NotificationHandler

NotificationHandler defines one subscriber for an Notification type. Several handler classes may subscribe to the same notification.

from abc import ABC, abstractmethod

from pymediate import Notification, NotificationHandler

class NotificationHandler[NotificationT: Notification](ABC):
    @abstractmethod
    async def __call__(self, notification: NotificationT) -> None: ...

Definition-time validation

When Python defines a notification-handler subclass, PyMediate checks that:

• NotificationT is an Notification subclass;
• the subclass defines __call__ with exactly one parameter besides self;
• that parameter is annotated with the exact NotificationT class;
• the return annotation is None; and
• __call__ is async def for pymediate.NotificationHandler and plain def for pymediate.sync.NotificationHandler.

Validation can raise InvalidNotificationTypeError or InvalidHandlerSignatureError. Valid handler classes are recorded in class-definition order. Unlike request handlers, notification handlers do not have a one-handler-per-type restriction.

The asynchronous and synchronous handler classes write to the same process-wide subscription list. Every handler for one exact notification type must use one API; do not mix pymediate.NotificationHandler and pymediate.sync.NotificationHandler for that notification. The asynchronous mediator expects awaitable handlers, while the synchronous mediator calls handlers directly.

Example

from pymediate import NotificationHandler


class RecordOrderMetric(NotificationHandler[OrderPlaced]):
    async def __call__(self, notification: OrderPlaced) -> None:
        print(f"recorded order {notification.order_id}")

Register a RecordOrderMetric instance with the service provider used by the mediator before publishing OrderPlaced.

Introspection method

get_notification_type() returns the declared notification class, or None on an unparameterized base.

See also

• Notification — the published message
• Mediator.publish() — resolves and invokes subscribers
• Notifications — concurrency, ordering, and failure handling
• Errors — validation and dispatch errors