Provided by: python3-flask-socketio_5.0.1-1_all 
NAME
flask-socketio - Flask-SocketIO Documentation
Flask-SocketIO gives Flask applications access to low latency bi-directional communications between the
clients and the server. The client-side application can use any of the SocketIO official clients
libraries in Javascript, C++, Java and Swift, or any compatible client to establish a permanent
connection to the server.
INSTALLATION
You can install this package in the usual way using pip:
pip install flask-socketio
REQUIREMENTS
Flask-SocketIO is compatible with Python 3.6+. The asynchronous services that this package relies on can
be selected among three choices:
• eventlet is the best performant option, with support for long-polling and WebSocket transports.
• gevent is supported in a number of different configurations. The long-polling transport is fully
supported with the gevent package, but unlike eventlet, gevent does not have native WebSocket support.
To add support for WebSocket there are currently two options. Installing the gevent-websocket package
adds WebSocket support to gevent or one can use the uWSGI web server, which comes with WebSocket
functionality. The use of gevent is also a performant option, but slightly lower than eventlet.
• The Flask development server based on Werkzeug can be used as well, with the caveat that it lacks the
performance of the other two options, so it should only be used to simplify the development workflow.
This option only supports the long-polling transport.
The extension automatically detects which asynchronous framework to use based on what is installed.
Preference is given to eventlet, followed by gevent. For WebSocket support in gevent, uWSGI is
preferred, followed by gevent-websocket. If neither eventlet nor gevent are installed, then the Flask
development server is used.
If using multiple processes, a message queue service is used by the processes to coordinate operations
such as broadcasting. The supported queues are Redis, RabbitMQ, Kafka, and any other message queues
supported by the Kombu package.
On the client-side, the official Socket.IO Javascript client library can be used to establish a
connection to the server. There are also official clients written in Swift, Java and C++. Unofficial
clients may also work, as long as they implement the Socket.IO protocol. The python-socketio package
includes a Python client.
VERSION COMPATIBILITY
The Socket.IO protocol has been through a number of revisions, and some of these introduced backward
incompatible changes, which means that the client and the server must use compatible versions for
everything to work.
The version compatibility chart below maps versions of this package to versions of the JavaScript
reference implementation and the versions of the Socket.IO and Engine.IO protocols.
┌────────────┬───────────┬───────────┬────────────────┬─────────────────┬─────────────────┐
│JavaScript │ Socket.IO │ Engine.IO │ Flask-SocketIO │ python-socketio │ python-engineio │
│Socket.IO │ protocol │ protocol │ version │ version │ version │
│version │ revision │ revision │ │ │ │
├────────────┼───────────┼───────────┼────────────────┼─────────────────┼─────────────────┤
│0.9.x │ 1, 2 │ 1, 2 │ Not supported │ Not supported │ Not supported │
├────────────┼───────────┼───────────┼────────────────┼─────────────────┼─────────────────┤
│1.x and 2.x │ 3, 4 │ 3 │ 4.x │ 4.x │ 3.x │
└────────────┴───────────┴───────────┴────────────────┴─────────────────┴─────────────────┘
│3.x │ 5 │ 4 │ 5.x │ 5.x │ 4.x │
└────────────┴───────────┴───────────┴────────────────┴─────────────────┴─────────────────┘
INITIALIZATION
The following code example shows how to add Flask-SocketIO to a Flask application:
from flask import Flask, render_template
from flask_socketio import SocketIO
app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret!'
socketio = SocketIO(app)
if __name__ == '__main__':
socketio.run(app)
The init_app() style of initialization is also supported. To start the web server simply execute your
script. Note the way the web server is started. The socketio.run() function encapsulates the start up of
the web server and replaces the app.run() standard Flask development server start up. When the
application is in debug mode the Werkzeug development server is still used and configured properly inside
socketio.run(). In production mode the eventlet web server is used if available, else the gevent web
server is used. If eventlet and gevent are not installed, the Werkzeug development web server is used.
The flask run command introduced in Flask 0.11 can be used to start a Flask-SocketIO development server
based on Werkzeug, but this method of starting the Flask-SocketIO server is not recommended due to lack
of WebSocket support. Previous versions of this package included a customized version of the flask run
command that allowed the use of WebSocket on eventlet and gevent production servers, but this
functionality has been discontinued in favor of the socketio.run(app) startup method shown above which is
more robust.
The application must serve a page to the client that loads the Socket.IO library and establishes a
connection:
<script src="//cdnjs.cloudflare.com/ajax/libs/socket.io/2.2.0/socket.io.js" integrity="sha256-yr4fRk/GU1ehYJPAs8P4JlTgu0Hdsp4ZKrx8bDEDC3I=" crossorigin="anonymous"></script>
<script type="text/javascript" charset="utf-8">
var socket = io();
socket.on('connect', function() {
socket.emit('my event', {data: 'I\'m connected!'});
});
</script>
RECEIVING MESSAGES
When using SocketIO, messages are received by both parties as events. On the client side Javascript
callbacks are used. With Flask-SocketIO the server needs to register handlers for these events, similarly
to how routes are handled by view functions.
The following example creates a server-side event handler for an unnamed event:
@socketio.on('message')
def handle_message(data):
print('received message: ' + data)
The above example uses string messages. Another type of unnamed events use JSON data:
@socketio.on('json')
def handle_json(json):
print('received json: ' + str(json))
The most flexible type of event uses custom event names. The message data for these events can be string,
bytes, int, or JSON:
@socketio.on('my event')
def handle_my_custom_event(json):
print('received json: ' + str(json))
Custom named events can also support multiple arguments:
@socketio.on('my_event')
def handle_my_custom_event(arg1, arg2, arg3):
print('received args: ' + arg1 + arg2 + arg3)
When the name of the event is a valid Python identifier that does not collide with other defined symbols,
the @socketio.event provides a more compact syntax that takes the event name from the decorated function:
@socketio.event
def my_custom_event(arg1, arg2, arg3):
print('received args: ' + arg1 + arg2 + arg3)
Named events are the most flexible, as they eliminate the need to include additional metadata to describe
the message type. The names message, json, connect and disconnect are reserved and cannot be used for
named events.
