Python Event Source Library
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README: Event Source Library for Python

This library implements W3C Draft's on event-source:

It enables a halfduplex communication from server to client, but initiated by the client, through standard HTTP(S) communication.


  • Fairly recent python (tested with 2.7)
  • Fairly recent tornado (tested with 2.2.1)


  1. Launch the server:

    eventsource-server -P 8888 -i -k 50000
  2. Launch the client:

    eventsource-client 42:42:42:42:42:42 -r 5000 -P 8888
  3. Send requests:

    eventsource-request 42:42:42:42:42:42 ping "42"
    eventsource-request 42:42:42:42:42:42 close

Command Line arguments

  • eventsource/ or eventsource-server:

    usage: eventsource/ [-h] [-H HOST] [-P PORT] [-d]
                                                [-j] [-k KEEPALIVE] [-i]
    Event Source Listener
    optional arguments:
    -h, --help            show this help message and exit
    -H HOST, --host HOST  Host to bind on
    -P PORT, --port PORT  Port to bind on
    -d, --debug           enables debug output
    -j, --json            to enable JSON Event
    -k KEEPALIVE, --keepalive KEEPALIVE
                            Keepalive timeout, in milliseconds
    -i, --id              to generate identifiers
  • eventsource/ or eventsource-client:

    usage: eventsource/ [-h] [-H HOST] [-P PORT] [-d]
                                            [-r RETRY]
    Event Source Client
    positional arguments:
    token                 Token to be used for connection
    optional arguments:
    -h, --help            show this help message and exit
    -H HOST, --host HOST  Host to connect to
    -P PORT, --port PORT  Port to be used connection
    -d, --debug           enables debug output
    -r RETRY, --retry RETRY
                            Reconnection timeout
  • eventsource/ or eventsource-request:

    usage: eventsource/ [-h] [-H HOST] [-P PORT] [-j]
                                        token action [data]
    Generates event for Event Source Library
    positional arguments:
    token                 Token to be used for connection
    action                Action to send
    data                  Data to be sent
    optional arguments:
    -h, --help            show this help message and exit
    -H HOST, --host HOST  Host to connect to
    -P PORT, --port PORT  Port to be used connection
    -j, --json            Treat data as JSON


You can install it by getting it from pypi:

pip install event-source-library

or by getting this repository and install it manually:

python install


To develop, you can use buildout:

pip install zc.buildout # if you haven't got it
ls bin/

Or you can do:

python develop

which will deploy the commands globally like an install, but still linked to the current sources.


On the server side, basically all you have to do is to add the following to your code:

from eventsource import listener

application = tornado.web.Application([
    (r"/(.*)/(.*)", listener.EventSourceHandler,



  • PORT is an integer for the port to bind to
  • KEEPALIVE is an integer for the timeout between two keepalive messages (to protect from disconnections), in milliseconds
  • EVENT is a eventsource.listener.Event based class, either one you made or
    • eventsource.listener.StringEvent : Each event gets and resends multiline strings
    • eventsource.listener.StringIdEvent : Each event gets and resends multiline strings, with an unique id for each event
    • eventsource.listener.JSONEvent : Each event gets and resends JSON valid strings
    • eventsource.listener.JSONIdEvent : Each event gets and resends JSON valid string, with an unique id for each event

See for more details.


To extend the behaviour of the event source library, without breaking eventsource definition, the Event based classes implements all processing elements that shall be done on events.

There is two abstract classes that defines Event: * eventsource.listener.Event : defines the constructor of an Event * eventsource.listener.EventId : defines an always incrementing id handler

here is an example to create a new Event that takes multiline data and join it in a one line string seperated with semi-colons.

class OneLineEvent(Event):
    ACTIONS = ["ping",Event.FINISH]

    """Property to enable multiline output of the value"""
    def get_value(self):
        # replace carriage returns by semi-colons
        # this method shall always return a list (even if one value)
        return [";".join([line for line in self._value.split('\n')])]

    value = property(get_value,set_value)

And now, I want to add basic id support to OneLineEvent, in OneLineEventId, nothing is easier

class OneLineEventId(OneLineEvent,EventId):
    id = property(EventId.get_value)

Or if I want the id to be a timestamp:

import time
class OneLineTimeStampEvent(OneLineEvent):
    id = property(lambda s: "%f" % (time.time(),))

You can change the behaviour of a few things in a Event-based class:

  • Event.LISTEN contains the GET action to open a connection (per default "poll")
  • Event.FINISH contains the POST action to close a connection (per default "close")
  • Event.RETRY contains the POST action to define the timeout after reconnecting on network disconnection (per default "0", which means disabled)
  • in the Event.ACTIONS list, you define what POST actions are allowed, per default, only Event.FINISH is allowed.
  • Event.content_type contains the "content_type" that will be asked for every form (it is not enforced).

To change the way events are generated, you can directly call EventSourceHandler.buffer_event() to create a new event to be sent. But the post action is best, at least while WSGI can't handle correctly long polling connections.


Python Event Source Library

(c) 2012 Bernard Pratz
Patches by Ian Whyman, Коренберг Марк and Max Suraev

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3 of the License.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see