pyee is a rough port of node.js’s EventEmitter. Unlike its namesake, it includes a number of subclasses useful for implementing async and threaded programming in python, such as async/await as seen in python 3.5+.


You can install this project into your environment of choice using pip:

pip install pyee

API Docs:

pyee supplies a BaseEventEmitter class that is similar to the EventEmitter class from Node.js. In addition, it supplies the subclasses AsyncIOEventEmitter, TwistedEventEmitter and ExecutorEventEmitter for supporting async and threaded execution with asyncio, twisted, and concurrent.futures Executors respectively, as supported by the environment.


In [1]: from pyee import BaseEventEmitter

In [2]: ee = BaseEventEmitter()

In [3]: @ee.on('event')
   ...: def event_handler():
   ...:     print('BANG BANG')

In [4]: ee.emit('event')

In [5]:
class pyee.BaseEventEmitter[source]

The base event emitter class. All other event emitters inherit from this class.

Most events are registered with an emitter via the on and once methods, and fired with the emit method. However, pyee event emitters have two special events:

  • new_listener: Fires whenever a new listener is created. Listeners for this event do not fire upon their own creation.

  • error: When emitted raises an Exception by default, behavior can be overriden by attaching callback to the event.

    For example:

    def on_error(message):
    ee.emit('error', Exception('something blew up'))

All callbacks are handled in a synchronous, blocking manner. As in node.js, raised exceptions are not automatically handled for you—you must catch your own exceptions, and treat them accordingly.

emit(event, *args, **kwargs)[source]

Emit event, passing *args and **kwargs to each attached function. Returns True if any functions are attached to event; otherwise returns False.


ee.emit('data', '00101001')

Assuming data is an attached function, this will call data('00101001')'.


Returns a list of all listeners registered to the event.

on(event, f=None)[source]

Registers the function f to the event name event.

If f isn’t provided, this method returns a function that takes f as a callback; in other words, you can use this method as a decorator, like so:

def data_handler(data):

In both the decorated and undecorated forms, the event handler is returned. The upshot of this is that you can call decorated handlers directly, as well as use them in remove_listener calls.

once(event, f=None)[source]

The same as ee.on, except that the listener is automatically removed after being called.


Remove all listeners attached to event. If event is None, remove all listeners on all events.

remove_listener(event, f)[source]

Removes the function f from event.

class pyee.AsyncIOEventEmitter(loop=None)[source]

An event emitter class which can run asyncio coroutines in addition to synchronous blocking functions. For example:

async def async_handler(*args, **kwargs):
    await returns_a_future()

On emit, the event emitter will automatically schedule the coroutine using asyncio.ensure_future and the configured event loop (defaults to asyncio.get_event_loop()).

Unlike the case with the BaseEventEmitter, all exceptions raised by event handlers are automatically emitted on the error event. This is important for asyncio coroutines specifically but is also handled for synchronous functions for consistency.

When loop is specified, the supplied event loop will be used when scheduling work with ensure_future. Otherwise, the default asyncio event loop is used.

For asyncio coroutine event handlers, calling emit is non-blocking. In other words, you do not have to await any results from emit, and the coroutine is scheduled in a fire-and-forget fashion.

class pyee.TwistedEventEmitter[source]

An event emitter class which can run twisted coroutines and handle returned Deferreds, in addition to synchronous blocking functions. For example:

def async_handler(*args, **kwargs):
    yield returns_a_deferred()


async def async_handler(*args, **kwargs):
    await returns_a_deferred()

When async handlers fail, Failures are first emitted on the failure event. If there are no failure handlers, the Failure’s associated exception is then emitted on the error event. If there are no error handlers, the exception is raised. For consistency, when handlers raise errors synchronously, they’re captured, wrapped in a Failure and treated as an async failure. This is unlike the behavior of BaseEventEmitter, which have no special error handling.

For twisted coroutine event handlers, calling emit is non-blocking. In other words, you do not have to await any results from emit, and the coroutine is scheduled in a fire-and-forget fashion.

Similar behavior occurs for “sync” functions which return Deferreds.

class pyee.ExecutorEventEmitter(executor=None)[source]

An event emitter class which runs handlers in a concurrent.futures executor. If using python 2, this will fall back to trying to use the futures backported library (caveats there apply).

By default, this class creates a default ThreadPoolExecutor, but a custom executor may also be passed in explicitly to, for instance, use a ProcessPoolExecutor instead.

This class runs all emitted events on the configured executor. Errors captured by the resulting Future are automatically emitted on the error event. This is unlike the BaseEventEmitter, which have no error handling.

The underlying executor may be shut down by calling the shutdown method. Alternately you can treat the event emitter as a context manager:

with ExecutorEventEmitter() as ee:
    # Underlying executor open

    def handler(data):


# Underlying executor closed

Since the function call is scheduled on an executor, emit is always non-blocking.

No effort is made to ensure thread safety, beyond using an executor.


Call shutdown on the internal executor.


alias of pyee._compat.CompatEventEmitter

class pyee._compat.CompatEventEmitter(scheduler=<function ensure_future>, loop=None)[source]

An EventEmitter exposed for compatibility with prior versions of pyee. This functionality is deprecated; you should instead use either AsyncIOEventEmitter, TwistedEventEmitter, ExecutorEventEmitter, or BaseEventEmitter.

This class is similar to the AsyncIOEventEmitter class, but also allows for overriding the scheduler function (ensure_future by default as in ASyncIOEventEmitter) and does duck typing checks to handle Deferreds. In other words, by setting scheduler to twisted.internet.defer.ensureDeferred this will support twisted use cases for coroutines.

When calling synchronous handlers, raised exceptions are ignored - as with the BaseEventEmitter, you must capture and handle your own exceptions. However, for coroutine functions, exceptions are handled by emitting them on the error event. Note that when using with twisted, the error event will emit Failures, not Exceptions.

This class will also successfully import in python 2, but without coroutine support.