Rules¶

Rules encapsulate reusable, composable predicates for validation and business logic.

ValidationRule¶

ValidationRule is an abstract base class for synchronous validation rules. Each rule inspects an arbitrary value and returns a list of RuleViolationError objects — empty when the value is valid.

Contract¶

class ValidationRule(ABC):
    @abstractmethod
    def validate(self, value: Any) -> list[RuleViolationError]:
        ...

Characteristics¶

Stateless — rules carry no mutable state and can be reused safely across threads and coroutines
Side-effect-free — validate must not perform I/O or mutate the input
Composable — rules can be combined into composite chains that report every failure

When to use¶

Subclass ValidationRule for any synchronous validation check. Return RuleViolationError instances carrying ErrorMessage and ErrorMetadata with a field name and error code. Compose multiple rules with CompositeValidationRule to validate several constraints at once.