Raising Custom Exceptions

This snippet shows how to define and raise custom exceptions in Python. Custom exceptions allow you to create specific error types tailored to your application's needs, improving code clarity and maintainability.

Code Example

We define a custom exception class InsufficientFundsError that inherits from the built-in Exception class. The __init__ method initializes the exception with a custom error message. The withdraw function checks if the withdrawal amount exceeds the balance. If it does, it raises the InsufficientFundsError with a descriptive message. The try-except block catches the custom exception and prints the error message.

class InsufficientFundsError(Exception):
    def __init__(self, message):
        super().__init__(message)
        self.message = message


def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError("Insufficient funds in your account.")
    new_balance = balance - amount
    print("Withdrawal successful. New balance:", new_balance)
    return new_balance


balance = 100
try:
    balance = withdraw(balance, 150)
except InsufficientFundsError as e:
    print("Error:", e.message)

# Error: Insufficient funds in your account.

Concepts Behind the Snippet

Custom exceptions provide a way to signal specific error conditions within your application domain. By creating meaningful exception types, you can make your code more readable and easier to debug. Inheriting from the base Exception class ensures that your custom exceptions behave like standard Python exceptions.

Real-Life Use Case

In an e-commerce application, you might define custom exceptions like ProductNotFoundError, InvalidOrderError, or PaymentFailedError to handle specific problems during product retrieval, order processing, and payment transactions.

Best Practices

• Choose descriptive names for your custom exception classes that clearly indicate the error condition they represent.
• Include informative error messages that help users understand the problem and take appropriate action.
• Document your custom exceptions so that other developers understand when and how to use them.

Interview Tip

Be prepared to explain the benefits of using custom exceptions over generic exceptions. Demonstrate your understanding of how to define and raise custom exceptions in Python.

When to Use Them

Use custom exceptions when you need to signal specific error conditions that are unique to your application or domain. This helps to make your code more expressive and easier to understand.

Alternatives

You could use generic exceptions and rely on error codes or messages to differentiate between different error conditions. However, custom exceptions provide a more structured and type-safe approach.

Pros

• Improved code readability and maintainability.
• More specific error handling.
• Better type safety.

Cons

• Requires more code to define custom exception classes.