
        return sum(self.marks) / len(self.marks)

    def add_mark(self, mark):
        self.marks.append(mark)

    def report(self):
        return f"{self.name}: average {self.average():.1f} from {len(self.marks)} tests"

meera = Student("Meera", [91, 88])
kabir = Student("Kabir", [64, 70, 75])
print(meera.report())
print(kabir.report())

meera.add_mark(79)
print("After a new test:", meera.report())
print("Kabir unchanged:", kabir.report())

Output

Meera: average 89.5 from 2 tests
Kabir: average 69.7 from 3 tests
After a new test: Meera: average 86.0 from 3 tests
Kabir unchanged: Kabir: average 69.7 from 3 tests

Try it yourself

Class vocabulary

Match each term to its meaning.

Class
Object
Attribute
Method
__init__

Common mistakes

Forgetting self as the first parameter of a method.
Writing name = name instead of self.name = name in __init__, so nothing is stored.
Calling a method without brackets: meera.average instead of meera.average().

Check yourself

1. What does __init__ do?

2. In a method, self refers to…

3. model = LinearRegression() in scikit-learn creates…

Summary

A class is a blueprint; objects are made from it.
Attributes store data; methods are the class's functions.
__init__ sets up objects and self is the current object.