The Socket.IO Clients¶

This package contains two Socket.IO clients:

a “simple” client, which provides a straightforward API that is sufficient for most applications
an “event-driven” client, which provides access to all the features of the Socket.IO protocol

Each of these clients comes in two variants: one for the standard Python library, and another for asynchronous applications built with the asyncio package.

Installation¶

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]"

Using the Simple Client¶

The advantage of the simple client is that it abstracts away the logic required to maintain a Socket.IO connection. This client handles disconnections and reconnections in a completely transparent way, without adding any complexity to the application.

Creating a Client Instance¶

The easiest way to create a Socket.IO client is to use the context manager interface:

import socketio

# standard Python
with socketio.SimpleClient() as sio:
    # ... connect to a server and use the client
    # ... no need to manually disconnect!

# asyncio
async with socketio.AsyncSimpleClient() as sio:
    # ... connect to a server and use the client
    # ... no need to manually disconnect!

With this usage the context manager will ensure that the client is properly disconnected before exiting the with or async with block.

If preferred, a client can be manually instantiated:

import socketio

# standard Python
sio = socketio.SimpleClient()

# asyncio
sio = socketio.AsyncSimpleClient()

Connecting to a Server¶

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

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

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

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

By default the client first connects to the server using the long-polling transport, and then attempts to upgrade the connection to use WebSocket. To connect directly using WebSocket, use the transports argument:

sio.connect('http://localhost:5000', transports=['websocket'])

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

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

The Socket.IO transport that is used in the connection can be obtained from the transport attribute:

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

The transport is given as a string, and can be either 'websocket' or 'polling'.

TLS/SSL Support¶

The client supports TLS/SSL connections. To enable it, use a https:// connection URL:

sio.connect('https://example.com')

Or when using asyncio:

await sio.connect('https://example.com')

The client verifies server certificates by default. Consult the documentation for the event-driven client for information on how to customize this behavior.

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 arguments provided to the method are the name of the event to emit and the optional data that is passed on to the server. The data can be of type str, bytes, dict, list or tuple. When sending a list or a tuple, the elements in it need to be of any allowed types except tuple. When a tuple is used, the elements of the tuple will be passed as individual arguments to the server-side event handler function.

Receiving Events¶

The client can wait for the server to emit an event with the receive() method:

event = sio.receive()
print(f'received event: "{event[0]}" with arguments {event[1:]}')

When using asyncio, this method needs to be awaited:

event = await sio.receive()
print(f'received event: "{event[0]}" with arguments {event[1:]}')

The return value of receive() is a list. The first element of this list is the event name, while the remaining elements are the arguments passed by the server.

With the usage shown above, the receive() method will return only when an event is received from the server. An optional timeout in seconds can be passed to prevent the client from waiting forever:

from socketio.exceptions import TimeoutError

try:
    event = sio.receive(timeout=5)
except TimeoutError:
    print('timed out waiting for event')
else:
    print('received event:', event)

Or with asyncio:

from socketio.exceptions import TimeoutError

try:
    event = await sio.receive(timeout=5)
except TimeoutError:
    print('timed out waiting for event')
else:
    print('received event:', event)

Disconnecting from the Server¶

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

sio.disconnect()

For the asyncio client this is a coroutine:

await sio.disconnect()

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.

Using the Event-Driven 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:

@sio.event
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:

@sio.event
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 and Namespace 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:

@sio.on('*')
def any_event(event, sid, data):
     pass

Asyncio servers can also use a coroutine:

@sio.on('*')
async def any_event(event, sid, data):
    pass

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.

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

Similarily, a “catch-all” namespace handler is invoked for any connected namespaces that do not have an explicitly defined event handler. As with catch-all events, '*' is used in place of a namespace:

@sio.on('my_event', namespace='*')
def my_event_any_namespace(namespace, sid, data):
    pass

For these events, the namespace is passed as first argument, followed by the regular arguments of the event.

Lastly, it is also possible to define a “catch-all” handler for all events on all namespaces:

@sio.on('*', namespace='*')
def any_event_any_namespace(event, namespace, sid, data):
    pass

Event handlers with catch-all events and namespaces receive the event name and the namespace as first and second arguments.