Flask-SocketIO also supports SocketIO namespaces, which allow the client to multiplex several independent
connections on the same physical socket:
@socketio.on('my event', namespace='/test')
def handle_my_custom_namespace_event(json):
print('received json: ' + str(json))
When a namespace is not specified a default global namespace with the name '/' is used.
For cases when a decorator syntax isn't convenient, the on_event method can be used:
def my_function_handler(data):
pass
socketio.on_event('my event', my_function_handler, namespace='/test')
Clients may request an acknowledgement callback that confirms receipt of a message they sent. Any values
returned from the handler function will be passed to the client as arguments in the callback function:
@socketio.on('my event')
def handle_my_custom_event(json):
print('received json: ' + str(json))
return 'one', 2
In the above example, the client callback function will be invoked with two arguments, 'one' and 2. If a
handler function does not return any values, the client callback function will be invoked without
arguments.
SENDING MESSAGES
SocketIO event handlers defined as shown in the previous section can send reply messages to the connected
client using the send() and emit() functions.
The following examples bounce received events back to the client that sent them:
from flask_socketio import send, emit
@socketio.on('message')
def handle_message(message):
send(message)
@socketio.on('json')
def handle_json(json):
send(json, json=True)
@socketio.on('my event')
def handle_my_custom_event(json):
emit('my response', json)
Note how send() and emit() are used for unnamed and named events respectively.
When working with namespaces, send() and emit() use the namespace of the incoming message by default. A
different namespace can be specified with the optional namespace argument:
@socketio.on('message')
def handle_message(message):
send(message, namespace='/chat')
@socketio.on('my event')
def handle_my_custom_event(json):
emit('my response', json, namespace='/chat')
To send an event with multiple arguments, send a tuple:
@socketio.on('my event')
def handle_my_custom_event(json):
emit('my response', ('foo', 'bar', json), namespace='/chat')
SocketIO supports acknowledgment callbacks that confirm that a message was received by the client:
def ack():
print 'message was received!'
@socketio.on('my event')
def handle_my_custom_event(json):
emit('my response', json, callback=ack)
When using callbacks, the Javascript client receives a callback function to invoke upon receipt of the
message. After the client application invokes the callback function the server invokes the corresponding
server-side callback. If the client-side callback is invoked with arguments, these are provided as
arguments to the server-side callback as well.
BROADCASTING
Another very useful feature of SocketIO is the broadcasting of messages. Flask-SocketIO supports this
feature with the broadcast=True optional argument to send() and emit():
@socketio.on('my event')
def handle_my_custom_event(data):
emit('my response', data, broadcast=True)
When a message is sent with the broadcast option enabled, all clients connected to the namespace receive
it, including the sender. When namespaces are not used, the clients connected to the global namespace
receive the message. Note that callbacks are not invoked for broadcast messages.
In all the examples shown until this point the server responds to an event sent by the client. But for
some applications, the server needs to be the originator of a message. This can be useful to send
notifications to clients of events that originated in the server, for example in a background thread.
The socketio.send() and socketio.emit() methods can be used to broadcast to all connected clients:
def some_function():
socketio.emit('some event', {'data': 42})
Note that socketio.send() and socketio.emit() are not the same functions as the context-aware send() and
emit(). Also note that in the above usage there is no client context, so broadcast=True is assumed and
does not need to be specified.
ROOMS
For many applications it is necessary to group users into subsets that can be addressed together. The
best example is a chat application with multiple rooms, where users receive messages from the room or
rooms they are in, but not from other rooms where other users are. Flask-SocketIO supports this concept
of rooms through the join_room() and leave_room() functions:
from flask_socketio import join_room, leave_room
@socketio.on('join')
def on_join(data):
username = data['username']
room = data['room']
join_room(room)
send(username + ' has entered the room.', room=room)
@socketio.on('leave')
def on_leave(data):
username = data['username']
room = data['room']
leave_room(room)
send(username + ' has left the room.', room=room)
The send() and emit() functions accept an optional room argument that cause the message to be sent to all
the clients that are in the given room.
All clients are assigned a room when they connect, named with the session ID of the connection, which can
be obtained from request.sid. A given client can join any rooms, which can be given any names. When a
client disconnects it is removed from all the rooms it was in. The context-free socketio.send() and
socketio.emit() functions also accept a room argument to broadcast to all clients in a room.
Since all clients are assigned a personal room, to address a message to a single client, the session ID
of the client can be used as the room argument.
CONNECTION EVENTS
Flask-SocketIO also dispatches connection and disconnection events. The following example shows how to
register handlers for them:
@socketio.on('connect')
def test_connect():
emit('my response', {'data': 'Connected'})
@socketio.on('disconnect')
def test_disconnect():
print('Client disconnected')
The connection event handler can return False to reject the connection, or it can also raise
ConectionRefusedError. This is so that the client can be authenticated at this point. When using the
exception, any arguments passed to the exception are returned to the client in the error packet.
Examples:
from flask_socketio import ConnectionRefusedError
@socketio.on('connect')
def connect():
if not self.authenticate(request.args):
raise ConnectionRefusedError('unauthorized!')
Note that connection and disconnection events are sent individually on each namespace used.
