Class and Instance Example

In this lesson, let's explore the relationship between Class and Instance (or object) using a "bank account" as an example.

The code below defines a 'BankAccount' class and shows how to create multiple account instances to manage each account's unique attributes and behaviors.

The BankAccount class represents a bank account and includes the following attributes and methods:

• Attributes: owner, balance

• Methods: deposit, withdraw, display_balance

The (__init__) is a constructor method called when an account instance is created from the class, setting the account object’s owner and initial balance.

The balance has a default value of 0 (balance=0).

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class BankAccount:
    # Constructor method
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    # Deposit method
    def deposit(self, amount):
        self.balance += amount
        print(f"{amount} has been deposited.")

    # Withdraw method
    def withdraw(self, amount):
        if self.balance >= amount:
            self.balance -= amount
            print(f"{amount} has been withdrawn.")
        else:
            print("Insufficient balance.")

    # Balance inquiry method
    def display_balance(self):
        print(f"Account balance: {self.balance}")

Below is an example of using the BankAccount class to create account instances account1 and account2, and performing deposit, withdrawal, and balance inquiry for each account.

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# Instance creation
account1 = BankAccount("CodeFriends", 1000)
account2 = BankAccount("GeekHouse", 2000)

# Depositing, withdrawing, and checking balance for account1
account1.deposit(500)
# 500 has been deposited.
account1.withdraw(200)
# 200 has been withdrawn.
account1.display_balance()
# Account balance: 1300

# Depositing, withdrawing, and checking balance for account2
account2.deposit(1000)
# 1000 has been deposited.
account2.withdraw(500)
# 500 has been withdrawn.
account2.display_balance()
# Account balance: 2500

Each account (account1, account2) is an instance of the BankAccount class, having independent attributes (owner, balance) and methods (deposit, withdraw, display_balance).

Instances created from a class maintain their own data, and even when using the same methods, each instance can produce different results.

For example, calling the display_balance method on the account results in a balance of 1300, while calling the same method on account2 results in a balance of 1500.

In Python, the self keyword refers to the current instance within a class method.

When defining a method within a class, the first parameter is self, allowing the method to refer to the current instance.

The self keyword serves two primary roles:

• Accessing Instance Attributes: self is used to access and modify the attributes of the current instance within a method. For example, self.balance refers to the balance attribute of the current instance.

• Method Calls: self is used to call other methods within the same instance. For example, self.deposit(amount) calls the deposit method within the same instance.