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This small modules implements a Publisher class to handle events in the HTTP Server-Sent-Events protocol.

It allows a number of subscribers to get notifications when events happen in certain feed channels. The common use case is for a Javascript client to subscribe to these feed using an EventSource instance, and the events be sent by a Python server like Flask.

You can run the module as a Python script to start an example chat, available at http://localhost:5000 .


  • Simple Publisher with publish and subscribe methods

  • Optional channels support

  • Optional subscriber differentiation (subscribers have individual properties that can be used to generate custom events)

  • Optional initial data for subscribers

  • Server type agnostic

API Overview


Creates a new publisher with an empty list of subscribers. All following functions are methods of Publisher.

subscribe(channel='default channel', properties=None, initial_data=[])

Subscribes to the channel(s), returning an infinite generator of Server-Sent-Events.

If properties is passed, these will be used for differentiation if a callable object is published (see Publisher.publish).

If the list initial_data is passed, all data there will be sent before the regular channel process starts.

publish(data, channel='default channel')

Publishes data to all subscribers of the given channel(s).

If data is callable, the return of data(properties) will be published instead, for the properties object of each subscriber. This allows for customized events.

get_subscribers(channel='default channel')

Returns a generator of all subscribers in the given channel(s).

Note on channel

channel can either be a channel name (e.g. 'secret room') or a list of channel names (e.g. ['chat', 'global messages']). It defaults to the channel named 'default channel'.


This is a minimal example using Flask as the server. Every viewer receives a notification when a new viewer visits the page. Open many simultaneous tabs to see the result.

For simplicity reasons it uses threads to serve the event streams. To build actual products you probably want to use gevents/gunicorn.

import flask
from datetime import datetime
from sse import Publisher

app = flask.Flask(__name__)
publisher = Publisher()

def subscribe():
    return flask.Response(publisher.subscribe(),

def root():
    ip = flask.request.remote_addr
    publisher.publish('New visit from {} at {}!'.format(ip,

    return """
        Open this page in new tabs to see the real time visits.
        <div id="events" />
        var eventSource = new EventSource('/subscribe');
        eventSource.onmessage = function(e) {
            document.getElementById('events').innerHTML += + '<br>';
""", threaded=True)

This example can be run at


Python library for Server-Sent-Events







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