Python Classes & Objects: Complete Guide to Object-Oriented Programming (OOP)

Object-Oriented Programming (OOP) is a software design paradigm that structures code into real-world entities modeled as objects. An object bundles both state (data held in attributes) and behavior (functions known as methods) together into a single cohesive unit. Python is fully object-oriented from the ground up.

Figure 31.1: The Student class blueprint instantiating two independent objects with unique memory IDs.

1. Defining a Class and the __init__ Constructor

Classes are declared using the class keyword. The __init__() dunder method is the class constructor:

class Student:
    # Class Attribute (shared across all student instances):
    university_name = "Delhi University"

    def __init__(self, name: str, roll_no: int, marks: float):
        # Instance Attributes (unique to each student object):
        self.name = name
        self.roll_no = roll_no
        self.marks = marks

    # Instance Method:
    def display_details(self):
        print(f"Roll #{self.roll_no} | Name: {self.name} | Marks: {self.marks}/100")

    def has_passed(self) -> bool:
        return self.marks >= 40.0

2. Instantiating and Using Objects

To create an object, call the class name like a function (which triggers __init__):

# Creating two distinct student objects:
s1 = Student("Aman Kumar", 101, 88.5)
s2 = Student("Priya Sharma", 102, 34.0)

s1.display_details()
print("Aman Passed?:", s1.has_passed())  # True

s2.display_details()
print("Priya Passed?:", s2.has_passed())  # False

print("University (Class Attribute):", Student.university_name)

3. Class Attributes vs Instance Attributes (BCA Viva Alert!)

4. Class Methods (@classmethod) & Static Methods (@staticmethod)

class Employee:
    standard_raise = 1.05

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    @classmethod
    def set_raise_percentage(cls, new_raise):
        cls.standard_raise = new_raise

    @staticmethod
    def is_workday(day_name):
        return day_name.lower() not in ["saturday", "sunday"]

# Calling class & static methods:
Employee.set_raise_percentage(1.08)
print("Is Monday a workday?:", Employee.is_workday("Monday"))  # True

Frequently Asked Questions (FAQ)