LEARN COMPLETE PYTHON IN 24 HOURS

2. Classes and Objects – Basic Building Blocks

Classes and objects are the foundation of Object-Oriented Programming in Python. A class is like a blueprint, and an object is the actual thing built from that blueprint.

2.1 Creating Classes (class keyword)

A class is defined using the class keyword followed by the class name (use CamelCase by convention).

Basic syntax:

Python

class ClassName: # class body (attributes and methods go here) pass

Simple example – Empty class

Python

class Car: pass # empty class (placeholder)

With a comment (good practice)

Python

class Student: """This class represents a student in school.""" pass

Naming convention (PEP 8):

• Use CamelCase (CapitalizeEachWord)

• Meaningful names: BankAccount, LibraryBook, EmployeeProfile

2.2 Creating Objects (Instances)

An object (or instance) is created by calling the class name like a function.

Syntax:

Python

object_name = ClassName(arguments_if_any)

Example

Python

class Car: pass # Creating objects car1 = Car() # object 1 car2 = Car() # object 2 print(car1) # <__main__.Car object at 0x...> print(car2) # different memory address print(car1 == car2) # False – different objects

Analogy: Car is the blueprint (class). car1 and car2 are two actual cars built from the same blueprint.

2.3 The init Method and self Parameter

init is a special method (constructor) that runs automatically when an object is created. It is used to initialize (set up) the object’s attributes.

self is a reference to the current object (instance). Python automatically passes it — you don’t need to send it.

Example – Initialize attributes

Python

class Car: def init(self, brand, color, year): self.brand = brand # instance attribute self.color = color self.year = year def drive(self): return f"{self.color} {self.brand} ({self.year}) is driving!" # Create objects with data car1 = Car("Maruti", "Red", 2025) car2 = Car("Honda", "Blue", 2024) print(car1.drive()) # Red Maruti (2025) is driving! print(car2.drive()) # Blue Honda (2024) is driving!

Key points about self:

• self is just a convention — you can name it anything (but never do!)

• First parameter of every instance method must be self

• Use self.attribute to access or set instance-specific data

2.4 Instance Variables vs Class Variables

TypeDefined where?Shared by all objects?Example in codeUse caseInstance VariableInside init (with self)No – unique per objectself.brand = "Maruti"Data specific to each object (color, year)Class VariableDirectly inside class bodyYes – shared by allwheels = 4 (outside any method)Common properties for all instances

Code Example

Python

class Car: wheels = 4 # class variable (shared) def init(self, brand, color): self.brand = brand # instance variable (unique) self.color = color car1 = Car("Maruti", "Red") car2 = Car("Honda", "Blue") print(car1.wheels) # 4 print(car2.wheels) # 4 print(Car.wheels) # 4 (access via class) Car.wheels = 6 # changes for all objects print(car1.wheels) # 6 print(car2.wheels) # 6 car1.wheels = 8 # creates instance variable for car1 only print(car1.wheels) # 8 (car1 has its own) print(car2.wheels) # 6 (car2 still uses class variable)

Rule: Use class variables for constants/shared data. Use instance variables for data unique to each object.

2.5 str and repr – Beautifully Printing Objects

By default, printing an object shows memory address — not useful.

Use special methods to control how objects are printed:

• str → human-readable string (used by print())

• repr → developer-friendly, unambiguous (used in REPL/debugger)

Example

Python

class Car: def init(self, brand, color, year): self.brand = brand self.color = color self.year = year def str(self): return f"{self.color} {self.brand} ({self.year})" def repr(self): return f"Car(brand='{self.brand}', color='{self.color}', year={self.year})" car = Car("Maruti", "Red", 2025) print(car) # Red Maruti (2025) ← str print(repr(car)) # Car(brand='Maruti', color='Red', year=2025) ← repr

Best practice:

• Always implement both

• str for users/readability

• repr for debugging (should ideally allow recreating the object)

Mini Summary Project – Student Class

Python

class Student: school = "XYZ International School" # class variable def init(self, name, roll_no, marks): self.name = name self.roll_no = roll_no self.marks = marks def str(self): return f"{self.name} (Roll: {self.roll_no}) - Marks: {self.marks}%" def get_grade(self): if self.marks >= 90: return "A+" elif self.marks >= 80: return "A" else: return "B or below" s1 = Student("Anshuman", 101, 92) s2 = Student("Rahul", 102, 85) print(s1) # Anshuman (Roll: 101) - Marks: 92% print(s1.school) # XYZ International School print(s1.get_grade()) # A+

This completes the full Classes and Objects – Basic Building Blocks section — the heart of OOP in Python!

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