custom-exceptions.md

Python by Example: Custom Exceptions

When built-in exceptions aren't specific enough, define your own. Inherit from Exception (or a subclass like ValueError) and add attributes for extra context. Callers can then catch your exception specifically and access the details. Custom exceptions make error handling clearer and more maintainable.

What you'll learn:

• Defining custom exception classes
• Adding attributes for context
• Raising and catching them

class ValidationError(Exception):
    """Raised when input validation fails."""

    def __init__(self, message, field=None):
        super().__init__(message)
        self.field = field


def validate_age(age):
    if not isinstance(age, int) or age < 0 or age > 150:
        raise ValidationError("Invalid age", field="age")
    return age


try:
    validate_age(-5)
except ValidationError as e:
    print(f"Error: {e}")
    print(f"Field: {e.field}")

Call super().__init__(message) so the base Exception stores the message. Add custom attributes like field for callers to use. Name exceptions with an Error suffix.

To run this program:

$ python source/custom-exceptions.py
Error: Invalid age
Field: age

Tip: Inherit from the most specific built-in that fits—e.g., ValueError for bad values, RuntimeError for runtime conditions.

Try it: Create a FileNotFoundError-style exception that includes the filename.

Source: custom-exceptions.py

Next: Modules