How to create a custom exception?

Custom exceptions are created by inheriting from the Exception class (or its subclasses).

Simple custom exception:

class InsufficientFundsError(Exception):
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError("Insufficient funds")
    return balance - amount

try:
    withdraw(100, 200)
except InsufficientFundsError as e:
    print(e)  # Insufficient funds

Exception with additional data:

class ValidationError(Exception):
    def __init__(self, field, message):
        self.field = field
        self.message = message
        super().__init__(f"{field}: {message}")

try:
    raise ValidationError("email", "Invalid format")
except ValidationError as e:
    print(e.field)    # email
    print(e.message)  # Invalid format

When to create custom exceptions:

• When built-in exceptions don't describe the error accurately enough.
• To separate business logic errors (e.g., UserNotFoundError, PermissionDeniedError).
• To easily catch a group of related errors using a common base class.

class AppError(Exception):
    """Base application exception"""
    pass

class NotFoundError(AppError):
    pass

class AccessDeniedError(AppError):
    pass