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Instance variables and methods

Instance variables and methods

In this course, we’ll dive into the exciting world of Python programming and explore the concepts of instance variables and methods. These are fundamental building blocks in Python that allow us to create dynamic and interactive programs. We’ll start from scratch, assuming no prior knowledge, and gradually build up our understanding through clear explanations, fun examples, and engaging practice questions.

1. Introduction to Objects:

  • Objects: In programming, an object is a collection of data (variables) and methods (functions) that act on the data. Objects are used to model real-world entities and concepts in a program.
  • Classes: A class is like a blueprint or template for creating objects. It defines the structure and behavior of objects of that type.

2. Instance Variables:

  • Instance Variables: Also known as attributes, instance variables are unique to each object of a class. They store data that is specific to each instance of the class.
  • Declaration and Initialization: To create an instance variable, you declare it within the class definition and initialize it using the special method __init__() (constructor) when creating an object.
  • Role of Instance Variables: Instance variables hold information that distinguishes one object from another, allowing objects of the same class to have different states.
  • Let’s see an example:

class Car:
    def __init__(self, brand, model):
        self.brand = brand  # Instance variable
        self.model = model  # Instance variable

# Creating instances of Car
car1 = Car("Toyota", "Camry")
car2 = Car("Honda", "Accord")

print(car1.brand)  # Output: Toyota
print(car2.model)  # Output: Accord

3. Instance Methods:

  • Instance Methods: Instance methods are functions defined within a class that operate on instance variables. They can access and manipulate the data stored in instance variables.
  • Declaration and Calling: Instance methods are defined like regular functions within the class definition, with the first parameter conventionally named self. They are called on objects of the class using dot notation.
  • Manipulating Instance Variables: Instance methods often work with instance variables to perform specific actions or calculations based on the object’s state.
  • Let’s define a class with an instance method:

class Person:
    def __init__(self, name): = name  # Instance variable

    def greet(self):
        return f"Hello, my name is {}"

# Creating instances of Person
person1 = Person("Alice")
person2 = Person("Bob")

print(person1.greet())  # Output: Hello, my name is Alice
print(person2.greet())  # Output: Hello, my name is Bob

Practice Questions:

  1. Question 1: Write a Python class Rectangle with instance variables length and width. Define an instance method calculate_area() that returns the area of the rectangle.

class Rectangle:
    def __init__(self, length, width):
        self.length = length
        self.width = width

    def calculate_area(self):
        return self.length * self.width

# Test the class
rectangle1 = Rectangle(5, 4)
print("Area of rectangle:", rectangle1.calculate_area())  # Output: 20

  1. Question 2: Create a Python class BankAccount with instance variables balance and account_number. Define instance methods deposit(amount) and withdraw(amount) to deposit and withdraw money from the account. Ensure that the balance cannot go below zero during a withdrawal.

class BankAccount:
    def __init__(self, account_number, initial_balance=0):
        self.account_number = account_number
        self.balance = initial_balance

    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        if self.balance >= amount:
            self.balance -= amount
            return amount
            print("Insufficient funds!")

# Test the class
account1 = BankAccount("12345", 100)
print("Initial balance:", account1.balance)  # Output: 100

print("After deposit:", account1.balance)  # Output: 150

withdrawn_amount = account1.withdraw(70)
print("Withdrawn amount:", withdrawn_amount)  # Output: 70
print("After withdrawal:", account1.balance)  # Output: 80

account1.withdraw(1000)  # Output: Insufficient funds!

  1. Question 3:
# Create a class called 'Dog' with an instance variable 'name'.
# Initialize the 'name' variable with a value "Buddy".
# Define an instance method 'bark()' that prints "Woof! My name is <name>."

# Your code here

class Dog:
    def __init__(self, name): = name

    def bark(self):
        print("Woof! My name is",

# Test the class
my_dog = Dog("Buddy")
my_dog.bark()  # Output: Woof! My name is Buddy

  1. Question 4:
# Create a class called 'Circle' with instance variables 'radius' and 'color'.
# Initialize 'radius' with a value 5 and 'color' with "red".
# Define an instance method 'area()' that calculates and returns the area of the circle.

# Your code here

import math

class Circle:
    def __init__(self, radius, color):
        self.radius = radius
        self.color = color

    def area(self):
        return math.pi * self.radius ** 2

# Test the class
my_circle = Circle(5, "red")
print("Area of the circle:", my_circle.area())  # Output: Area of the circle: 78.53981633974483

By the end of this course, you will have a solid understanding of instance variables and methods in Python. You’ll be able to create your own classes, define instance variables to store data unique to each object, and implement instance methods to perform specific actions within your programs. With practice, you’ll become proficient in using these fundamental concepts to build more complex and interactive Python applications.

Now, let’s dive into the exciting world of Python programming and unleash your creativity!