CLASS-BASED NAMESPACES
As an alternative to the decorator-based event handlers described above, the event handlers that belong
to a namespace can be created as methods of a class. The flask_socketio.Namespace is provided as a base
class to create class-based namespaces:
from flask_socketio import Namespace, emit
class MyCustomNamespace(Namespace):
def on_connect(self):
pass
def on_disconnect(self):
pass
def on_my_event(self, data):
emit('my_response', data)
socketio.on_namespace(MyCustomNamespace('/test'))
When class-based namespaces are used, any events received by the server are dispatched to a method named
as the event name with the on_ prefix. For example, event my_event will be handled by a method named
on_my_event. If an event is received for which there is no corresponding method defined in the namespace
class, then the event is ignored. All event names used in class-based namespaces must use characters that
are legal in method names.
As a convenience to methods defined in a class-based namespace, the namespace instance includes versions
of several of the methods in the flask_socketio.SocketIO class that default to the proper namespace when
the namespace argument is not given.
If an event has a handler in a class-based namespace, and also a decorator-based function handler, only
the decorated function handler is invoked.
ERROR HANDLING
Flask-SocketIO can also deal with exceptions:
@socketio.on_error() # Handles the default namespace
def error_handler(e):
pass
@socketio.on_error('/chat') # handles the '/chat' namespace
def error_handler_chat(e):
pass
@socketio.on_error_default # handles all namespaces without an explicit error handler
def default_error_handler(e):
pass
Error handler functions take the exception object as an argument.
The message and data arguments of the current request can also be inspected with the request.event
variable, which is useful for error logging and debugging outside the event handler:
from flask import request
@socketio.on("my error event")
def on_my_event(data):
raise RuntimeError()
@socketio.on_error_default
def default_error_handler(e):
print(request.event["message"]) # "my error event"
print(request.event["args"]) # (data,)
DEBUGGING AND TROUBLESHOOTING
To help you debug issues, the server can be configured to output logs to the terminal:
socketio = SocketIO(logger=True, engineio_logger=True)
The logger argument controls logging related to the Socket.IO protocol, while engineio_logger controls
logs that originate in the low-level Engine.IO transport. These arguments can be set to True to output
logs to stderr, or to an object compatible with Python's logging package where the logs should be emitted
to. A value of False disables logging.
Logging can help identify the cause of connection problems, 400 responses, bad performance and other
issues.
ACCESS TO FLASK'S CONTEXT GLOBALS
Handlers for SocketIO events are different than handlers for routes and that introduces a lot of
confusion around what can and cannot be done in a SocketIO handler. The main difference is that all the
SocketIO events generated for a client occur in the context of a single long running request.
In spite of the differences, Flask-SocketIO attempts to make working with SocketIO event handlers easier
by making the environment similar to that of a regular HTTP request. The following list describes what
works and what doesn't:
• An application context is pushed before invoking an event handler making current_app and g available to
the handler.
• A request context is also pushed before invoking a handler, also making request and session available.
But note that WebSocket events do not have individual requests associated with them, so the request
context that started the connection is pushed for all the events that are dispatched during the life of
the connection.
• The request context global is enhanced with a sid member that is set to a unique session ID for the
connection. This value is used as an initial room where the client is added.
• The request context global is enhanced with namespace and event members that contain the currently
handled namespace and event arguments. The event member is a dictionary with message and args keys.
• The session context global behaves in a different way than in regular requests. A copy of the user
session at the time the SocketIO connection is established is made available to handlers invoked in the
context of that connection. If a SocketIO handler modifies the session, the modified session will be
preserved for future SocketIO handlers, but regular HTTP route handlers will not see these changes.
Effectively, when a SocketIO handler modifies the session, a "fork" of the session is created
exclusively for these handlers. The technical reason for this limitation is that to save the user
session a cookie needs to be sent to the client, and that requires HTTP request and response, which do
not exist in a SocketIO connection. When using server-side sessions such as those provided by the
Flask-Session or Flask-KVSession extensions, changes made to the session in HTTP route handlers can be
seen by SocketIO handlers, as long as the session is not modified in the SocketIO handlers.
• The before_request and after_request hooks are not invoked for SocketIO event handlers.
• SocketIO handlers can take custom decorators, but most Flask decorators will not be appropriate to use
for a SocketIO handler, given that there is no concept of a Response object during a SocketIO
connection.
AUTHENTICATION
A common need of applications is to validate the identity of their users. The traditional mechanisms
based on web forms and HTTP requests cannot be used in a SocketIO connection, since there is no place to
send HTTP requests and responses. If necessary, an application can implement a customized login form that
sends credentials to the server as a SocketIO message when the submit button is pressed by the user.
However, in most cases it is more convenient to perform the traditional authentication process before the
SocketIO connection is established. The user's identity can then be recorded in the user session or in a
cookie, and later when the SocketIO connection is established that information will be accessible to
SocketIO event handlers.
Using Flask-Login with Flask-SocketIO
Flask-SocketIO can access login information maintained by Flask-Login. After a regular Flask-Login
authentication is performed and the login_user() function is called to record the user in the user
session, any SocketIO connections will have access to the current_user context variable:
@socketio.on('connect')
def connect_handler():
if current_user.is_authenticated:
emit('my response',
{'message': '{0} has joined'.format(current_user.name)},
broadcast=True)
else:
return False # not allowed here
Note that the login_required decorator cannot be used with SocketIO event handlers, but a custom
decorator that disconnects non-authenticated users can be created as follows:
import functools
from flask import request
from flask_login import current_user
from flask_socketio import disconnect, emit
def authenticated_only(f):
@functools.wraps(f)
def wrapped(*args, **kwargs):
if not current_user.is_authenticated:
disconnect()
else:
return f(*args, **kwargs)
return wrapped
@socketio.on('my event')
@authenticated_only
def handle_my_custom_event(data):
emit('my response', {'message': '{0} has joined'.format(current_user.name)},
broadcast=True)
DEPLOYMENT
There are many options to deploy a Flask-SocketIO server, ranging from simple to the insanely complex. In
this section, the most commonly used options are described.
