PythonConstructors and `__init__`

Constructors and __init__ in Python

In Python, a constructor is a special method that is automatically called when an object is created. It allows you to initialize the object’s attributes (state).

In Python, the constructor method is named:

__init__()

What is __init__?

  • __init__ is the initializer or constructor.

  • It is called when you create a new object from a class.

  • You can use it to:

    • Define object attributes
    • Set default values
    • Run startup code when an object is created

Syntax of __init__

constructor_syntax.py
class ClassName:
    def __init__(self, parameters):
        self.attribute = value

Example 1: Basic Constructor

example1_basic_constructor.py
class Person:
    def __init__(self, name):
        self.name = name
 
    def greet(self):
        print(f"Hello, my name is {self.name}.")
 
p1 = Person("Alice")
p1.greet()
output.txt
Hello, my name is Alice.

Example 2: Multiple Parameters in Constructor

example2_multiple_params.py
class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model
 
    def display(self):
        print(f"Car: {self.brand} {self.model}")
 
car1 = Car("Tesla", "Model S")
car1.display()
output.txt
Car: Tesla Model S

Example 3: Constructor with Default Values

example3_default_values.py
class Student:
    def __init__(self, name, grade="A"):
        self.name = name
        self.grade = grade
 
    def show(self):
        print(f"Student: {self.name}, Grade: {self.grade}")
 
s1 = Student("John")
s2 = Student("Sara", "B")
 
s1.show()
s2.show()
output.txt
Student: John, Grade: A
Student: Sara, Grade: B

Example 4: Constructor Logic with Calculations

example4_constructor_logic.py
class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = self.width * self.height
 
    def show_area(self):
        print(f"Area of rectangle: {self.area}")
 
r1 = Rectangle(10, 5)
r1.show_area()
output.txt
Area of rectangle: 50

Example 5: Constructor for Bank Account Object

example5_bank_account.py
class BankAccount:
    def __init__(self, holder_name, initial_balance=0):
        self.holder_name = holder_name
        self.balance = initial_balance
 
    def show_balance(self):
        print(f"{self.holder_name}'s balance is ₹{self.balance}")
 
acc = BankAccount("Emma", 5000)
acc.show_balance()
output.txt
Emma's balance is ₹5000

Key Points about __init__

It is automatically called when an object is created. The first parameter is always self — refers to the current object. You can pass parameters to initialize attributes. You can set default values. You can perform logic or validations inside __init__.

Summary

  • __init__ is Python’s constructor.

  • It allows you to initialize attributes when creating objects.

  • You can define:

    • Multiple attributes
    • Default values
    • Logic on object creation

  • Constructors help ensure objects are properly initialized with valid data.

Next, we will cover Inheritance — a powerful feature that allows classes to reuse and extend functionality from other classes.

Classes and ObjectsInheritance