Add C# += / -= event subscriptions to Python using the Events package

watch_this.py

import functools

from events import Events  # Note you must pip install events


class set_event:
    """Define a class-based setter method decorator."""

    def __init__(self, func, event):
        super().__init__()
        functools.update_wrapper(self, func)
        self._func = func
        self._event = event
        self._last = None

    def __call__(self, *args, **kwargs):
        if len(args) == 2:
            attr = args[1]
        elif "value" in kwargs:
            attr = kwargs["value"]
        else:
            raise RuntimeError("Not a simple property assignment.")
        self._func(*args, **kwargs)
        if attr != self._last:
            self._event(args[0])
        self._last = attr


def add_events(fields: dict):
    """Define a class decorator with arguments."""

    def decorator(klass):

        original_init = klass.__init__

        @functools.wraps(original_init)
        def init(self, *args, **kwargs):
            original_init(self, *args, **kwargs)
            self.__dict__["_events"] = events = Events(tuple(fields.values()))
            for event_name in fields.values():
                self.__dict__[event_name] = getattr(events, event_name)
            for field, event_name in fields.items():
                class_field = type(self).__dict__[field]
                # Attempt to decorate the instance property...
                self.__dict__[field] = property(
                    class_field.fget,
                    set_event(class_field.fset, getattr(events, event_name)),
                    class_field.fdel,
                )

        klass.__init__ = init

        return klass

    return decorator


@add_events(
    {"age": "on_age_changed", "name": "on_name_changed", "city": "on_location_changed"}
)
class Person:
    def __init__(self, name: str, age: int, city: str):
        self.__name = name
        self.__age = age
        self.__city = city

    @property
    def age(self):
        return self.__age

    @age.setter
    def age(self, value):
        self.__age = value

    @property
    def name(self):
        return self.__name

    @name.setter
    def name(self, value):
        self.__name = value

    @property
    def city(self):
        return self.__city

    @city.setter
    def city(self, value):
        self.__city = value


def main():
    p1 = Person("Michael", 47, "Portland")
    p2 = Person("Zoe", 50, "Orlando")
    p1.on_name_changed += lambda p: print(f"Person changed name to {p.name}.")
    p1.on_location_changed += location_changed
    p2.on_location_changed += location_changed

    # Two moves for Zoe
    p2.city = "Seattle"
    p2.city = "LA"
    # One move for Michael
    p1.city = "Vancouver"

    # One name change for Michael to Mike
    p1.name = "Mike"

    # No longer watching for moves for Michael
    p1.on_location_changed -= location_changed
    p1.city = "Portland"


def location_changed(person: Person):
    print(f"Looks like {person.name} has moved to {person.city}")


if __name__ == "__main__":
    main()