Embedded Server
The simplest deployment strategy is to have eventlet or gevent installed, and start the web server by
calling socketio.run(app) as shown in examples above. This will run the application on the eventlet or
gevent web servers, whichever is installed.
Note that socketio.run(app) runs a production ready server when eventlet or gevent are installed. If
neither of these are installed, then the application runs on Flask's development web server, which is not
appropriate for production use.
Unfortunately this option is not available when using gevent with uWSGI. See the uWSGI section below for
information on this option.
Gunicorn Web Server
An alternative to socketio.run(app) is to use gunicorn as web server, using the eventlet or gevent
workers. For this option, eventlet or gevent need to be installed, in addition to gunicorn. The command
line that starts the eventlet server via gunicorn is:
gunicorn --worker-class eventlet -w 1 module:app
If you prefer to use gevent, the command to start the server is:
gunicorn -k gevent -w 1 module:app
When using gunicorn with the gevent worker and the WebSocket support provided by gevent-websocket, the
command that starts the server must be changed to select a custom gevent web server that supports the
WebSocket protocol. The modified command is:
gunicorn -k geventwebsocket.gunicorn.workers.GeventWebSocketWorker -w 1 module:app
In all these commands, module is the Python module or package that defines the application instance, and
app is the application instance itself.
Due to the limited load balancing algorithm used by gunicorn, it is not possible to use more than one
worker process when using this web server. For that reason, all the examples above include the -w 1
option.
uWSGI Web Server
When using the uWSGI server in combination with gevent, the Socket.IO server can take advantage of
uWSGI’s native WebSocket support.
A complete explanation of the configuration and usage of the uWSGI server is beyond the scope of this
documentation. The uWSGI server is a fairly complex package that provides a large and comprehensive set
of options. It must be compiled with WebSocket and SSL support for the WebSocket transport to be
available. As way of an introduction, the following command starts a uWSGI server for the example
application app.py on port 5000:
$ uwsgi --http :5000 --gevent 1000 --http-websockets --master --wsgi-file app.py --callable app
Using nginx as a WebSocket Reverse Proxy
It is possible to use nginx as a front-end reverse proxy that passes requests to the application.
However, only releases of nginx 1.4 and newer support proxying of the WebSocket protocol. Below is a
basic nginx configuration that proxies HTTP and WebSocket requests:
server {
listen 80;
server_name _;
location / {
include proxy_params;
proxy_pass http://127.0.0.1:5000;
}
location /static {
alias <path-to-your-application>/static;
expires 30d;
}
location /socket.io {
include proxy_params;
proxy_http_version 1.1;
proxy_buffering off;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";
proxy_pass http://127.0.0.1:5000/socket.io;
}
}
The next example adds the support for load balancing multiple Socket.IO servers:
upstream socketio_nodes {
ip_hash;
server 127.0.0.1:5000;
server 127.0.0.1:5001;
server 127.0.0.1:5002;
# to scale the app, just add more nodes here!
}
server {
listen 80;
server_name _;
location / {
include proxy_params;
proxy_pass http://127.0.0.1:5000;
}
locaton /static {
alias <path-to-your-application>/static;
expires 30d;
}
location /socket.io {
include proxy_params;
proxy_http_version 1.1;
proxy_buffering off;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";
proxy_pass http://socketio_nodes/socket.io;
}
}
While the above examples can work as an initial configuration, be aware that a production install of
nginx will need a more complete configuration covering other deployment aspects such as serving static
file assets and SSL support.
Using Multiple Workers
Flask-SocketIO supports multiple workers behind a load balancer starting with release 2.0. Deploying
multiple workers gives applications that use Flask-SocketIO the ability to spread the client connections
among multiple processes and hosts, and in this way scale to support very large numbers of concurrent
clients.
There are two requirements to use multiple Flask-SocketIO workers:
• The load balancer must be configured to forward all HTTP requests from a given client always to the
same worker. This is sometimes referenced as "sticky sessions". For nginx, use the ip_hash directive to
achieve this. Gunicorn cannot be used with multiple workers because its load balancer algorithm does
not support sticky sessions.
• Since each of the servers owns only a subset of the client connections, a message queue such as Redis
or RabbitMQ is used by the servers to coordinate complex operations such as broadcasting and rooms.
When working with a message queue, there are additional dependencies that need to be installed:
• For Redis, the package redis must be installed (pip install redis).
• For RabbitMQ, the package kombu must be installed (pip install kombu).
• For Kafka, the package kafka-python must be installed (pip install kafka-python).
• For other message queues supported by Kombu, see the Kombu documentation to find out what dependencies
are needed.
• If eventlet or gevent are used, then monkey patching the Python standard library is normally required
to force the message queue package to use coroutine friendly functions and classes.
For eventlet, monkey patching is done with:
import eventlet
eventlet.monkey_patch()
For gevent, you can monkey patch the standard library with:
from gevent import monkey
monkey.patch_all()
In both cases it is recommended that you apply the monkey patching at the top of your main script, even
above your imports.
To start multiple Flask-SocketIO servers, you must first ensure you have the message queue service
running. To start a Socket.IO server and have it connect to the message queue, add the message_queue
argument to the SocketIO constructor:
socketio = SocketIO(app, message_queue='redis://')
The value of the message_queue argument is the connection URL of the queue service that is used. For a
redis queue running on the same host as the server, the 'redis://' URL can be used. Likewise, for a
default RabbitMQ queue the 'amqp://' URL can be used. For Kafka, use a kafka:// URL. The Kombu package
has a documentation section that describes the format of the URLs for all the supported queues.
