The Basics of Exception Handling

Before we venture into exception handling in classes, let's refresh our understanding of the fundamental concepts of exception handling in Python:

try:

# Code that may trigger an exception

result = 10 / 0

except ZeroDivisionError:

# Handle the specific exception

print("Cannot divide by zero!")

except Exception as e:

# Handle any other exceptions

print(f"An error occurred: {e}")

else:

# Code that executes if no exceptions were raised

print(f"The result is: {result}")

finally:

# Code that always executes, regardless of exceptions

print("This will always execute.")

The try block encompasses the code that may raise an exception, while the except blocks are designated for handling specific exceptions. The else block runs if no exceptions occur, and the finally block executes at the end, irrespective of whether an exception was raised.

Handling Exceptions in Classes

Next, let's explore how to manage exceptions within Python classes. We will design a simple BankAccount class to address potential exceptions during fund withdrawals or deposits.

class BankAccount:

def __init__(self, balance=0):

self.balance = balance

def withdraw(self, amount):

try:

if amount > self.balance:

raise ValueError("Insufficient funds")

self.balance -= amount

print(f"Withdrew {amount}. New balance: {self.balance}")

except ValueError as e:

print(f"Error: {e}")

def deposit(self, amount):

try:

if amount <= 0:

raise ValueError("Amount must be positive")

self.balance += amount

print(f"Deposited {amount}. New balance: {self.balance}")

except ValueError as e:

print(f"Error: {e}")

In this example, the withdraw method checks if the requested withdrawal exceeds the account balance, raising a ValueError if it does. The deposit method ensures that the deposited amount is positive, raising a ValueError otherwise.

Here’s how you can utilize this class:

account = BankAccount(1000)

account.withdraw(500) # Withdrew 500. New balance: 500.0

account.deposit(-200) # Error: Amount must be positive

account.withdraw(700) # Error: Insufficient funds

Handling Multiple Exceptions

At times, it may be necessary to manage various exception types within the same try block. This can be accomplished by adding multiple except clauses or by using a tuple to denote multiple exception types.

try:

# Code that might raise an exception

result = int("not a number")

except (ValueError, TypeError) as e:

# Handle both ValueError and TypeError

print(f"An error occurred: {e}")

In this case, the except clause will capture both ValueError and TypeError exceptions.

Raising Custom Exceptions

Python also permits the creation and raising of your own custom exceptions, which can be beneficial when addressing particular error conditions in your code.

class NegativeAmountError(Exception):

pass

class BankAccount:

def __init__(self, balance=0):

self.balance = balance

def withdraw(self, amount):

try:

if amount < 0:

raise NegativeAmountError("Cannot withdraw a negative amount")

if amount > self.balance:

raise ValueError("Insufficient funds")

self.balance -= amount

print(f"Withdrew {amount}. New balance: {self.balance}")

except NegativeAmountError as e:

print(f"Error: {e}")

except ValueError as e:

print(f"Error: {e}")

In this revised BankAccount class, we introduce a custom NegativeAmountError exception, which is raised if the withdrawal amount is negative. This custom exception is then handled separately from the built-in ValueError.