Event emitter in python

event-emitter.py

from collections import defaultdict
from typing import Callable, Optional

HandlerFunction = Callable[..., None]


class EventHandler:
    def __init__(self):
        self.handlers = defaultdict(list)

    def on(self, event: str) -> Callable[[HandlerFunction], HandlerFunction]:
        def register_handler(handler: HandlerFunction) -> HandlerFunction:
            self.handlers[event].append(handler)

            return handler

        return register_handler

    def once(self, event: str) -> Callable[[HandlerFunction], HandlerFunction]:
        def register_handler(handler: HandlerFunction) -> HandlerFunction:
            def wrapper(*args, **kwargs) -> None:
                handler(*args, **kwargs)

                self.off(event, wrapper)

            self.handlers[event].append(wrapper)

            return handler

        return register_handler

    def off(self, event: str, handler: Optional[HandlerFunction] = None) -> None:
        if handler is None:
            self.handlers[event] = []
        else:
            self.handlers[event].remove(handler)

    def emit(self, event: str, *args, **kwargs) -> None:
        for handler in self.handlers[event]:
            handler(*args, **kwargs)


if __name__ == "__main__":
    event_handler = EventHandler()

    @event_handler.on("test")
    def test_handler_add(a: int, b: int) -> None:
        print(a + b)

    @event_handler.once("test")
    def test_handler_sub(a: int, b: int) -> None:
        print(a - b)

    # Both should run
    event_handler.emit("test", 1, 2)

    # After running the `once`
    event_handler.emit("test", 1, 2)

    # None should run
    event_handler.off("test", test_handler_add)

    event_handler.emit("test", 1, 2)