Emitting from an External Process
For many types of applications, it is necessary to emit events from a process that is not the SocketIO
server, for an example a Celery worker. If the SocketIO server or servers are configured to listen on a
message queue as shown in the previous section, then any other process can create its own SocketIO
instance and use it to emit events in the same way the server does.
For example, for an application that runs on an eventlet web server and uses a Redis message queue, the
following Python script broadcasts an event to all clients:
socketio = SocketIO(message_queue='redis://')
socketio.emit('my event', {'data': 'foo'}, namespace='/test')
When using the SocketIO instance in this way, the Flask application instance is not passed to the
constructor.
The channel argument to SocketIO can be used to select a specific channel of communication through the
message queue. Using a custom channel name is necessary when there are multiple independent SocketIO
services sharing the same queue.
Flask-SocketIO does not apply monkey patching when eventlet or gevent are used. But when working with a
message queue, it is very likely that the Python package that talks to the message queue service will
hang if the Python standard library is not monkey patched.
It is important to note that an external process that wants to connect to a SocketIO server does not need
to use eventlet or gevent like the main server. Having a server use a coroutine framework, while an
external process is not a problem. For example, Celery workers do not need to be configured to use
eventlet or gevent just because the main server does. But if your external process does use a coroutine
framework for whatever reason, then monkey patching is likely required, so that the message queue
accesses coroutine friendly functions and classes.
CROSS-ORIGIN CONTROLS
For security reasons, this server enforces a same-origin policy by default. In practical terms, this
means the following:
• If an incoming HTTP or WebSocket request includes the Origin header, this header must match the scheme
and host of the connection URL. In case of a mismatch, a 400 status code response is returned and the
connection is rejected.
• No restrictions are imposed on incoming requests that do not include the Origin header.
If necessary, the cors_allowed_origins option can be used to allow other origins. This argument can be
set to a string to set a single allowed origin, or to a list to allow multiple origins. A special value
of '*' can be used to instruct the server to allow all origins, but this should be done with care, as
this could make the server vulnerable to Cross-Site Request Forgery (CSRF) attacks.
Upgrading to Flask-SocketIO 5.x from the 4.x releases
The Socket.IO protocol recently introduced a series of backwards incompatible changes. The 5.x releases
of Flask-SocketIO adopted these changes, and for that reason it can only be used with clients that have
also been updated to the current version of the protocol. In particular, this means that the JavaScript
client must be upgraded to a 3.x release, and if your client hasn't been upgraded to the latest version
of the Socket.IO protocol, then you must use a Flask-SocketIO 4.x release.
The following protocol changes are of importance, as they may affect existing applications:
• The default namespace '/' is not automatically connected anymore, and is now treated in the same way as
other namespaces.
• Each namespace connection has its own sid value, different from the others and different from the
Engine.IO sid.
• Flask-SocketIO now uses the same ping interval and timeout values as the JavaScript reference
implementation, which are 25 and 5 seconds respectively.
• The ping/pong mechanism has been reversed. In the current version of the protocol, the server issues a
ping and the client responds with a pong.
• The default allowed payload size for long--polling packets has been lowered from 100MB to 1MB.
• The io cookie is not sent to the client anymore by default.
API REFERENCE
class flask_socketio.SocketIO(app=None, **kwargs)
Create a Flask-SocketIO server.
Parameters
• app -- The flask application instance. If the application instance isn't known at the
time this class is instantiated, then call socketio.init_app(app) once the application
instance is available.
• manage_session -- If set to True, this extension manages the user session for Socket.IO
events. If set to False, Flask's own session management is used. When using Flask's
cookie based sessions it is recommended that you leave this set to the default of True.
When using server-side sessions, a False setting enables sharing the user session between
HTTP routes and Socket.IO events.
• message_queue -- A connection URL for a message queue service the server can use for
multi-process communication. A message queue is not required when using a single server
process.
• channel -- The channel name, when using a message queue. If a channel isn't specified, a
default channel will be used. If multiple clusters of SocketIO processes need to use the
same message queue without interfering with each other, then each cluster should use a
different channel.
• path -- The path where the Socket.IO server is exposed. Defaults to 'socket.io'. Leave
this as is unless you know what you are doing.
• resource -- Alias to path.
• kwargs -- Socket.IO and Engine.IO server options.
The Socket.IO server options are detailed below:
Parameters
• client_manager -- The client manager instance that will manage the client list. When this
is omitted, the client list is stored in an in-memory structure, so the use of multiple
connected servers is not possible. In most cases, this argument does not need to be set
explicitly.
• logger -- To enable logging set to True or pass a logger object to use. To disable
logging set to False. The default is False. Note that fatal errors will be logged even
when logger is False.
• json -- An alternative json module to use for encoding and decoding packets. Custom json
modules must have dumps and loads functions that are compatible with the standard library
versions. To use the same json encoder and decoder as a Flask application, use
flask.json.
• async_handlers -- If set to True, event handlers for a client are executed in separate
threads. To run handlers for a client synchronously, set to False. The default is True.
• always_connect -- When set to False, new connections are provisory until the connect
handler returns something other than False, at which point they are accepted. When set to
True, connections are immediately accepted, and then if the connect handler returns False
a disconnect is issued. Set to True if you need to emit events from the connect handler
and your client is confused when it receives events before the connection acceptance. In
any other case use the default of False.
The Engine.IO server configuration supports the following settings:
Parameters
• async_mode -- The asynchronous model to use. See the Deployment section in the
documentation for a description of the available options. Valid async modes are
threading, eventlet, gevent and gevent_uwsgi. If this argument is not given, eventlet is
tried first, then gevent_uwsgi, then gevent, and finally threading. The first async mode
that has all its dependencies installed is then one that is chosen.
