The Socket.IO Client

This package contains two Socket.IO clients:

The methods in the two clients are the same, with the only difference that in the asyncio client most methods are implemented as coroutines.


To install the standard Python client along with its dependencies, use the following command:

pip install "python-socketio[client]"

If instead you plan on using the asyncio client, then use this:

pip install "python-socketio[asyncio_client]"

Creating a Client Instance

To instantiate an Socket.IO client, simply create an instance of the appropriate client class:

import socketio

# standard Python
sio = socketio.Client()

# asyncio
sio = socketio.AsyncClient()

Defining Event Handlers

The Socket.IO protocol is event based. When a server wants to communicate with a client it emits an event. Each event has a name, and a list of arguments. The client registers event handler functions with the socketio.Client.event() or socketio.Client.on() decorators:

def message(data):
    print('I received a message!')

@sio.on('my message')
def on_message(data):
    print('I received a message!')

In the first example the event name is obtained from the name of the handler function. The second example is slightly more verbose, but it allows the event name to be different than the function name or to include characters that are illegal in function names, such as spaces.

For the asyncio client, event handlers can be regular functions as above, or can also be coroutines:

async def message(data):
    print('I received a message!')

If the server includes arguments with an event, those are passed to the handler function as arguments.

Catch-All Event Handlers

A “catch-all” event handler is invoked for any events that do not have an event handler. You can define a catch-all handler using '*' as event name:

def catch_all(event, data):

Asyncio clients can also use a coroutine:

async def catch_all(event, data):

A catch-all event handler receives the event name as a first argument. The remaining arguments are the same as for a regular event handler.

Connect, Connect Error and Disconnect Event Handlers

The connect, connect_error and disconnect events are special; they are invoked automatically when a client connects or disconnects from the server:

def connect():
    print("I'm connected!")

def connect_error(data):
    print("The connection failed!")

def disconnect():
    print("I'm disconnected!")

The connect_error handler is invoked when a connection attempt fails. If the server provides arguments, these are passed on to the handler. The server can use an argument to provide information to the client regarding the connection failure.

The disconnect handler is invoked for application initiated disconnects, server initiated disconnects, or accidental disconnects, for example due to networking failures. In the case of an accidental disconnection, the client is going to attempt to reconnect immediately after invoking the disconnect handler. As soon as the connection is re-established the connect handler will be invoked once again.

The connect, connect_error and disconnect events have to be defined explicitly and are not invoked on a catch-all event handler.

Connecting to a Server

The connection to a server is established by calling the connect() method:


In the case of the asyncio client, the method is a coroutine:

await sio.connect('http://localhost:5000')

Upon connection, the server assigns the client a unique session identifier. The applicaction can find this identifier in the sid attribute:

print('my sid is', sio.sid)

Emitting Events

The client can emit an event to the server using the emit() method:

sio.emit('my message', {'foo': 'bar'})

Or in the case of asyncio, as a coroutine:

await sio.emit('my message', {'foo': 'bar'})

The single argument provided to the method is the data that is passed on to the server. The data can be of type str, bytes, dict, list or tuple. When sending a tuple, the elements in it need to be of any of the other four allowed types. The elements of the tuple will be passed as multiple arguments to the server-side event handler function.

The emit() method can be invoked inside an event handler as a response to a server event, or in any other part of the application, including in background tasks.

Event Callbacks

When a server emits an event to a client, it can optionally provide a callback function, to be invoked as a way of acknowledgment that the server has processed the event. While this is entirely managed by the server, the client can provide a list of return values that are to be passed on to the callback function set up by the server. This is achieved simply by returning the desired values from the handler function:

def my_event(sid, data):
    # handle the message
    return "OK", 123

Likewise, the client can request a callback function to be invoked after the server has processed an event. The socketio.Server.emit() method has an optional callback argument that can be set to a callable. If this argument is given, the callable will be invoked after the server has processed the event, and any values returned by the server handler will be passed as arguments to this function.


The Socket.IO protocol supports multiple logical connections, all multiplexed on the same physical connection. Clients can open multiple connections by specifying a different namespace on each. Namespaces use a path syntax starting with a forward slash. A list of namespaces can be given by the client in the connect() call. For example, this example creates two logical connections, the default one plus a second connection under the /chat namespace:

sio.connect('http://localhost:5000', namespaces=['/chat'])

To define event handlers on a namespace, the namespace argument must be added to the corresponding decorator:

def my_custom_event(sid, data):

@sio.on('connect', namespace='/chat')
def on_connect():
    print("I'm connected to the /chat namespace!")

Likewise, the client can emit an event to the server on a namespace by providing its in the emit() call:

sio.emit('my message', {'foo': 'bar'}, namespace='/chat')

If the namespaces argument of the connect() call isn’t given, any namespaces used in event handlers are automatically connected.

Class-Based Namespaces

As an alternative to the decorator-based event handlers, the event handlers that belong to a namespace can be created as methods of a subclass of socketio.ClientNamespace:

class MyCustomNamespace(socketio.ClientNamespace):
    def on_connect(self):

    def on_disconnect(self):

    def on_my_event(self, data):
        self.emit('my_response', data)


For asyncio based servers, namespaces must inherit from socketio.AsyncClientNamespace, and can define event handlers as coroutines if desired:

class MyCustomNamespace(socketio.AsyncClientNamespace):
    def on_connect(self):

    def on_disconnect(self):

    async def on_my_event(self, data):
        await self.emit('my_response', data)


When class-based namespaces are used, any events received by the client 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 socketio.Client and socketio.AsyncClient classes that default to the proper namespace when the namespace argument is not given.

In the case that an event has a handler in a class-based namespace, and also a decorator-based function handler, only the standalone function handler is invoked.

Disconnecting from the Server

At any time the client can request to be disconnected from the server by invoking the disconnect() method:


For the asyncio client this is a coroutine:

await sio.disconnect()

Managing Background Tasks

When a client connection to the server is established, a few background tasks will be spawned to keep the connection alive and handle incoming events. The application running on the main thread is free to do any work, as this is not going to prevent the functioning of the Socket.IO client.

If the application does not have anything to do in the main thread and just wants to wait until the connection with the server ends, it can call the wait() method:


Or in the asyncio version:

await sio.wait()

For the convenience of the application, a helper function is provided to start a custom background task:

def my_background_task(my_argument):
    # do some background work here!

task = sio.start_background_task(my_background_task, 123)

The arguments passed to this method are the background function and any positional or keyword arguments to invoke the function with.

Here is the asyncio version:

async def my_background_task(my_argument):
    # do some background work here!

task = sio.start_background_task(my_background_task, 123)

Note that this function is not a coroutine, since it does not wait for the background function to end. The background function must be a coroutine.

The sleep() method is a second convenience function that is provided for the benefit of applications working with background tasks of their own:


Or for asyncio:

await sio.sleep(2)

The single argument passed to the method is the number of seconds to sleep for.

Debugging and Troubleshooting

To help you debug issues, the client can be configured to output logs to the terminal:

import socketio

# standard Python
sio = socketio.Client(logger=True, engineio_logger=True)

# asyncio
sio = socketio.AsyncClient(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, unexpected disconnections and other issues.