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 ofon+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_handlerAn 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 and the accessors[**TArgs] type are provided to support this feature:
from events import accessors, event, EventHandler
class EventPropertyExample:
@event[str, object]
def item_changed() -> accessors[str, object]:
def add(self: Self, value: EventHandler[str, object]) -> None: ...
def remove(self: Self, value: EventHandler[str, object]) -> None: ...
return (add, remove)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:
__event_item_changed: Final = object()
def __init__(self) -> None:
self.__events = EventHandlerList()
@event # [str, object] is inferred
def item_changed(): # -> accessors[str, object] is inferred
def add(self: Self, value: EventHandler[str, object]) -> None:
self.__events.add_handler(self.__event_item_changed, value)
def remove(self: Self, value: EventHandler[str, object]) -> None:
self.__events.remove_handler(self.__event_item_changed, value)
return (add, remove)
def _on_item_changed(self, key: str, value: object) -> None:
handler = self.__events[self.__event_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)Install using pip:
pip install cs-events