• ping_interval -- The interval in seconds at which the server pings the client. The
default is 25 seconds. For advanced control, a two element tuple can be given, where the
first number is the ping interval and the second is a grace period added by the server.
• ping_timeout -- The time in seconds that the client waits for the server to respond
before disconnecting. The default is 5 seconds.
• max_http_buffer_size -- The maximum size of a message when using the polling transport.
The default is 1,000,000 bytes.
• allow_upgrades -- Whether to allow transport upgrades or not. The default is True.
• http_compression -- Whether to compress packages when using the polling transport. The
default is True.
• compression_threshold -- Only compress messages when their byte size is greater than this
value. The default is 1024 bytes.
• cookie -- If set to a string, it is the name of the HTTP cookie the server sends back tot
he client containing the client session id. If set to a dictionary, the 'name' key
contains the cookie name and other keys define cookie attributes, where the value of each
attribute can be a string, a callable with no arguments, or a boolean. If set to None
(the default), a cookie is not sent to the client.
• cors_allowed_origins -- Origin or list of origins that are allowed to connect to this
server. Only the same origin is allowed by default. Set this argument to '*' to allow all
origins, or to [] to disable CORS handling.
• cors_credentials -- Whether credentials (cookies, authentication) are allowed in requests
to this server. The default is True.
• monitor_clients -- If set to True, a background task will ensure inactive clients are
closed. Set to False to disable the monitoring task (not recommended). The default is
True.
• engineio_logger -- To enable Engine.IO logging set to True or pass a logger object to
use. To disable logging set to False. The default is False. Note that fatal errors are
logged even when engineio_logger is False.
on(message, namespace=None)
Decorator to register a SocketIO event handler.
This decorator must be applied to SocketIO event handlers. Example:
@socketio.on('my event', namespace='/chat')
def handle_my_custom_event(json):
print('received json: ' + str(json))
Parameters
• message -- The name of the event. This is normally a user defined string, but a
few event names are already defined. Use 'message' to define a handler that takes
a string payload, 'json' to define a handler that takes a JSON blob payload,
'connect' or 'disconnect' to create handlers for connection and disconnection
events.
• namespace -- The namespace on which the handler is to be registered. Defaults to
the global namespace.
on_error(namespace=None)
Decorator to define a custom error handler for SocketIO events.
This decorator can be applied to a function that acts as an error handler for a namespace.
This handler will be invoked when a SocketIO event handler raises an exception. The handler
function must accept one argument, which is the exception raised. Example:
@socketio.on_error(namespace='/chat')
def chat_error_handler(e):
print('An error has occurred: ' + str(e))
Parameters
namespace -- The namespace for which to register the error handler. Defaults to the
global namespace.
on_error_default(exception_handler)
Decorator to define a default error handler for SocketIO events.
This decorator can be applied to a function that acts as a default error handler for any
namespaces that do not have a specific handler. Example:
@socketio.on_error_default
def error_handler(e):
print('An error has occurred: ' + str(e))
on_event(message, handler, namespace=None)
Register a SocketIO event handler.
on_event is the non-decorator version of 'on'.
Example:
def on_foo_event(json):
print('received json: ' + str(json))
socketio.on_event('my event', on_foo_event, namespace='/chat')
Parameters
• message -- The name of the event. This is normally a user defined string, but a
few event names are already defined. Use 'message' to define a handler that takes
a string payload, 'json' to define a handler that takes a JSON blob payload,
'connect' or 'disconnect' to create handlers for connection and disconnection
events.
• handler -- The function that handles the event.
• namespace -- The namespace on which the handler is to be registered. Defaults to
the global namespace.
event(*args, **kwargs)
Decorator to register an event handler.
This is a simplified version of the on() method that takes the event name from the
decorated function.
Example usage:
@socketio.event
def my_event(data):
print('Received data: ', data)
The above example is equivalent to:
@socketio.on('my_event')
def my_event(data):
print('Received data: ', data)
A custom namespace can be given as an argument to the decorator:
@socketio.event(namespace='/test')
def my_event(data):
print('Received data: ', data)
emit(event, *args, **kwargs)
Emit a server generated SocketIO event.
This function emits a SocketIO event to one or more connected clients. A JSON blob can be
attached to the event as payload. This function can be used outside of a SocketIO event
context, so it is appropriate to use when the server is the originator of an event, outside
of any client context, such as in a regular HTTP request handler or a background task.
Example:
@app.route('/ping')
def ping():
socketio.emit('ping event', {'data': 42}, namespace='/chat')
Parameters
• event -- The name of the user event to emit.
• args -- A dictionary with the JSON data to send as payload.
• namespace -- The namespace under which the message is to be sent. Defaults to the
global namespace.
• to -- Send the message to all the users in the given room. If this parameter is
not included, the event is sent to all connected users.
• include_self -- True to include the sender when broadcasting or addressing a room,
or False to send to everyone but the sender.
• skip_sid -- The session id of a client to ignore when broadcasting or addressing a
room. This is typically set to the originator of the message, so that everyone
except that client receive the message. To skip multiple sids pass a list.
• callback -- If given, this function will be called to acknowledge that the client
has received the message. The arguments that will be passed to the function are
those provided by the client. Callback functions can only be used when addressing
an individual client.
send(data, json=False, namespace=None, to=None, callback=None, include_self=True, skip_sid=None,
**kwargs)
Send a server-generated SocketIO message.
This function sends a simple SocketIO message to one or more connected clients. The message
can be a string or a JSON blob. This is a simpler version of emit(), which should be
preferred. This function can be used outside of a SocketIO event context, so it is
appropriate to use when the server is the originator of an event.
Parameters
• data -- The message to send, either a string or a JSON blob.
• json -- True if message is a JSON blob, False otherwise.
• namespace -- The namespace under which the message is to be sent. Defaults to the
global namespace.