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:

@sio.event
def connect():
    print("I'm connected!")

@sio.event
def connect_error(data):
    print("The connection failed!")

@sio.event
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:

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

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 application can find this identifier in the sid attribute:

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

The Socket.IO transport that is used in the connection can be obtained from the transport attribute:

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

The transport is given as a string, and can be either 'websocket' or 'polling'.

TLS/SSL Support¶

The client supports TLS/SSL connections. To enable it, use a https:// connection URL:

sio.connect('https://example.com')

Or when using asyncio:

await sio.connect('https://example.com')

The client will verify the server certificate by default. To disable certificate verification, or to use other less common options such as client certificates, the client must be initialized with a custom HTTP session object that is configured with the desired TLS/SSL options.

The following example disables server certificate verification, which can be useful when connecting to a server that uses a self-signed certificate:

http_session = requests.Session()
http_session.verify = False
sio = socketio.Client(http_session=http_session)
sio.connect('https://example.com')

And when using asyncio:

connector = aiohttp.TCPConnector(ssl=False)
http_session = aiohttp.ClientSession(connector=connector)
sio = socketio.AsyncClient(http_session=http_session)
await sio.connect('https://example.com')

Instead of disabling certificate verification, you can provide a custom certificate authority bundle to verify the certificate against:

http_session = requests.Session()
http_session.verify = '/path/to/ca.pem'
sio = socketio.Client(http_session=http_session)
sio.connect('https://example.com')

And for asyncio:

ssl_context = ssl.create_default_context()
ssl_context.load_verify_locations('/path/to/ca.pem')
connector = aiohttp.TCPConnector(ssl=ssl_context)
http_session = aiohttp.ClientSession(connector=connector)
sio = socketio.AsyncClient(http_session=http_session)
await sio.connect('https://example.com')

Below you can see how to use a client certificate to authenticate against the server:

http_session = requests.Session()
http_session.cert = ('/path/to/client/cert.pem', '/path/to/client/key.pem')
sio = socketio.Client(http_session=http_session)
sio.connect('https://example.com')

And for asyncio:

ssl_context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
ssl_context.load_cert_chain('/path/to/client/cert.pem',
                            '/path/to/client/key.pem')
connector = aiohttp.TCPConnector(ssl=ssl_context)
http_session = aiohttp.ClientSession(connector=connector)
sio = socketio.AsyncClient(http_session=http_session)
await sio.connect('https://example.com')

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 arguments provided to the method are the name of the event to emit and the optional data that is passed on to the server. The data can be of type str, bytes, dict, list or tuple. When sending a list or a tuple, the elements in it need to be of any allowed types except tuple. When a tuple is used, the elements of the tuple will be passed as individual 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:

@sio.event
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.

Namespaces¶

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:

@sio.event(namespace='/chat')
def my_custom_event(sid, data):
    pass

@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):
        pass

    def on_disconnect(self):
        pass

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

sio.register_namespace(MyCustomNamespace('/chat'))

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):
        pass

    def on_disconnect(self):
        pass

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

sio.register_namespace(MyCustomNamespace('/chat'))

A catch-all class-based namespace handler can be defined by passing '*' as the namespace during registration:

sio.register_namespace(MyCustomNamespace('*'))

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:

sio.disconnect()

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:

sio.wait()

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!
    pass

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!
    pass

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:

sio.sleep(2)

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.

python-socketio

Navigation

Related Topics

The Socket.IO Clients¶

Installation¶

Using the Simple Client¶

Creating a Client Instance¶

Connecting to a Server¶

TLS/SSL Support¶

Emitting Events¶

Receiving Events¶

Disconnecting from the Server¶

Debugging and Troubleshooting¶

Using the Event-Driven Client¶

Creating a Client Instance¶

Defining Event Handlers¶

Catch-All Event and Namespace Handlers¶

Connect, Connect Error and Disconnect Event Handlers¶

Connecting to a Server¶

TLS/SSL Support¶

Emitting Events¶

Event Callbacks¶

Namespaces¶

Class-Based Namespaces¶

Disconnecting from the Server¶

Managing Background Tasks¶

Debugging and Troubleshooting¶