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Library for emitting events and handling them in a decoupled way.

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EventSail is a minimal observer pattern package which is performant as well as supports both sync and async operations. Born out of utility and curiosity to replicate Javascript EventEmitter, here we are with library with similar functionalities.

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Package can be installed from PyPi using below command.

python -m pip install eventsail

If using poetry or pipenv for packaging, use below command.

poetry add eventsail
pipenv install eventsail


EventSail has a very minimal configuration to get started. It exports Event class which consumes Emitter under the hood for firing callbacks for the events. For easing, creation of Event, EventSail also exports event method for instantiation.

from eventsail import event, Event
test_event: Event = event('test_event_name')

Resultant test_event will be a synchronous event type. If you want an asynchronous event, you can instantiate it as.

test_event: Event = event("test", is_sync=False)

Now test_event would be of type asynchronous event.

Subscribing a callback

To register callbacks/handlers, you can either call subscribe method on Event instance passing callback/function or you can also use it as decorator over your method.

def foo(*args,**kwargs): ...


def foo(*args,**kwargs): ...

For Async event, all the callbacks will be delegated to threads thus preventing any operations to be blocked.

If you want to leverage power of async/await you will have to hint Event class to do so by passing use_asyncio flag to True.

test_event = event("test", is_sync=False, use_asyncio=True)

This way it will entertain coroutines as well by creating Task and placing them over running event loop. For non coroutine methods, it will execute them over loop in a Thread Pool executor, thus preventing blocking of Event loop.

If you want to call a callback/listener only once, you can use once method exposed by Event object. Post trigger, listener will be automatically unsubscribed.

def foo(*args,**kwargs): ...

Above callback will only be fired once.

Unsubscribing a callback

To unsubscribe a callback, you can simply call unsubscribe method on Object passing callback you want to unsubscribe.


Calling this method on already unsubscribed callback would do no harm.

To remove all the listeners/callbacks, you can call clear method.


Event class is also singleton class, so you will always get same instance for same set of arguments. This is intentionally kept in place so that we do not loose listeners subscribed.

Waiting for async tasks

There can be instances where we would need to wait for async emitted tasks to complete before shutdown as this can be critical. Event class exposes certain properties containing async tasks.

  • Event.all_async_tasks : This will return all the async tasks scheduled over Event loop for the Emitter.
  • Event.own_async_tasks : This will only return async tasks corresponding to the immediate parent Emitter of the Event.

To wait for async tasks to complete, Event class exposes awaitable API to wait for tasks to complete. This can be called as below.

await test_event.wait_for_async_tasks()

This will wait for all the async tasks. This also has a default timeout for 10 seconds post which it will error out.


To run tests, simply run below command:

pytest -s --cov=.


EventSail is released under the MIT License. See the bundled LICENSE file for details.

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