Raising and Defining Exceptions

You can raise exceptions yourself with raise when something is wrong with the input. This is how good functions reject bad data loudly instead of returning a confusing result.

def set_age(age):
    if age < 0:
        raise ValueError("age cannot be negative")
    return age

For your own libraries it is often clearer to define a custom exception. You do that by subclassing the built-in Exception class. Even with no body it is fully usable:

class ValidationError(Exception):
    pass

def register(name):
    if not name:
        raise ValidationError("name is required")
    return name.upper()

Callers can then catch your specific type:

try:
    register("")
except ValidationError as e:
    print(f"rejected: {e}")

A custom type makes your errors self-documenting and lets callers handle your failures separately from built-in ones. Because it inherits from Exception, a broad except Exception still catches it too.