• to -- Send the message only to the users in the given room. If this parameter is
not included, the message is sent to all connected users.
• include_self -- True to include the sender when broadcasting or addressing a room,
or False to send to everyone but the sender.
• skip_sid -- The session id of a client to ignore when broadcasting or addressing a
room. This is typically set to the originator of the message, so that everyone
except that client receive the message. To skip multiple sids pass a list.
• callback -- If given, this function will be called to acknowledge that the client
has received the message. The arguments that will be passed to the function are
those provided by the client. Callback functions can only be used when addressing
an individual client.
close_room(room, namespace=None)
Close a room.
This function removes any users that are in the given room and then deletes the room from
the server. This function can be used outside of a SocketIO event context.
Parameters
• room -- The name of the room to close.
• namespace -- The namespace under which the room exists. Defaults to the global
namespace.
run(app, host=None, port=None, **kwargs)
Run the SocketIO web server.
Parameters
• app -- The Flask application instance.
• host -- The hostname or IP address for the server to listen on. Defaults to
127.0.0.1.
• port -- The port number for the server to listen on. Defaults to 5000.
• debug -- True to start the server in debug mode, False to start in normal mode.
• use_reloader -- True to enable the Flask reloader, False to disable it.
• extra_files -- A list of additional files that the Flask reloader should watch.
Defaults to None
• log_output -- If True, the server logs all incomming connections. If False logging
is disabled. Defaults to True in debug mode, False in normal mode. Unused when
the threading async mode is used.
• kwargs -- Additional web server options. The web server options are specific to
the server used in each of the supported async modes. Note that options provided
here will not be seen when using an external web server such as gunicorn, since
this method is not called in that case.
stop() Stop a running SocketIO web server.
This method must be called from a HTTP or SocketIO handler function.
start_background_task(target, *args, **kwargs)
Start a background task using the appropriate async model.
This is a utility function that applications can use to start a background task using the
method that is compatible with the selected async mode.
Parameters
• target -- the target function to execute.
• args -- arguments to pass to the function.
• kwargs -- keyword arguments to pass to the function.
This function returns an object compatible with the Thread class in the Python standard
library. The start() method on this object is already called by this function.
sleep(seconds=0)
Sleep for the requested amount of time using the appropriate async model.
This is a utility function that applications can use to put a task to sleep without having
to worry about using the correct call for the selected async mode.
test_client(app, namespace=None, query_string=None, headers=None, flask_test_client=None)
The Socket.IO test client is useful for testing a Flask-SocketIO server. It works in a
similar way to the Flask Test Client, but adapted to the Socket.IO server.
Parameters
• app -- The Flask application instance.
• namespace -- The namespace for the client. If not provided, the client connects to
the server on the global namespace.
• query_string -- A string with custom query string arguments.
• headers -- A dictionary with custom HTTP headers.
• flask_test_client -- The instance of the Flask test client currently in use.
Passing the Flask test client is optional, but is necessary if you want the Flask
user session and any other cookies set in HTTP routes accessible from Socket.IO
events.
flask_socketio.emit(event, *args, **kwargs)
Emit a SocketIO event.
This function emits a SocketIO event to one or more connected clients. A JSON blob can be attached
to the event as payload. This is a function that can only be called from a SocketIO event handler,
as in obtains some information from the current client context. Example:
@socketio.on('my event')
def handle_my_custom_event(json):
emit('my response', {'data': 42})
Parameters
• event -- The name of the user event to emit.
• args -- A dictionary with the JSON data to send as payload.
• namespace -- The namespace under which the message is to be sent. Defaults to the
namespace used by the originating event. A '/' can be used to explicitly specify the
global namespace.
• callback -- Callback function to invoke with the client's acknowledgement.
• broadcast -- True to send the message to all clients, or False to only reply to the
sender of the originating event.
• to -- Send the message to all the users in the given room. If this argument is not set
and broadcast is False, then the message is sent only to the originating user.
• include_self -- True to include the sender when broadcasting or addressing a room, or
False to send to everyone but the sender.
• skip_sid -- The session id of a client to ignore when broadcasting or addressing a room.
This is typically set to the originator of the message, so that everyone except that
client receive the message. To skip multiple sids pass a list.
• ignore_queue -- Only used when a message queue is configured. If set to True, the event
is emitted to the clients directly, without going through the queue. This is more
efficient, but only works when a single server process is used, or when there is a single
addressee. It is recommended to always leave this parameter with its default value of
False.
flask_socketio.send(message, **kwargs)
Send a SocketIO message.
This function sends a simple SocketIO message to one or more connected clients. The message can be
a string or a JSON blob. This is a simpler version of emit(), which should be preferred. This is a
function that can only be called from a SocketIO event handler.
Parameters
• message -- The message to send, either a string or a JSON blob.
• json -- True if message is a JSON blob, False otherwise.
• namespace -- The namespace under which the message is to be sent. Defaults to the
namespace used by the originating event. An empty string can be used to use the global
namespace.
• callback -- Callback function to invoke with the client's acknowledgement.
• broadcast -- True to send the message to all connected clients, or False to only reply to
the sender of the originating event.
• to -- Send the message to all the users in the given room. If this argument is not set
and broadcast is False, then the message is sent only to the originating user.
• include_self -- True to include the sender when broadcasting or addressing a room, or
False to send to everyone but the sender.
• skip_sid -- The session id of a client to ignore when broadcasting or addressing a room.
This is typically set to the originator of the message, so that everyone except that
client receive the message. To skip multiple sids pass a list.
• ignore_queue -- Only used when a message queue is configured. If set to True, the event
is emitted to the clients directly, without going through the queue. This is more
efficient, but only works when a single server process is used, or when there is a single
addressee. It is recommended to always leave this parameter with its default value of
False.
flask_socketio.join_room(room, sid=None, namespace=None)
Join a room.
