C#-Style Event Handling Mechanism for Python

C# provides a very simple syntax using the observer pattern for its event handling system. The aim of this project is to implement the pattern in python as similarly as possible.

In C#, an "event" is a field or a property of the delegate type EventHandler. Since delegates in C# can be combined and removed with += and -= operators, event handlers can easily subscribe to or unsubscribe from the event using those operators.

Python does not support an addition of two Callable types. So the Event[**TArgs] class is provided to mimic delegates:

from events import Event

item_changed = Event[str, object]()

C# naming convention prefers present/past participles (changing/changed) instead of on+infinitive (on_change) for events.

Handlers can subscribe to and unsubscribe from the event with the same syntax:

def item_changed_handler(key: str, value: object) -> None:
    ...

item_changed += item_changed_handler
item_changed -= item_changed_handler

An event can be raised by simply invoking it with the arguments:

item_changed("info", obj)

Since Event acts just like a delegate from C#, it is not required to be bound to a class or an instance object. This is the major difference to other packages that try to implement the C#-style event system, which usually revolve around a container object for events.

An example class with event fields may look like this:

class EventExample:
    def __init__(self) -> None:
        self.__value = ""
        self.updated: Event[str] = Event()

    def update(self, value: str) -> None:
        if self.__value != value:
            self.__value = value
            self.updated(value)

obj = EventExample()
obj.updated += lambda value: print(f"obj.{value=}")
obj.update("new value")

A class decorator @events is provided as a shortcut for event fields:

from events import Event, events

@events
class EventFieldsExample:
    item_added: Event[object]
    item_removed: Event[object]
    item_updated: Event[str, object]

C# also provides event properties with add and remove accessors:

public event EventHandler<ItemChangedEventArgs> ItemChanged
{
    add { ... }
    remove { ... }
}

This feature is useful for classes that do not actually own the events, but need to forward the subscriptions to the underlying object that do own the events.

The @event[**TArgs] decorator is provided to support this feature:

from events import accessors, event, EventHandler

class EventPropertyExample:
    @event[str, object]
    def item_changed():
        def add(self: Self, value: EventHandler[str, object]) -> None: ...
        def remove(self: Self, value: EventHandler[str, object]) -> None: ...
        return (add, remove)

Note: This is an experimental syntax, and may change in later versions.

Furthermore, the EventHandlerCollection interface is provided to support the functionalities of System.ComponentModel.EventHandlerList class from C#, along with the two implementations EventHandlerList and EventHandlerDict using a linked list and a dictionary respectively. The behaviour of EventHandlerList is exactly the same as of its counterpart from C#.

A typical usage of EventHandlerList in C# can be translated directly into the python code:

class EventPropertyExample:
    def __init__(self) -> None:
        self.__events = EventHandlerList()

    @event  # [str, object] is inferred
    def item_changed():
        def add(self: Self, value: EventHandler[str, object]) -> None:
            self.__events.add_handler("item_changed", value)

        def remove(self: Self, value: EventHandler[str, object]) -> None:
            self.__events.remove_handler("item_changed", value)

        return (add, remove)

    def _on_item_changed(self, key: str, value: object) -> None:
        handler = self.__events["item_changed"]
        if handler:
            handler(key, value)

The class decorator @events also provides a shortcut for event properties. The above code can be shortened to:

@events(collection="__events")
class EventPropertyExample:
    item_changed: event[str, object]

    def __init__(self) -> None:
        self.__events = EventHandlerList()

    def _on_item_changed(self, key: str, value: object) -> None:
        self.__events.invoke("item_changed", key, value)

Installation

Install using pip:

pip install cs-events