Custom Exceptions

As your Python projects grow in size and complexity, you will encounter situations where the standard exceptions, such as KeyError or IndexError, are too general for your application's needs. When you want to signal a specific error that is unique to your program's logic—such as an invalid user operation, a failed business rule, or a domain-specific constraint—a custom exception provides the right tool.

Custom exceptions allow you to:

• Clearly communicate the nature of an error by giving it a descriptive name;
• Make your codebase easier to debug by pinpointing exactly where and why a problem occurred;
• Separate different types of errors for more precise handling in your error management strategy;
• Document the expected error conditions in your application, making your code more maintainable and understandable.

By defining your own exceptions, you create a robust foundation for error handling that adapts to the unique requirements of larger, real-world Python programs.

How to Raise and Catch a Custom Exception

You raise a custom exception using the raise statement when a specific condition is met. To handle the exception, use a try-except block that targets your custom exception class.

Example:

Suppose you want to prevent an account balance from going negative. You define a custom exception called NegativeBalanceError. When a withdrawal would cause a negative balance, you raise this exception. Then you catch it to display a clear message to the user.

class NegativeBalanceError(Exception):
    """Raised when an operation would result in a negative account balance."""
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise NegativeBalanceError("Withdrawal would result in a negative balance.")
    return balance - amount

try:
    new_balance = withdraw(100, 150)
except NegativeBalanceError as e:
    print(f"Error: {e}")

Key points:

• Use the raise statement to signal an error with your custom exception;
• Catch the custom exception with an except block to control how your program responds;
• Provide a helpful message to make debugging and user feedback easier.