Python - Constructors (Complete Guide for Beginners)

In Python Object-Oriented Programming (OOP), a constructor is a special method that is automatically called when an object is created. Constructors are used to initialize object attributes and perform setup tasks when a new instance of a class is created.

Python uses the __init__() method as its constructor.

In this tutorial, you will learn everything about Python constructors with practical examples.

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What is a Constructor?

A constructor is a special method that:

• Runs automatically when an object is created
• Initializes object attributes
• Helps set default values
• Reduces repetitive code

The constructor method in Python is:

__init__()

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Why Use Constructors?

Without constructors, you would need to manually assign values after creating objects.

Constructors help:

• Initialize data automatically
• Improve code readability
• Create objects efficiently
• Ensure every object starts with required values

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Basic Constructor Example

Syntax

class ClassName:

    def __init__(self):
        pass

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Creating Your First Constructor

Example

class Student:

    def __init__(self):
        print("Student object created")

s1 = Student()

Output

Student object created

The constructor runs automatically when the object is created.

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Constructor with Parameters

Most constructors accept parameters to initialize object data.

Example

class Student:

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

s1 = Student("John")

print(s1.name)

Output

John

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Multiple Parameters in Constructor

Example

class Student:

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

s1 = Student("John", 20)

print(s1.name)
print(s1.age)

Output

John
20

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Understanding self in Constructors

The self parameter refers to the current object.

Example

class Car:

    def __init__(self, brand):
        self.brand = brand

car1 = Car("Toyota")

print(car1.brand)

Output

Toyota

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Creating Multiple Objects

Each object gets its own copy of instance attributes.

Example

class Employee:

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

e1 = Employee("Ali")
e2 = Employee("Sara")

print(e1.name)
print(e2.name)

Output

Ali
Sara

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Constructor with Default Values

You can provide default parameter values.

Example

class User:

    def __init__(self, name="Guest"):
        self.name = name

u1 = User()
u2 = User("John")

print(u1.name)
print(u2.name)

Output

Guest
John

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Constructor Initializing Multiple Attributes

Example

class Product:

    def __init__(self, name, price, stock):
        self.name = name
        self.price = price
        self.stock = stock

product = Product("Laptop", 800, 10)

print(product.name)
print(product.price)
print(product.stock)

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Constructor and Methods Together

Example

class Student:

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

    def display(self):
        print("Name:", self.name)
        print("Marks:", self.marks)

s1 = Student("John", 90)

s1.display()

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Real-World Example: Bank Account

class BankAccount:

    def __init__(self, owner, balance):
        self.owner = owner
        self.balance = balance

    def show_balance(self):
        print("Balance:", self.balance)

account = BankAccount("John", 1000)

account.show_balance()

Output

Balance: 1000

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Real-World Example: Car Management System

class Car:

    def __init__(self, brand, model, year):
        self.brand = brand
        self.model = model
        self.year = year

    def show_info(self):
        print(self.brand, self.model, self.year)

car1 = Car("Toyota", "Corolla", 2025)

car1.show_info()

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Constructor with Calculations

Example

class Rectangle:

    def __init__(self, length, width):
        self.length = length
        self.width = width
        self.area = length * width

rect = Rectangle(10, 5)

print(rect.area)

Output

50

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Difference Between Constructor and Normal Method

Constructor	Normal Method
Uses __init__()	Uses any method name
Called automatically	Called manually
Initializes objects	Performs actions
Runs once during creation	Runs whenever called

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Common Mistakes

Mistake 1: Misspelling init

❌ Wrong

class Test:

    def init(self):
        pass

✔ Correct

class Test:

    def __init__(self):
        pass

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Mistake 2: Forgetting self

❌ Wrong

def __init__(name):
    pass

✔ Correct

def __init__(self, name):
    pass

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Mistake 3: Wrong Number of Arguments

❌ Wrong

student = Student()

When constructor requires parameters.

✔ Correct

student = Student("John")

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Best Practices

• Always use meaningful parameter names.
• Initialize all required attributes in the constructor.
• Keep constructors simple.
• Avoid putting complex business logic inside constructors.
• Use default values when appropriate.

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Constructor Summary

Feature	Description
Method Name	__init__()
Purpose	Initialize objects
Called	Automatically
Uses	self
Accepts Parameters	Yes
Creates Attributes	Yes

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Practical Example: Library System

class Book:

    def __init__(self, title, author):
        self.title = title
        self.author = author

    def display(self):
        print("Title:", self.title)
        print("Author:", self.author)

book1 = Book("Python Basics", "John Smith")

book1.display()

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Conclusion

Constructors are one of the most important features of Python OOP. They help initialize objects automatically and ensure that every object starts with the required data.

You learned:

• What constructors are
• How __init__() works
• Constructors with parameters
• Default values
• Real-world examples
• Common mistakes and best practices

Understanding constructors is essential for building professional and scalable Python applications.

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Practice Exercises

Exercise 1

Create a Student class with a constructor that stores:

• Name
• Age
• Grade

Display all values.

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Exercise 2

Create a Car class with:

• Brand
• Model
• Year

Add a method to display car information.

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Exercise 3

Create a Rectangle class whose constructor calculates and stores the area.