This function puts the user in a room, under the current namespace. The user and the namespace are
obtained from the event context. This is a function that can only be called from a SocketIO event
handler. Example:
@socketio.on('join')
def on_join(data):
username = session['username']
room = data['room']
join_room(room)
send(username + ' has entered the room.', room=room)
Parameters
• room -- The name of the room to join.
• sid -- The session id of the client. If not provided, the client is obtained from the
request context.
• namespace -- The namespace for the room. If not provided, the namespace is obtained from
the request context.
flask_socketio.leave_room(room, sid=None, namespace=None)
Leave a room.
This function removes the user from a room, under the current namespace. The user and the
namespace are obtained from the event context. Example:
@socketio.on('leave')
def on_leave(data):
username = session['username']
room = data['room']
leave_room(room)
send(username + ' has left the room.', room=room)
Parameters
• room -- The name of the room to leave.
• sid -- The session id of the client. If not provided, the client is obtained from the
request context.
• namespace -- The namespace for the room. If not provided, the namespace is obtained from
the request context.
flask_socketio.close_room(room, namespace=None)
Close a room.
This function removes any users that are in the given room and then deletes the room from the
server.
Parameters
• room -- The name of the room to close.
• namespace -- The namespace for the room. If not provided, the namespace is obtained from
the request context.
flask_socketio.rooms(sid=None, namespace=None)
Return a list of the rooms the client is in.
This function returns all the rooms the client has entered, including its own room, assigned by
the Socket.IO server.
Parameters
• sid -- The session id of the client. If not provided, the client is obtained from the
request context.
• namespace -- The namespace for the room. If not provided, the namespace is obtained from
the request context.
flask_socketio.disconnect(sid=None, namespace=None, silent=False)
Disconnect the client.
This function terminates the connection with the client. As a result of this call the client will
receive a disconnect event. Example:
@socketio.on('message')
def receive_message(msg):
if is_banned(session['username']):
disconnect()
else:
# ...
Parameters
• sid -- The session id of the client. If not provided, the client is obtained from the
request context.
• namespace -- The namespace for the room. If not provided, the namespace is obtained from
the request context.
• silent -- this option is deprecated.
class flask_socketio.Namespace(namespace=None)
trigger_event(event, *args)
Dispatch an event to the proper handler method.
In the most common usage, this method is not overloaded by subclasses, as it performs the
routing of events to methods. However, this method can be overriden if special dispatching
rules are needed, or if having a single method that catches all events is desired.
emit(event, data=None, room=None, include_self=True, namespace=None, callback=None)
Emit a custom event to one or more connected clients.
send(data, room=None, include_self=True, namespace=None, callback=None)
Send a message to one or more connected clients.
close_room(room, namespace=None)
Close a room.
class flask_socketio.SocketIOTestClient(app, socketio, namespace=None, query_string=None, headers=None,
flask_test_client=None)
This class is useful for testing a Flask-SocketIO server. It works in a similar way to the Flask
Test Client, but adapted to the Socket.IO server.
Parameters
• app -- The Flask application instance.
• socketio -- The application's SocketIO instance.
• namespace -- The namespace for the client. If not provided, the client connects to the
server on the global namespace.
• query_string -- A string with custom query string arguments.
• headers -- A dictionary with custom HTTP headers.
• flask_test_client -- The instance of the Flask test client currently in use. Passing the
Flask test client is optional, but is necessary if you want the Flask user session and
any other cookies set in HTTP routes accessible from Socket.IO events.
is_connected(namespace=None)
Check if a namespace is connected.
Parameters
namespace -- The namespace to check. The global namespace is assumed if this
argument is not provided.
connect(namespace=None, query_string=None, headers=None)
Connect the client.
Parameters
• namespace -- The namespace for the client. If not provided, the client connects to
the server on the global namespace.
• query_string -- A string with custom query string arguments.
• headers -- A dictionary with custom HTTP headers.
Note that it is usually not necessary to explicitly call this method, since a connection is
automatically established when an instance of this class is created. An example where it
this method would be useful is when the application accepts multiple namespace connections.
disconnect(namespace=None)
Disconnect the client.
Parameters
namespace -- The namespace to disconnect. The global namespace is assumed if this
argument is not provided.
emit(event, *args, **kwargs)
Emit an event to the server.
Parameters
• event -- The event name.
• *args --
The event arguments.
• callback -- True if the client requests a callback, False if not. Note that
client-side callbacks are not implemented, a callback request will just tell the
server to provide the arguments to invoke the callback, but no callback is
invoked. Instead, the arguments that the server provided for the callback are
returned by this function.
• namespace -- The namespace of the event. The global namespace is assumed if this
argument is not provided.
send(data, json=False, callback=False, namespace=None)
Send a text or JSON message to the server.
Parameters
• data -- A string, dictionary or list to send to the server.
• json -- True to send a JSON message, False to send a text message.
• callback -- True if the client requests a callback, False if not. Note that
client-side callbacks are not implemented, a callback request will just tell the
server to provide the arguments to invoke the callback, but no callback is
invoked. Instead, the arguments that the server provided for the callback are
returned by this function.
• namespace -- The namespace of the event. The global namespace is assumed if this
argument is not provided.
get_received(namespace=None)
Return the list of messages received from the server.
Since this is not a real client, any time the server emits an event, the event is simply
stored. The test code can invoke this method to obtain the list of events that were
received since the last call.
Parameters
namespace -- The namespace to get events from. The global namespace is assumed if
this argument is not provided.
AUTHOR
Miguel Grinberg
COPYRIGHT
2014, Miguel Grinberg
Dec 24, 2020 FLASK-SOCKETIO(1)